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Identification of Research Areas inResponse to the Fukushima AccidentJanuary 2013

Report of the SNETP

Fukushima Task Group

This document has been prepared by theFukushima Task Group establishedwithin the Sustainable Nuclear Energy

Technology Platform.

Contact: [email protected]

R e p o r t o f t h e S N E T P F u k u s h i m a T a s k G r o u p 3

Jozef Misak,Chairman of the Fukushima Task Group

R e p o r t o f t h e S N E T P Fu k u s h i m a Ta s k G r o u p

In response to the Fukushima accident, onMarch 31st, 2011, the Governing Board ofthe Sustainable Nuclear Energy Technology

Platform decided to establish a Task Group withthe main objective to assess implications of theaccident on the medium and long term researchand development Platform’s programme. It wasclear that the most meaningful response toFukushima is to learn and convert the lessonslearned into better knowledge and subsequentsafety improvements of current and futurereactor systems, and that the research anddevelopment should be one of the maincontributors to achieving this goal.

Large and fast growing volume of relevantinformation has been collected and evaluated asa basis for the Group’s work. The report focuseson research associated with safety of nuclearpower plants with emphasis on robustness of theplants against extreme hazards, on preventionand mitigation of severe accidents includingorganizational and societal factors, and on

mitigation of accident consequences. The Groupconcluded that although the Fukushimaaccident does not require completely newresearch directions, it is appropriate specifyingor updating priorities in certain areas. Findingsof the Group, with identification of specificresearch priorities for 13 thematic areas, aresummarized in this report. Many suggestionsformulated for existing plants are also applicableto new reactor designs.

The ideas included in the report have beenpresented by members of the Group in severaltechnical meetings, and already reflected in theupdated SNETP Strategic Research andInnovation Agenda. The Group is confidentthat the report will help the nuclear communityin better determining future research directions.It should be however understood thatinformation about the Fukushima accident isstill uncertain and not complete. The reportshould be therefore considered as live documentwhich may be subject to future updating.

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The Sustainable Nuclear Energy Technology Platform

Foreword by the chairman

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� 1. Introduction

The accident which occurred at theFukushima Dai-ichi nuclear power plant(NPP) on March 11, 2011, raised public

concern on nuclear energy and drew a newattention to the safety of NPPs, in particular incase of extremely severe external hazards.

A number of initiatives have been undertaken inmany countries and at international level inorder to take into account the lessons learnedfrom this accident for the improvement ofnuclear reactors design and the organization tomanage radiological accidents. It can be quotedthe OECD/NEA ministerial seminar on June 7,2011 the safety authorities’ forum on June 8,2011 and the ministerial conference organizedby the IAEA from 20 to 24 June, 2011 whichresulted in the Action Plan on Nuclear Safetyapproved by the Board of Governor andendorsed by the IAEA general conference.

Most of the countries operating nuclear reactorshave launched systematic reassessment of thesafety margins oftheir nuclear fleetunder severe naturalhazards, usuallycalled stress-tests. A comprehensivestress tests processhas been launchedby the EuropeanCouncil under the European Commissioncoordination.

SNETP gives nuclear safety the highest priorityin its vision and Strategic Research andInnovation Agenda (2013-SRIA). Europeantechnical safety organizations (TSOs) areinvolved in the activities of the platformgovernance and working groups. SNETP

promotes safety related research andharmonization at European level, for currentand future generation of nuclear fissiontechnologies. This is the reason why the SNETPGoverning Board decided to empower a TaskGroup to investigate how the first lessonslearned from Fukushima accident could impactsafety related R&D orientations and priorities.

In its investigations, the Task Groupconcentrated on medium and long term R&D,in particular on the developments, updating andvalidation of methods and tools for areas whichare not considered as adequately covered up tonow. Of course, it is understood that a broadranging multidisciplinary and synergic approachto reflect fully the lessons learned fromFukushima should be foreseen. An example ofan additional factor contributing to furtherimprovements in the safety margins is testing ofsystems and components aimed at a betterquantification of safety margins. Even if thetopic has been only marginally addressed in theTask Group’s work, identification of appropriateexisting – and future – experimental facilitiesand relevant tests should be also considered.

This report addresses high level orientationsthat will be further detailed by SNETPTechnology Task Groups (NUGENIA, ESNIIand NC2I). On longer term – in several years –the outcomes of the future expertise acquired onthe four reactors and fuel pools of FukushimaDai-ichi site will be very valuable for thequalification and validation of the results of theproposed R&D tasks.

This report should be thus considered as a firststep of the process, but it already catchesimportant features which are summarizedbelow.

Most Nuclearcountries have

launched systematicreassessment of the

safety margins

R e p o r t o f t h e S N E T P Fu k u s h i m a Ta s k G r o u p 7

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Executive summary

� 2. What are the main challenges revealed by the Fukushima accident?

The Fukushima accident was triggeredby the combination of two initiatingevents:

� An exceptional magnitude earthquake whichcaused the sudden total loss of almost all the off-site power supply. The reactors 1-2-3 which werein operation have been automatically shut-down.The residual heat removal systems were startedimmediately after, relying on electricity suppliedby emergency power sources (diesel generatorsand batteries).

� The associated tsunami caused the flooding of thesite by a wave about twice the size consideredpreviously in the safety evaluation. It led to boththe loss of all the emergency power supply systemsand of the heat sink.

The immediate challenge for the emergencyresponse team was to recover cooling capabilitiesin a situation where the off-site power supplyhas required about 11 days to be effective. Thissituation has affected all 4 reactors and the spentfuel pools. A detailed analysis of the systemdeficiencies which caused the loss of emergencycooling systems is still difficult to perform dueto the lack of information but nevertheless,according to the current knowledge of theaccident, the efficiency and the reliability of thedecision making process during this extremeevent were not optimal.

The challenges identified from the lessonslearned are the following:

� To enhance further defence-in-depth capabilitiesfor any type of initiating events, especially forsevere natural hazards and any theircombinations. It should be considered for existingreactors, future Gen III reactors as well as for thedevelopment of Gen IV reactors.

� To address more systematically at the design stagethe plant features for coping the design extensionconditions (beyond design basis accidents) toassure the robustness of the defence in-depth andto avoid cliff edge effects. The approach shouldinclude situations where several units on the samesite are affected by a beyond design basis event.

� To develop multiple and more robust lines ofdefence with respect to design basis events anddesign extension conditions to define additionalmeasures to be considered in the design.

A special emphasis has to be put on theemergency management which has beenchallenged during the accident due to:

� The concomitance of many events, the severeenvironmental conditions and the mutualinteraction between the affected units on site.

� The complexity and the difficulty of the decisionmaking process which has altered the effectivenessand the promptness of the actions and which hasgenerated both confusions and delays.

� The difficulties to recover a suitable and stableelectrical supply source during several days.

Improvements of the emergency preparednessand response should consider:

� The availability of more sophisticated tools toprovide the operators with more reliable andquick monitoring of the plant status andprediction of the accident progression to help inthe implementation of an appropriate recoverystrategy.

� The availability of redundant intervention meansin the vicinity of the site.

� An enhanced coordinated internationalcooperation/expertise which could provide help inthe plant status diagnosis, in the prognosis for thesituation evolution and on the mitigation strategy.

� 3. Identification of relevant research areas

Safety related R&D tasks are alreadyincluded in the main orientations of theSNETP 2009 SRA and, in particular for

the current reactors, specific actions areproposed related to the long term operation andto severe accident management. The R&Deffort is largely shared through EURATOMframework programme and, at broaderinternational level, through the OECD/NEAprogrammes. The R&D on severe accidents hasin particular the objective to produceknowledge, methods and codes to assess risks forbeyond design situations.

Ongoing investigation of the Fukushimaaccident is influencing the scale researchpriorities with a more specific focus on extremeexternal events and their combinations, oncommon mode failures and human behaviour,with subsequent assessment of the impact therobustness of the defence in depth.

R e p o r t o f t h e S N E T P F u k u s h i m a T a s k G r o u p8

The Task Group has identified 13 main areas ofresearch, focused on siting, design and operationof NPPs. These areas are briefly described below.

a. Systematic assessment ofvulnerabilities to defence-in-depth and safety margins for beyond design basis loads

The main objective of this area is to develop asystematic methodology for the determination ofevaluation safety margins and the evaluation riskof occurrence of cliff-edge effects for extremeevents beyond the design basis.

The methodology should include theidentification of rare extreme events potentiallyleading to common cause failures of multipleplants system, analysis of their consequences andthe effects of interrelation between plant systemsand operators actions. The R&D tasks shouldalso include the development of complex modelsfor the behaviour of NPPs under extreme loadsbeyond the design basis and the determination ofthe ultimate capacity of physical barriers and ofthe integrity of the reactor containment.

Further improvement in the safety margins couldbe achieved through extensive testing and/or re-testing systems and components against e.g.seismic loads. Identification of current and futureexperimental facilities able to host such testwould valuably contribute to safety.

b. Human and organizational factors under high stress and harmful conditions

Fukushima accident revealed the importance ofhuman and organizational factors under highstress and harsh working conditions, affectingthe decision-making process. The humanresponse to these challenging conditions shouldbe explored in more detail, in order to identifyfeasible and efficient ways to improve theemergency preparedness and the response to asevere nuclear accident, including developmentof supporting tools and procedures.

c. Improved methods for external event hazard evaluation

The focus of this area is to enhance andharmonize the methodologies for the

assessment of external hazards and for theircombination as well as their effects on NPPs.The methodologies should be updated on thebasis of the state of art knowledge in earthscience and may also consider man-madehazards (airplane crash, missile impacts, cyber-attacks, malevolent acts).

d. Use of the probabilistic methods to assess plant safety in relation to extreme events

To complement the deterministic approach,PSA is performed with a systematic approachmaking use of realistic assessments of theperformance of equipments and operators. PSAhas the potential to provide a deepunderstanding of the potential risk resultingfrom the operation of a NPP over wide range ofconditions.

The main objective is to extend the present PSAmethodologies to extreme events with very lowfrequency. It should take into consideration suchfactors as the availability of site infrastructure,the prolonged station black-out and thepotential loss of ultimate heat sink, which are“traditionally” out of the scope of PSA. Thistype of analysis should also consider humanreliability and behaviour under suchcircumstances.

e. Advanced deterministic methods to assess plant safety in relation to extreme events

The works in this area should focus onimprovement and harmonisation of thetraditional deterministic methods for theassessment of the damage to structures,components and systems of NPP under variousextreme loads beyond the design basis. Updatedmethods should address extreme loads and theircombinations, and determine the measures forthe elimination or mitigation of the damage.

f. Advanced safety systems

The Fukushima accident and in particular thelong term duration of the loss of electricitysupply highlighted the potential interest inpassive systems or, more generally, in safetyrelated equipment and components based on


R e p o r t o f t h e S N E T P F u k u s h i m a T a s k G r o u p

passive rules. The main objective of futureinvestigation should be to demonstrate theactual capability of such systems to guaranteethe residual heat removal under extremeaccident scenarios (prolonged station black-out,loss of infrastructures, loss of instrumentation,and reduced accessibility to the plant).

Research in this area should also address thedevelopment and the qualification of numericaltools for the 3D simulation of relevantphenomena like multiphase natural circulationand heat exchange where challenges still existfor getting reliable results.

g. Advanced materials for nuclear power

SNETP supports ongoing comprehensive R&Dprogrammes on advanced materials for nuclearpower which are performed under the umbrellaof the European Energy Research Association(EERA). The main objective is to develop, testand model the behaviour of existing and newstructural material for nuclear components,taking into account the harsh conditions(corrosive environment, high radiation doseexposure, and high frequency thermal andmechanical fatigue).

h. Advanced methods for the analysisof severe accidents

The main objective is to review the state of theart and to enhance and validate robust modelsand simulation platforms for the analysis ofsevere accidents.

Works in this area should continue in on-goingresearch programmes with focus on phenomenawhich are not yet adequately understood, such aspost accidental heat removal, coolability ofoverheated and partially relocated reactor core,in-vessel core melt progression, in-vessel moltencorium retention, corium stabilization incontainment, molten-core-concrete-interactionand hydrogen generation and behaviour in thecontainment.

i. Improved procedures formanagement of severe accidents

The main objective is to enhance the overalllevel of knowledge, skills, predictive tools and

strategies applicable for severe accidentmanagement (including conditions of long termloss of electricity supply, loss of heat sink). Thetasks include investigations on reliablemonitoring and communication tools underharsh severe conditions.

j. Assessment of the radiologicaleffects of the severe accidents

This area should provide for support to ofexisting R&D programmes in order to updateand validate models for determination of thesource term, for dissemination of radioactivesubstances and for assessment of the radiologicalimpact of the releases on the human health andon the environment. It may includeharmonization of the intervention levels forradiological accidents and reconsideration of theINES scale as a tool for communication with thepublic.

k. Improved modelling of fueldegradation in spent fuel pool

The main objective of this area is to improve theknowledge of the fuel degradation phenomenaand failure modes for fuel assemblies stored inthe spent fuel pools and to develop and validatethe modelling tools.

The R&D actions should consider the scenarioswith the dry out of the pool and the followingreflooding phase including the effect of thepossible presence of debris that may fall downinto the pool.

l. Methods for minimization ofcontamination in the NPPsurroundings and for treatment oflarge volume of radioactive waste

Assessment of the radiological conditions in awide area around the NPP and mitigation of theeffects of contamination on the population andthe environment have been key challenges sincethe beginning of the Fukushima accident to thepresent day, and will remain relevant for the nextyears.

The ultimate goal in this area covers thecomplete renovation and restoration ofcontaminated territory. Effectiveness of existingmethods for radiological environmental

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Figure 1 - Nuclear power plants in service in Japan (source " VGB, GRS)


monitoring as well as availability of suitabletechnical means for the rehabilitation ofpotentially contaminated territory should beevaluated and attention should be paid to themethods for the isolation and fixation of theradioactive materials released outside the NPP.

m.Accident management in the framework of the integrated rescue system

Problems encountered in the management ofFukushima event claim for updated methods forthe evaluation of the radiological status on thesite and in the surrounding, for supportingimportant decisions like evacuation and for theimprovement of the integrated rescue systems atthe national and international level. Also, theway to communicate the radiological data to thepublic needs to be improved for a more clearunderstanding of the actual level of the threat.

� 4. Conclusions

Despite Fukushima’s accident, thenuclear energy remains an importantcomponent for the European energy

mix and also represents a significantcontribution to cover the worldwide energy

needs. It is the responsibility of the nuclearenergy stakeholders including SNETP tobenefit from the lessons learned from theFukushima accident.

Research and development are essential tools fora better understanding of the phenomena and,thus, are necessary to derive practical indicationsto enhance the prevention and the mitigation ofsevere accidents.

No really new phenomena were revealed fromthe Fukushima accident and the basicorientations of the SRA are still valid. Howeverthe specific research areas identified in thisdocument shall be considered with theappropriate priority given to them in the updateof the SRA. In particular, the issues related torare extreme events severe accidents should beconsidered in a more comprehensive approachto safety in order to better quantify the designmargins and understand the behaviour of NPPsunder beyond design basis scenarios. The R&Dactivities identified in the current documentshould continue playing an essential role infuture utilization of nuclear power, supportingthe life extension of the current LWRs, whichshould be associated with comprehensive safetyassessment using up to date standards, thedeployment of the Generation III reactors andthe development of the Generation IVtechnologies, always keeping high safety level asutmost priority.

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Foreword by the chairman 5Executive summary 7Table of contents 13

1. Introduction 152. Overview of challenges identified during Fukushima accident 172.1 The Fukushima site 172.2 Accident scenario for reactors 1, 2 and 3 172.3 Spent fuel pools 192.4 Fukushima radiological issues 202.5 The current situation in Fukushima and recovery activities 21

3. Lessons learned from the Fukushima accident 233.1 Overview of Fukushima Lessons in Major International Activities and Documents 233.2 Lessons drawn on enhancement of safety infrastructure and culture 263.3 Lessons drawn on plant design and on evaluation of external hazards 263.4 Lessons drawn on mitigation of severe accidents 273.5 Lessons drawn on the emergency preparedness and management 273.6 Outcome of the stress-tests and R&D implication 293.7 Impact on the R&D directions and priorities 30

4. Identification of research areas associated with the lessons learned 334.1. Systematic assessment of vulnerabilities to defence-in-depth

and safety margins for beyond design basis loads 334.2. Human/organizational factors under high stress and harmful conditions 334.3. Improved methods for external event hazard evaluation 344.4. Use of the probabilistic methods to assess plant safety in relation to extreme events 354.5. Advanced deterministic methods to assess plant safety in relation to extreme events 364.6. Advanced safety systems for nuclear power plants 364.7. Advanced materials for nuclear power 374.8. Advanced methods for the analysis of severe accidents 384.9. Improved procedures for management of severe accidents 384.10. Assessment of the radiological effects of the severe accidents 394.11. Improved modelling of fuel degradation in the spent fuel pool 404.12. Methods for minimization of contamination in the NPP surroundings

and for treatment of large volume of radioactive waste 414.13. Accident management in the framework of the integrated rescue system 41

5. Basic insights related to research needs for Gen II and III reactors 435.1 Overview 435.2 Research areas 445.3 Implications for NUGENIA Technical Areas 45

6. Conclusions 517. References 53

Contributors to drafting and review 55Acknowledgements 55


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Table of contents

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1. Introduction

The Sustainable Nuclear Energy Technology Platform FT

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The accident which occurred on 11 March2011 led to the destruction of fournuclear reactors at the Fukushima Dai-

ichi nuclear site and again drew attention to thesafety of NPPs. Although the causes, course andconsequences of the accident were stronglylinked to the given location and to a specific typeof a boiling water reactor, utilizing lessonslearned about the possibility of a severe accidentwith radiological consequences, due to failureswith a common cause, is also relevant for othersites and for other types of reactors.

Following the accident, a number of initiativeshave been undertaken by all stakeholders of thenuclear-energy sector, including designers,manufacturers, operators and supervisory bodiesof nuclear facilities in many countries. Theseinitiatives were established in order to learnfrom the accident and to utilize the lessonslearned for further safety improvements. Inaccordance with the principles of feedback ofoperating experience, operators of nuclearfacilities have done and continue to performassessment of the safety margins and adoptmeasures to increase the safety level of the plantsnamed stress tests. National regulatory bodies,international organizations and EuropeanCommission (EC) were also extensivelyinvolved in reviewing the stress test reports.

It is clear that after these immediate steps, thelessons learned from Fukushima in the mediumand long term will be reflected also in the designrequirements for NPPs, in international safetystandards, regulations issued by nationalsupervisory authorities, operational procedures,emergency planning, etc. All of theseforthcoming steps should be based on theupdated level of knowledge, which should be theproduct of comprehensive research anddevelopment projects.

Nuclear safety related research and development(R&D) constitutes an important base for safetyimprovements and accident prevention. The

Sustainable Nuclear Energy TechnologyPlatform (SNETP), as the European platformwith the overall goal to enhance thesustainability of nuclear power by supportingtechnological development, should definitelyreflect implications of Fukushima into its ownactivities, in particular in nuclear safety relatedresearch. With this objective, the SNETPGoverning Board at its 7th meeting held inRome, 31 March 2011 decided to establish aTask Group in order to:

� Assess the lessons learned from the accident.� Assess the results of the stress tests.� Assess the implications for SNETP on Generation

II/III (Gen II/III), as well as on other componentsof the Platform.

� Make proposals to SNETP (and possibly in turn, tothe SNETP Board or to the EC).

The focus on R&D distinguishes the TaskGroup from the other groups and initiatives thatalready have issued reports on lessons learnedfrom the Fukushima accident.

Coherently with its mission, and inconsideration of the work already done by thevarious organisations, the Task Group hasconcentrated its attention on medium and longterm R&D tasks, in particular on development,updating and validation of methods andcomputational tools for those areas which arenot considered as sufficiently well understood.Of course, it is acknowledged that a broadranging multidisciplinary and synergic approachto reflect fully the lessons learned fromFukushima should be foreseen.

The results of the Task Group’s work aresummarized in the current document. Reflectingthe Fukushima accident, it is more focused onlight water reactor technology. Nevertheless, theauthors believe that many of the generalimplications and areas for additional researchand development are applicable for other


nuclear installations and reactor designs,including Gen IV reactors.

This document is intended to be a “high level”one, assuming that more detailed tasks will beelaborated by the three technological WGs ofthe SNETP, i.e. NUGENIA, ESNII and NC2I.

Nevertheless, in chapter 5, more attention isgiven to the feedback of the Fukushima accidenton the Gen II and III reactors, since this mayhave a much greater impact in the short term.

In addition to this introduction the documentconsists of 5 chapters.

� In chapter 2 a brief description of the Fukushimaaccident is summarized and an overview ofchallenges identified during the accident is given.

� Chapter 3 makes reference to other internationalactivities and documents issued and summarisesthe lessons learned from the accident for severalareas. Lessons from the European stress tests,which are relevant for various R&D areas, are alsogiven here.

� Chapter 4 contains proposals for 13 research areasidentified by the Task Group, as a reflection of thelessons learned from the Fukushima accident asdiscussed in chapter 3.

� Chapter 5 discusses cross-reference of newlyproposed R&D activities to other SNETP targetresearch areas.

� Finally, chapter 6 summarizes main conclusions ofthis report.


Figure 2 - Air dose over 1 m above ground level

2. Overview of challenges identified during Fukushima accident


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� 2.1 The Fukushima site

Fukushima Dai-ichi NPP is located in theNorthern region of Japan, in the FutabaCounty, Fukushima Prefecture, facing the

Pacific Ocean on the east side about 300 kmnorth-east of Tokyo. The site, operated by theTokyo Electric Power Company TEPCO, hosts6 nuclear units and a spent fuel storage pool aswell. Unit 1 has a BWR3 reactor and MARK-1containment, Units 2, 3 and 4 are BWR4MARK-1, Unit 5 is a BWR5 MARK-1 andUnit 6 is a BWR5 MARK-2. The nominalelectrical power of the reactors in the Fukushimasite is 460 MWe for Unit 1, 784 MWe for Units2-5 and 1100 MWe for Unit 6. When theearthquake struck Japan, at 2:46 p.m. JST (Japan Standard Time) on 11 March 2011,reactors 1, 2 and 3 were in full operation, whilereactors 4, 5 and 6 were at shut-down.

Figure 3 - Fukushima Dai-ichi NPP

� 2.2 Accident scenario for reactors 1, 2 and 3

The extremely strong earthquake - closeto level 9 on the Richter scale (10oMSK-64) – had its epicentre in the sea,

about 100 km offshore from the Fukushima site.It caused a sudden total loss of almost all the off-site power supplies. Reactors 1, 2 and 3, whichwere in full operation, were automatically shut-

down by control rod insertion. The residual heatremoval systems were started immediately after,relying on electricity supplied by emergencypower sources (diesel generators and batteries).

The Japanese tsunami alert system was activatedvery quickly. However, less than one hour laterthe first earthquake strike, the Fukushima siteinstallations were flooded by a tsunami wavetwice as high as the maximum provisional valueconsidered by TEPCO in its risk re-evaluationcarried out in the year 2000, and more thanthree times higher than the value predicted bythe tsunami alert.

The wave flooded the emergency powergenerators and destroyed the pumping stationsthus preventing the reactors and the fuel storagepools from being cooled normally. Nevertheless,cooling of the reactors continued to differentextents and by different means for several hoursthrough the emergency cooling systems, i.e.:

� In Reactor 1, through the Isolation Condenser (IC),which is able to condense the vapour from thereactor coolant system and inject it back to thesystem, thereby establishing heat removal bynatural circulation. The High Pressure CoreInjection (HPCI) System, which is designed toprovide water through the annular torus and/orthe storage pool, was unable to operate. TEPCOtried supplying water to the core cooling systemvia fire pumps, but their efficiency turned out to bequite low, possibly because some valves were stuckin the wrong position.

� In Reactor 2, the HPCI System was failing too, butthe cooling water was supplied until 1:30 p.m.March 14 through the Reactor Core IsolationCooling (RCIC) System, operating through aturbo-pump fed by the core outlet vapour.

� In Reactor 3, both emergency cooling systemsremained operational for several hours; the RCICprovided the core cooling until 11:36 March 12,when it shut-down; supplementary cooling wasensured by the HPCI, which remained operationaluntil 2:42, on March 13.


All emergency cooling systems wereprogressively lost for different reasons, so thatnone of them was in operation by the afternoonMarch 14.

The system deficiencies which caused the loss ofthe emergency cooling system are difficult toassess, as reliable information from the site isstill missing. Even, the circumstances whichprevented the operators from actuating injectionof cold water from outside the site (the sea, forinstance) to cool the reactor vessels down stillremain undisclosed1. Anyway, according tocurrent knowledge, the efficiency and reliabilityof the decision making process during thisextreme event was clearly not optimal.

When the cooling was lost, the residual power tobe removed was estimated to be in the range 5 to10 MW per reactor, which required supplyingapproximately 5 to 10 m3/h of fresh water toeach reactor. The water in the reactor startedvaporizing, which led to a sudden pressureincrease and caused the venting of vapour intothe suppression pool (wet-well), located in theannular torus. This is designed to be cooleddown under emergency conditions through aheat exchanger, which unfortunately was notoperating because the heat sink had been lost.The extensive vaporisation caused a reduction ofthe water level in the core, which quite quicklyreached the threshold for clad rupture(estimated to be about 2/3 of the fuel assemblyheight)2 .

Several depressurisation operations of thecontainments were carried out when the coredegradation had already started. Consequently,

large amounts of hydrogen were released outsidethe reactor containments, which resulted inviolent explosions within the reactor buildings3.These explosions partially destroyed thebuildings and also affected the fuel storage poolfacility. The detonations damaged the Reactor 1building first, then the Reactor 3, and they aremost likely to have caused damage to theReactor 2 torus.

Despite the extreme conditions due to the veryhigh radioactivity levels, the operators eventuallysucceeded in injecting water from the sea intothe vessels of the reactors, through the firepumps, which contributed to stabilising thesituation.

Until the connection to an external power sourcewas restored, 11 days after the initiating eventon March 22, all the attempts to recover theelectrical supply from outside the site failed,mainly due to electrical control panel short-circuiting.

Fresh water has been continuously injected intothe reactor cores since May 25.

On March 25, the operators discovered largeamounts of contaminated water in thebasements of the turbine buildings4. Thehandling of this contaminated water and theliquid radioactive waste remained one of themajor concerns on the site. Many efforts havebeen made by TEPCO including use of storagetanks for the contaminated water andinstallation of decontamination units.

This document does not deals in detail withbroad activities on reconstruction of the accident

1 - Actually, the vesselmaterial was quite old andthe embrittlement couldhave been significant.

2 - It is remembered thatthe LWR fuel clad startsdeteriorating at about 800 °C, then potentiallybreaks out; eventually thefuel starts melting ataround 2300 °C througheutectic interactions. InFukushima, the oxidationreaction of the claddings athigh temperature under theaction of the waterproduced large amounts ofzirconium oxide andhydrogen, which mixed withvapour and built-up in theuppermost region of theprincipal cooling circuit,then within thecontainment. The chemicalreaction was accelerated bythe temperature increase inparticular when thetemperature exceeded 1200 °C. As much as 50%more hydrogen could havebeen delivered, comparedto conventional PWRs, as aconsequence of the largeamount of zirconium due tothe fuel assembly design.

3 - During the fueldegradation, the productionof hydrogen has beencoupled with the emissionof volatile fission products.Thus the venting released tothe environment largeamounts of volatileproducts, such as the noblegases, the iodine - in gasform -, and aerosols,including Caesium 137.Luckily, part of thecontaminants had beentrapped in the wet-well, soreducing the environmentalcontamination.

Figure 4 - Causes of the accident and plant damages


scenario, which have been carried-out, and arestill ongoing, because of limited informationavailable up to now. Nevertheless it should beunderlined that all activities aimed at aninvestigation of the phenomenology of theevents and available data are extremelyimportant to enhance the knowledge andimprove the reliability of computation throughvalidation, especially for the severe accidentssimulation tools.

� 2.3 Spent fuel pools

When the accident happened, the fuelstorage pool of reactor 4 - which hadbeen shut-down since December

2010 for maintenance operations - had got themaximum load with 3 full cores, including thejust unloaded one.

On March 15, a series of events - either fires orhydrogen fast combustion (deflagrations) -affected the Reactor 4 building and causedserious structural damage. According to currentevaluations, these events could not haveoriginated from either thermal or neutronicsevents in the pool5.

Several assumptions have been made to explainthese deflagrations, although all of them requirefurther confirmation. Two of the main ones notdirectly related to the event in the pools were:

� The build-up of hydrogen originating fromReactor 3 through the existing connections,because the two plants share the same stack6.

� The rupture of a hydrogen circuit in the reactorbuilding as a direct consequence of theearthquake.

Other assumptions more directly linked to thepools include:

� The locally ineffective venting and cooling down -due to work degradation – which could havecontributed to building up hydrogen pocketsoriginating from radiolysis in the pool7.

� The large production of hydrogen as aconsequence of insufficient cooling and fueldegradation.

According to in-situ inspections (through films,water radioactivity monitoring, etc), it is nowwidely acknowledged that no significant fueldegradation occurred in the Reactor 4 storagepool, which supports the explanations relying onoutside originating events. The temperature ofthe water in the Reactor 4 fuel storage pool was

4 - The sea water originallyentered into the basementof the Reactor 4 building, asa direct consequence of thetsunami flooding. It hasbeen later contaminatedthrough injection of waterfrom the reactorcontainment. In the case ofReactor 2, the leakageprobably originated fromthe failures in the torus,damaged by the March 15explosion. The strongcontamination demonstratesthe efficiency of the pools tomitigate the air releases.

5 - Considering the loadingand the fuel burn-up, theoverall residual heat in thefuel pool of reactor 4 hasbeen evaluated ex-post atabout 3 MWth. Accordingly,accounting for the largethermal inertia and theinitial water level, thecoolant temperature shouldnot have reached theboiling threshold beforeone/two days after theinitiating event. Thus someexperts claim that the firstassembly heads could havestarted uncovering onMarch 18, at the earliest.This position is notunanimously shared.Moreover, Japanese recentlyprovided experimentalevidence to counter it.

6 - Further experimentalevidence has been providedlater on, which stronglysupports this explanation.Actually, analysis of theactivity levels of thedifferential air filtersdemonstrated a one-directional hydrogen flowfrom Unit 3 to Unit 4.

7 - This position is notwidely shared based on theclaim that the radiolysiscould not be able toproduce such large amountsof hydrogen.

Figure 5 - Recovery actions

Figure 6 : Motor of fuel loading machine in spent fuelpool (source “TEPCO, WNN”)


rising until a complementary cooling capacity wasprovided (e.g. for Reactor 3 pool throughhelicopter supply, from March 17 and later on byfire truck). In the case of Reactor 4 fuel pool onMarch 20, the supply of water was first providedthrough a water hose and then through a concreteduct from above through an articulated arm.

From then on an additional continuous supplyhas been necessary to compensate forevaporation and leakage. Luckily, the civil workshave accommodated the combustion anddeflagration of hydrogen without any majorconsequence. Nevertheless, it is obvious that,had a severe degradation of the fuel happened,the situation would have deteriorated quitequickly, thus causing large early releases to theenvironment.

� 2.4 Fukushima radiological issues

The incumbent risk of core melting wasrecognised very quickly, as aconsequence of the possible failure of

water injection, even if precise information onthe availability and the operability of corecooling systems on each unit was missing.Before the explosion of the Unit 1 building,none of the environmental radioactivitymeasurements identified large releases, evenafter the first venting of the Unit 1 containment.

Nevertheless, the severity of the situation in case

of a long-lasting blackout was promptly realised.The following issues and necessary, urgentactions were identified to protect the plants:

� The primary circuit had to be discharged into thereactor containment via the suppression pool tocontrol the pressure. Cold water injection couldhave prevented a fast increase of the containmentpressure.

� Without actuation of the emergency cooling, thewater in the suppression pool would haveprogressively warmed-up and started boiling.

� The reactor containment pressure would haveincreased quickly because of the very small size ofthe containment, compared to PWRs.

� The containment should have been ventedperiodically to avoid rupture.

� In case of core dewatering and melt, thecontaminants would have been released from thefuel to the primary circuit, then to the reactorcontainment through the suppression pool andeventually partially released to the environmentvia the containment venting line (if any).

� The large amount of hydrogen produced by theclad oxidation during the melting would havecaused a very high risk of hydrogen combustion inthe reactor building in case of leakage from thecontainment vessel.

� The hydrogen combustion inside the secondarycontainment could have damaged the structures,jeopardized the pool structures, destroyed the roofand created a bypass to the turbine hall.

� Publicly available calculations were predictingvery high release in such conditions.

Figure 7 - Radioactivity release and radiation levels


The assessment of the radiological conditionscan be described based on documents [1-4]. Theexplosion of Unit 1 on March 12, 15:36, wasquickly identified as a hydrogen explosion. Thatprovided evidence that the core had alreadystarted melting. It was therefore obvious thatpart of the gaseous radioactive elements (noblegas, iodine) had already spread out in theenvironment. The explosions in Unit 2 & 3buildings definitely demonstrated how dramaticthe sequence of events for these plants could be,in case of long-term station black-out.

The early atmospheric releases were transportedtowards the north and then towards the ocean.Most of the meteorological stations and the doserate measurement stations were out of useduring this period. The only station which wasable to detect the radioactive plume from Unit 1was the Minami Soma one, located on the shoreapproximately 25 kilometres north of thenuclear site.

Within a domain of dozens of kilometres, themajor contamination of the environmentoriginated from the releases from Unit 2, thecore of which had already started melting onMarch 14, followed by the resulting explosion inthe torus room of the primary containmentvessel, on March 15.

This event and the subsequent ventingoperations from midnight March 15 onwardsengendered massive atmospheric releases too,which initially went south, then progressivelyswitched to the west and finally to north-west asthe wind direction changed. Consequently awide area westward of the site was impacted byatmospheric releases from Unit 2.

Shortly after 8 p.m., heavy rain started movingfrom the north-west towards the site (in theopposite direction of the plume). The major

period of rain occurred from 9 p.m. to midnight.The consequential wash-out of the radioactiveplume produced a large deposition in theenvironment.

The Tokyo area was impacted mainly by thereleases from Unit 2. The high dose rate areafirst spread to the southern region ofFukushima, towards Tokyo and then movedtowards the north-west in the direction ofFukushima city and Itate.

Unexplained atmospheric releases were observedfrom March 19. The releases were transportedfirst towards the north-west, then directly to theocean. During the night of March 20, a large-scale meteorological circulation transportedfission products which were over the ocean, andthen back towards Japan. From March 21 theTokyo area was affected again with at least threedays of rain in the area. The combination of rainand fission products in the atmosphere led tocontamination of the Tokyo area. (See figurebelow)

� 2.5 The current situation in Fukushima and recovery activities

The exact extent of the damage in theFukushima reactors it not yet knownsince it is impossible to directly observe

the pressure vessel and the primarycontainment. This is only expected to bepossible several years from now. However,according to simulations performed by TEPCOon the basis of the sequence of the events and ofthe released radioactivity [5], it is assumed thatthe corium was able to melt through the ReactorPressure Vessel (RPV) in Unit 1, while it isexpected to have been retained inside the RPVin Units 2 and 3. The estimation of radioactivityrelease given by the Japanese government inJune 2011 was about 15% of the Chernobyl

Figure 8 - Meteorological conditions March 15th and 16th


release; this value is questionable, however noupdated official estimations are available.

On December 16, 2011, the plant was officiallydeclared in a state of “cold shutdown”. Areasonable interpretation of this statement isthat the core and corium debris were beingstably cooled, and that the coolant pressure andtemperature were low and under control. Astable cooling condition was achieved also forthe spent fuel pools, by installation of dedicatedheat exchangers and devices for maintaining thewater level.

The contaminated debris outside the buildingshave been removed and stored in containers.The radioactivity emissions have decreased to anacceptable level and the dose rate level on thesite boundary is between 10 and 100 Sv/h.However inside reactor buildings the radiationlevel varies from about 10 mSv/h to more than 1Sv/h. A lot of debris is still there and part of ithas high radiation dose rate. Dose maps havebeen compiled and attention is devoted toreduce worker exposure during works on thesite, with the installation of dedicated facilitiesfor hosting them on the site and for improvingthe health care.

Several actions have been performed and areongoing to prevent further contamination. Thecovering of Unit 1 has been completed.Inhibitors have been sprayed to preventspreading of radioactive dust. Support structureshave been installed at the bottom of the spentfuel pool in Unit 4. A temporary barrier hasbeen installed as a countermeasure againsttsunamis and a water shielding wall is underconstruction to prevent underground waterleakage towards the ocean.

According to the medium-long termdecommissioning plan, fuel removal from thespent fuel pools is expected to start in two yearsand removal of fuel debris from the reactors inten years, with completion in 25 to 30 years.

During this period, several development actionswill be required to support the decontaminationand decommissioning activities. A list of theseactions has already been identified by TEPCO:

� Development of inspection methods and devicesfor inspection of leakage points in thecontainment vessel, and for operation in high-temperature, high humidity and high-doseenvironments.

� Development of remote sampling anddecontamination methods.

� Development of technologies and methods torepair leakage points, including underwaterrepair.

� Development of systems to prevent the dispersalof radioactive materials.

Also the technologies for core defueling and fueldebris removal will have to be improvedcompared to the previous experience in TMI,since in the case of TMI there was no fuel in thecontainment. Finally it will be necessary todevelop technologies for managing and treatingradioactive waste which do not belong to thepresent waste categories. A hot laboratory willhave to be installed on the site since it is expectedthat the decontamination and fuel debris removalwill generate non-transportable samples. Specificmethods for waste treatment, packaging, storageand disposal will likely be necessary.

Figure 9 - Dose rate comparisons - Tokyo city centre (a) and in Hitachi� miya (b) - Source REF.4

Figure 10 - Decontamination using synthetic resinemulsion (source “TEPCO, WNN”)



� 3.1 Overview of Fukushima Lessons in Major International Activities and Documents

As was the case for reactor accidents inthe past, after the Fukushima event thenuclear industry reacted promptly to

identify areas for safety improvements. Thisprocess started at the level of each individualorganisation, but very soon efforts have beencoordinated at international level in order todevelop a shared vision of what happened andwhat implications it has for the future.

This section provides a short review of a limitednumber of relevant documents and meetingoutcomes, representing the view of differentinstitutions:

1. The IAEA expert mission report [6].2. The Japan government report [7].3. The Near Term U S Nuclear Regulatory

Commission report [8].4. The OECD/NEA Forum on Fukushima accident;

this was actually a meeting of the NEA Committeeon Nuclear Regulatory Activities (CNRA) [9].

5. The ETSON position paper on research needs forGEN 2 and GEN 3 NNPs [10].

6. The IRSN Special report on Fukushima [11].7. The IAEA International Experts’ Meeting on

Reactor and Spent Fuel Safety in the Light of theAccident at the Fukushima Daiichi Nuclear PowerPlant, March 19-22, 2012 [12].

8. The report of the Fukushima Nuclear AccidentIndependent Investigation Commission (NAIIC),established by the National Diet of Japan [13].

Most documents have no explicit suggestions forR&D programmes, but they provide with thegeneral framework allowing identification of theR&D topics with the highest priority. Inaddition some of them also give explicit

indications for R&D directions, for example theETSON report [10]. A short description of thecontent and the most relevant conclusions of thedocuments is provided below.

3.1.1 The IAEA expert mission report

The IAEA expert mission in Japan from May24 to June 2, 2011 led to the issuing of an

important report, which influenced all actionslaunched at international and European level toassess and to further enhance the safety ofnuclear installations. The report's findings werepresented at a ministerial conference in Viennafrom 20 to 24 June, with the aim of providinginitial information about the accident, assessingthe level of emergency preparedness at theinternational level, in order to reinforce anddiscuss the implications of the incident for thesafety of all nuclear facilities, identifying anyareas to be reviewed, lessons learned and futureactions.

The report includes the following parts:

a) A reconstruction of the accident sequences. Thispart has been superseded by more detaileddocuments issued later, for instance the INPOreport [14].

b) A series of 15 conclusions, derived from the IAEAFundamental Safety Principles. The main findingis that IAEA Fundamental Safety Principlesprovide a robust basis in relation to thecircumstances of the Fukushima accident and coverall the areas of lessons learned from the accident.In Fukushima there were insufficient defence-in-depth provisions against tsunami hazards, andaccident management provisions were notadequate to cope with severe accidents in generaland with multiple plant failures in particular. Thereport suggests that a periodic updating ofnational regulations and guidance tointernationally established standards andguidance should be considered. In addition a

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3. Lessons learned from the Fukushima accident

23

review of the IAEA Safety Standards in order tocover multi-unit sites may be advisable.

c) Lessons learned. Sixteen lessons grouped into fourareas were identified, generally applicable for alloperators of NPPs: • nuclear system organization, safety infrastructure

and safety culture,• assessment of risk from external events and

prevention measures, • phenomenology, management and mitigation of

severe accidents,• emergency preparedness and management.The main issues identified in the report were: • when evaluating the risk from extreme external

events, it is important to pay careful attention topossible cliff edge effects, which require a specificsafety approach, because they can widely overrunthe safety margins, thus engendering verydetrimental potential consequences, despite theirvery low probability. This topic should beaddressed considering the occurrence of a fully-new class of probabilistically negligible, butpotentially extremely damaging situations,

• for accident management it is necessary to takethe right decisions in due time; the Fukushimaexperience has shown that the decision processesappropriate for normal operation may beinadequate during emergencies,

• the emergency and evacuation plans should beconsidered not only for the first phase of theaccident, but should be comprehensive in order toensure the health of the population in the mediumand long term.

3.1.2 The report of the Japanesegovernment.

The report issued by the InvestigationCommittee established by the Japanese

government, has a very wide scope.

It includes an overview of the nuclear regulatorysystem in Japan, a detailed assessment of theexternal events (earthquake and tsunami) withtheir impact on the Fukushima plant and of theaccident evolution. It also investigates how theemergency was managed and evaluates whatamount of radioactive materials was released tothe environment and the level of radiationexposure. It also discusses the co-operation withinternational institutions and foreignorganisations, how communication wasmanaged and the expected future remediationwork.

A detailed list of 28 lessons was generated. Someof these lessons are specific to Japan, inparticular those dealing with the regulatoryindependence, which in the Japan nuclearsystem was not fully guaranteed. Other lessonsare related to issues applicable and alreadyconsidered in most nuclear countries. Probablythe most interesting lessons for the wholecommunity are those addressing the issue ofemergency preparedness and management, sinceJapan is assumed to be one of the best preparedcountries for natural disasters.

3.1.3 The NRC Near Term Report

Soon after the accident, NRC constituted aTask Force which issued on July 12th, 2011a “Near Term” report [8] containing a series ofrecommendations. These are mainly addressedto NRC itself, suggesting improvements in theregulatory framework in order to enhance thesafety level of US nuclear reactors. Similar to theIAEA lessons, these recommendations aremainly dealing with the strengthening of themitigation capability in case of severe accidents,especially in case of multi-unit sites, and withthe enhancement of the preparedness to manageunexpected situations.

Similarly to the situation in Europe, USlicensees are requested to re-evaluate andupgrade as necessary the seismic and floodingprotection for all operating reactors. Aregulatory system based on balanced risk-informed and deterministic defence-in-depthapproach should be maintained andstrengthened.

3.1.4 The CNRA Forum on the Fukushima Accident

The OECD/NEA Committee for NuclearRegulatory Activities met in Paris on June

8, 2011. The Forum noted that after Fukushima,in-depth assessments of plant safety had alreadybeen carried out by many regulatory authoritiesin NEA member states and associated countries,and invited remaining regulatory authoritiesresponsible for the oversight of nuclearinstallations to launch similar reviews andanalyses as soon as possible.

The Forum also declared its commitment tosystematically advance the knowledge neededfor plant designs and post-accident situations.The identified priority areas include extreme


external natural hazards and resistance againstthese hazards and combined events, ability ofsafety systems to withstand severe accidents,emergency response and managementcapabilities, crisis communication and siterecovery plans and their implementation.

Overall, the Forum underlined the importanceof co-ordinated and comprehensive actions byall regulatory authorities.

3.1.5 The ETSON position paper

This document - the development of whichhad already started before the Fukushima

event - had the more general purpose tocontribute to the SNETP project definition andlaunching, by identifying research priorities forGEN II and III reactors, with specific focus onsafety research. Nevertheless, the Fukushimaevent - which occurred during the final stage ofthe preparation - has unavoidably stronglyinfluenced the document.

The ETSON position paper includes anappendix dedicated to the Fukushima accident.There the paper identifies some lessons learned,in particular in the field of crisis managementand lists a series of possible improvements insafety methodologies. The appendix providesspecific indications for future research, in orderto address the following requirements:

� To evaluate in the best estimate way thebehaviour of the plant systems for beyond designbasis accidents.

� To evaluate the ultimate capacity of the systemswith respect to the load applied and to identifywhen the level of damage becomes non-linear orcatastrophic.

These objectives may require a large extension ofphysical modelling and computer tooldevelopment in different areas and in particularin the area of severe accidents and containmentsystem simulation.

The paper also includes a series of suggestions forresearch submitted by JNES, the Japanese TSO,specifically addressing improving the knowledgeof the Fukushima accident, its environmentalimpact and to support the site restoration process.

3.1.6 The IRSN Special report

IRSN dedicated the full January 2012 issue oftheir Repères Magazine to the Fukushimaaccident and the lessons learned.

In the editorial paper, IRSN announced the willto promote the concept of “hardened coresafety” at the international level. This is based on“an additional level of safety-in-depth at nuclearfacilities to ensure that their vital safetyfunctions remain operational over a sufficientperiod of time in the event of any physicallypossible environmental hazard”.

As far as research is concerned, the issuesidentified by IRSN are:

� Better characterisation of natural events, likeearthquakes, floods, etc., including methodologiesfor dealing with rare events.

� Protection of persons and the environment,including better and faster environmentalmonitoring, better models for contaminationpredictions, health effects of low doses, and effectof contamination on the environment – inparticular the marine environment.

� Broader consideration of social sciences, both atfacility level and outside the boundary of the plant.

3.1.7 The IAEA Expert Meeting, March 2012

During the IAEA International ExpertsMeeting on Reactor and Spent Fuel Safety

in the Light of the Accident at the FukushimaDaiichi Nuclear Power Plant, held in Vienna onMarch 19-22, 2012, all the relevant technicalaspects of reactor and spent fuel safety in light ofthe Fukushima accident were discussed, tacklingthe results of several national assessments ofvulnerability of their NPPs. In some of thesestudies the IAEA's complementary methodologyreleased in November 2011, was applied.

The meeting showed that the effort of theMember States and the IAEA was comprehen-sive and thoughtful, and that good progress inimproving safety has been made. From theR&D point of view, it has to be mentioned thatexperts also proposed the undertaking of furtherresearch about the phenomenology that charac-terises the accident progress.

3.1.8 The report of the NAII Commission (National Diet of Japan)

This report lists several errors and signs ofnegligence that made the Fukushima plant

unprepared for the events of March 11,identifying deficiencies and failures in the


response to the accident by TEPCO, theregulator and the government.

The Commission identifies as the main cause ofthe accident the conventions existing in Japaneseculture and mindset: the attitude to theobedience, the reluctance to question theauthority, the devotion to the company or to the“group”. It underlines the lack of a clearidentification of the responsibilities of theregulator and of the promoters of the nuclearenergy, as well as the lack of control of the civilsociety over the nuclear institutions.

The Commission also ascribes to TEPCOorganisational problems, identifies shortcomingsin the law and regulations, and spurs for nocosmetic solutions.

� 3.2 Lessons drawn on enhancement of safety infrastructure and culture

The different outcomes from Fukushimaevents investigations support severalrecommendations to improve the

current safety assessment methodology andpolicies, such as:

� Agreeing internationally on major safetyobjectives, e.g. through WENRA reference levelsand safety objectives;

� Searching for a common design certification,through extending and generalising the currentMDEP (Multinational Design Evaluation Program)process and gathering together regulatory bodiesand technical safety organisations;

� Adopting a common policy to support only certifieddesigns on the international markets, whilstprotecting the intellectual property of the designs;

� Generalising the scope and the practice of thestress-tests worldwide;

� Searching for harmonisation of the assessmentpractices and methodologies;

� Defining standards for competence evaluation inthe reactor safety field;

� For current and advanced reactor designs,extending the defence-in-depth application to anyaggression source, both internal and external,including the beyond design basis events either toexclude, if they are likely to lead to earlyradioactive releases, or at least to mitigate theirenvironmental impact;

� as far as the Gen IV reactor concepts areconcerned, strictly applying the generalisedexclusion principle that claims the absence of anytechnical need to evacuate the populationneighbouring the reactor site.

� 3.3 Lessons drawn on plant design and on evaluationof external hazards

The Fukushima accident was triggered bythe combination of two main initiatingevents - an exceptional magnitude

earthquake and the associated tsunami wave-that struck a site which already suffered fromsevere weather conditions (cold weather with icyclimate and snow). The combination of thesethree agents led to:

� The loss of almost all the off-site power supplies;� The flooding of the site by a wave twice the size of

that considered in the year 2000 risk evaluation,which led to both the loss of all the emergencypower supply system and the ultimate heat sink;

� The build-up of civil structures debris, which underthe effect of the massive damage caused by thetsunami and the severe weather conditions,combined with the radioactivity, prevented the sitefrom being effectively accessed on the ground;

� The situation affected all reactors on the samesite; it could have also affected some other sites(especially Fukushima Dai-ni).

The combined effects infer that the followingaspects need to be considered:

� Extending even further the in-depth safetyapproach to any type of hazards, in particularexternal ones, and accounting for any mode ofcombination of them;

� Systematically including the design extensionconditions (beyond design basis accidents) in thedefence-in-depth approach at the design stage;

� Developing wider and more robust lines ofdefence with respect to the design basis;

� Including systematically in the in-depth safetyapproach the situations where several plants onthe same site are affected by a beyond designbasis event;

� Taking into account in the design of additionalmeasures for beyond design basis events, whereseveral nuclear sites are concerned andcommunication means (e.g. roads) are severelydegraded.


� 3.4 Lessons drawn on mitigationof severe accidents

Severe accidents were not prescribed assafety design basis for current plants andthey are not even considered in full scope

for Gen III plants in some countries. Severeaccident protection has been taken into accountthrough “ad hoc” rules, addressed to helpoperators in managing hydrogen releases,prolonged station black-outs, anticipatedtransients without scram (ATWS) and otherspossible sequences. Nevertheless, severeaccidents are considered for existing plants asdesign extension conditions (beyond designbasis accidents).

The execution of several experimentalprogrammes has significantly increased theknowledge about phenomena which may occurin design extension conditions. The execution oflevel-2 PSA for many LWR plants and theanalysis of several sequences using up to datecodes like MAAP, ASTEC and MELCOR,provided very good insights into plant behaviourand helped in developing severe accidentmanagement procedures.

Nonetheless, the work performed till now didnot allow for sufficiently detailed simulation ofthe course of Fukushima accident, includingevaluation of the source term after the loss of therelevant infrastructures. It is therefore veryimportant that the future research in this field beaccompanied by a parallel effort to transfer theenhanced knowledge into actual improvementsof operating plants and to really protect thepublic against a low probability highconsequence severe accident, which cannot becompletely excluded to occur in a LWR plant.

Some items to be considered with priority are:

� Provide simple alternative power sources (such asmobile power, compressed air and water supplies),ensure available essential safety relatedparameters based on hardened instrumentationand secure communication lines for control rooms,emergency centres and other places on and off-site.

� Strengthen and integrate on-site emergencyresponse capabilities, ensure an adequate numberof experienced personnel who can deal with eachtype of reactor and can be called upon to supportthe affected sites, enhance the training forresponding to severe accidents.

� Revise the risk and implications of hydrogenexplosions, enhance the measures to prevent them

and implement mitigating systems, includinghardened containment venting arrangements.

� Enhance spent fuel pool makeup capability andinstrumentation for the spent fuel pool.

Last, but not least, modification of the reactordesign should be considered, which wouldinclude feeding the primary circuits of NPPs,through additional nozzles in the main coolantsystem. These design modifications could bedefined based on PSA results within a risk-informed overall approach. It should be carefullychecked, that additional measures would notresult in increased risk due to large complexity ofthe design or use of unproven design solutions.

� 3.5 Lessons drawn on theemergency preparednessand management

The main difficulties and drawbacks inthe management of the Fukushimaaccident came from:

� The concomitance and overlapping of many non-nuclear and nuclear events. The effects of both theearthquake and the tsunami on the survival andliving conditions of the population in thesurrounding regions attracted the prime attentionof the authorities and officials at the very earlystage of the crisis.

� The complexity and difficulty of the decisionmaking process which affected the effectivenessand promptness of the actions and generated bothconfusion and delays.

� The location and lay-out of the site, the severeenvironmental conditions and the build-up ofdebris consequent to the earthquake and tsunamidevastation, which prevented intervention fromthe ground, e.g. to replace the failing generatorsor perform a successful change of shift (possibilityof shift members affected by the disaster while noton duty).

� The simultaneity of the aggression on severalunits and their pools on the site, the status of eachone of them continuously impacting the others,and vice versa, and sometimes preventing theworkers from intervening.

� The practical impossibility to efficiently anddurably recover a suitable and stable electricalsupply source.

It is now widely agreed that the involvement ofseveral units and reactor fuel pools in the crisismanagement situation created unacceptable levels


of stress for the operators, already affected by theextreme working conditions and the extendedlack of communication with their families andrelatives which added to the stress. The humanfactor has turned out to be a fundamental issue inthe Fukushima crisis management. Specifictraining in abnormal and extreme situationmanagement of qualified intervention groups(including e.g. military troops) has beenadvocated to address these issues.

International collaboration can also effectivelyhelp. Any country affected by such a hugecatastrophe should be allowed to seek help andsupport from neighbouring countries,Nevertheless to be effective, this support shouldnot conflict with national emergency practices.Accordingly, harmonisation of practices foremergency management should be undertakenworldwide to achieve enough uniformity andstandardisation in organisation, technicalcommunication, management methodologies,data acquisition methodologies and tools. Thatcould also help to overcome the practical barrierssuch as language, knowledge, and familiaritywith the design..

In the case of a major emergency theinternational experts, beyond direct involvementin the national organisation, could provide helpfrom abroad using long distance communicationon plant status diagnosis, prognosis of theaccident progression and on selection of themanagement strategy, provided that thenecessary organisation has been set up.

Moreover, the Fukushima events indicated thatemergency management means should preferablybe independent for each unit at a site, and alsosufficiently robust to cope with the local weatherand access conditions. This underlines theusefulness of relying on the availability of redundantintervention means in the vicinity of the site.

Some items to be considered with priority in thefuture are:

� Requiring that facility emergency plans addressprolonged station blackout and multiunit events.

� Organising large scale radiation protection forworkers on the site under severe accidentconditions.

� Improving and refining the existing methods andmodels to determine the source term, accuratelypredict the effect of released radioactive materialsand clearly define the criteria for wide-areaevacuation.

� Pursuing emergency preparedness, reinforcing theenvironmental monitoring, clarifying theallocation of roles between central & localorganisations in responding to the combinedsituation of a massive natural disaster and anuclear emergency.

� Assessing the concept of long term sheltering infavour of the concepts of ‘deliberate evacuation’and ‘evacuation-prepared area’.

� Enhancing communication to domestic andinternational communities regarding the accident;

� Establishing an internationally shared approachto emergency management, allowing effectiveassistance from other countries.

Figure 11 - Source : METI, WNN


8 - The text belowenumerates topics of R&Dwhich are more or lessderived from the lessonslearned from the stress testsand the subsequentinvestigations carried-out indifferent countries. The textis more aimed at providinga relevant list of topics thanmaking an explicitconnection to the stress-testoutcome.


� 3.6 Outcome of the stress-tests and R&D implication

Following the Fukushima Dai-ichiaccident, the European Council of 24-25March 2011 requested that a

comprehensive safety assessment wereperformed on all EU nuclear plants, in light ofthe preliminary lessons learned. The request ofthe Council included stress tests performed atnational level, complemented by a Europeanpeer review. Consensus on specifications of thestress tests was achieved by ENSREG and theEuropean Commission on 24 May 2011. Thismultilateral exercise of the stress tests coveredover 150 reactors in European countriesoperating NPPs. The stress tests and their peerreview focused on three topics:

1) Natural initiating events, including earthquake,flooding and extreme weather.

2) The loss of safety systems.3) Severe accident management.

The 15 European Union countries with NPPs aswell as Switzerland and Ukraine performed thestress tests and were subjected to the peer review.The stress tests and peer review consisted ofthree steps. In the first step the plant operatorsperformed an assessment and made proposals forsafety improvements, following the ENSREGspecifications. In the second step the nationalregulators performed an independent review ofthe operators’ assessments and issued additionalrequirements. The last step was a European peerreview of the national reports submitted by theregulators. The peer review, which started on the1st of January 2012 consisted of a desktop review(study of submitted national report), followed bya two week assessment of the report duringtopical review, completed by additionaldiscussions during the country reviews. Therewere about 80 reviewers from 24 Europeancountries participating in the peer review.Observers from several non-EU countries(Canada, Croatia, Japan, UAE and USA) as wellas the IAEA also attended.

As a result of the stress tests 17 national reports,a peer review report for each of the seventeenparticipating countries, and a final peer reviewreport (developed by the Stress Test Peer ReviewBoard and endorsed by ENSREG on 25 April2012) were produced [15]. The review focusedon the identification of strong features,

weaknesses and possible ways to increase plantrobustness in light of the preliminary lessonslearned from Fukushima.

Although the stress tests were based oninformation available at the time and were notprimarily intended to specify areas for futureresearch, they also indicated, explicitly orimplicitly8, the need for future studies anddevelopment in the following areas:

a) Plant design and identification of external hazards

� Development of approaches to natural hazarddefinition, techniques and data, and developmentof guidance on natural hazards assessments,including earthquake, flooding and extremeweather conditions.

� Development of guidance on the assessment ofmargins beyond the design basis and cliff-edgeeffects for extreme natural hazards.

� Development of a systematic approach to extremeweather challenges and a more consistentunderstanding of the possible design mitigationmeasures.

� Development of the approach for assessment ofthe secondary effects of natural hazards, such asflood or fires arising as a result of seismic events.

� Enhancement of PSA for natural hazards otherthan seismic (in particular extreme weather) anddevelopment of methods to determine marginsand identify potential plant improvements.

� Overall enhancement of PSA analysis, covering allplant states, external events and prolongedprocesses, for PSA levels 1 and 2.

b) Analysis of severe accidents

� Further detailed studies on progression of severeaccidents, allowing the determination of thetiming of cliff edges such as core melt, reactorpressure vessel failure, containment basemat meltor other modes of containment failure, uncoveryof spent fuel pool fuel.

� Further analysis of phenomena associated withreactor cavity flooding and related steamexplosion risks following potential reactor vesselpenetration by molten corium.

� Studies of long-term containment over-pressurisation due to excessive production ofsteam and non-condensable gases, and means forprotection of containment integrity, includingfiltered venting.

29

� Studies of potential re-criticality both in reactorcores as well as spent fuel pools, taking into accountpotential geometry and material compositionchanges caused either by external hazards or by theprogression of the severe accident.

� Further analysis of hydrogen production,distribution, deflagration and detonation incomplex containment geometries.

� Analysis of potential for migration of hydrogeninto spaces beyond where it is produced in theprimary containment, as well as hydrogenproduction in spent fuel pools and of measures toreduce the hydrogen risk.

� Analysis of severe accidents involving molten fuelin the spent fuel pools and measures for mitigationof the consequences, including venting of buildingsin case of coolant boiling in the spent fuel pools.

� Determination of expected radiological conditionsinside plant buildings and outside during severeaccidents, as well as the limitation of radiologicalreleases, including situations with the damagedcontainment.

c) Management of severe accidents

� Development of advanced instrumentation basedon simple physical principles (e.g. passivetemperature, pressure readers) able to operateand being used specifically in station black-outand loss of DC power.

� Systematic evaluation of the availability of safetyfunctions required for severe accidentmanagement under different circumstances.

� Feasibility of accident management actions forlong processes with duration of several days,involving accidents occurring in parallel on severalunits, and taking into account potentialinteractions between the reactor core and thespent fuel pool.

� Investigation of cooling modes for partiallyrelocated core prior to reactor vessel failure,including assessment of possible vessel failure dueto thermal shocks in older NPPs.

� Further assessment of the feasibility of variousstrategies for molten corium cooling, both in-vessel as well as ex-vessel, aimed at protectingcontainment integrity.

� Enhancement of the methods and tools for severeaccident management training and exercises,(such as desk-top training, use of multi-function orfull-scope simulators) including development ofnew training tools for NPP staff training.

d) Emergency management and radiological impact

� Technical and organisational strengthening of on-site emergency arrangements, including on-siteemergency centres protected against extremenatural hazards and contamination.

� Studies of the logistics of the external support andrelated arrangements (storage of equipment,equipment and manpower resources, use ofnational defence resources, etc.).

� Studies of feasibility of operations in the event ofwidespread damage, for example, following anearthquake, including the needs for differentequipment (e. g. bulldozers) and plans on how toclear the route to the most critical locations orequipment.

� Enhancement of methods for assessment ofradiological situations on site including the case ofmulti-unit accidents, in connection with radiationmonitoring, habitability and feasibility of accidentmanagement actions.

� Development of conceptual solutions for post-accident fixing of contamination and thetreatment of potentially large volumes ofcontaminated water.

� 3.7 Impact on the R&D directions and priorities

Prior to Fukushima, the R&D activitiesrelated to existing reactors werepredominantly focused on the issues

associated with the long-term operation. Whilethese issues remain equally important, greatattention should be paid to safety. In the area ofsafety the main objectives of the R&Dprogrammes for the reactors presently inoperation and under construction are:

� On the one hand, appreciating and improving theoperating and safety margins with respect to thedesign basis accidents.

� On the other hand, extending them to the beyonddesign basis situations, e.g. through the safetymargin approach which relies on the definition ofthe risk space which accurately accounts for anykind of generating events.

Capitalising on the first evaluations and relyingon the above mentioned considerations, it istoday widely acknowledged that the Fukushimaevents do not require either major re-orientationof current R&D programmes or their


cancellation at national and international levels.It also means that the current SNETP objectivesand content are only slightly challenged. Inparticular, the importance of the SARNETnetwork to share effort on severe accident R&Das well as projects such as the ASAMPSA2(best-practices for Level 2 PSA) is confirmedand should be continued with the objective toproduce knowledge, methods and codes able toassess risks experienced by NPP for all beyonddesign situations. Nevertheless, carefulinvestigation of the Fukushima event outcome islikely to engender a new scale of priority ofprogrammes, with a particular focus on externalextreme events and their combination, commonmode effects and human behaviour. In addition,although R&D on air oxidation of relevantmaterials is already under way, some more

attention needs to be paid to the safety of spentfuel pools.

Also it is worthwhile to gain a betterunderstanding of the behaviour of the systemsversus the intensity of the applied stress andload, and appreciating how and when it turns tonon-linear and/or catastrophic. This may call foran extension of the physical testing andmodelling, as well as the development ofimproved and physically-based computer toolsin different fields of endeavour.

In relation with every single area where lessonshave been drawn, and on the basis of the stresstest reports, the SNETP Task Force finallyidentified a series of R&D topics with relevantpriority, as listed in Table 1. For each of them adeeper discussion will be provided in chapter 4.


Table 1. List of research areas identified for each lesson learned area

LESSON LEARNED AREA RESEARCH AREAS

Nuclear system organisation, safety infrastructure and safety culture

1. Systematic assessment of vulnerabilities todefence-in-depth and safety margins forbeyond design basis loads

2. Human/organisational factors under highstress and harmful conditions

Assessment of risk from external events

and prevention measures

3. Improved methods for external event hazardevaluation

4. Use of the probabilistic methods to assess plantsafety in relation to extreme events

5. Advanced deterministic methods to assessplant safety in relation to extreme events

6. Advanced safety systems for nuclear powerplants

Phenomenology, management and mitigation of severe accidents

7. Material behaviour during severe accidents8. Advanced methods for the analysis of severe

accidents9. Improved procedures for management of

severe accidents10. Assessment of the radiological effects of the

severe accidents11. Improved modelling of fuel degradation in the

spent fuel pool

Emergency preparedness and management

12. Methods for minimization of contamination inthe NPP surroundings and for treatment oflarge volume of radioactive waste

13. Accident management in the framework of theintegrated rescue system


� 4.1.Systematic assessment of vulnerabilities to defence-in-depth and safety margins for beyond design basis loads

4.1.1. Lessons learned from the Fukushima accident

The Fukushima accident showed the need forquantification of safety margins (in

particular in the case of extreme externalhazards) beyond the design basis, up to coredamage and major releases of radioactivesubstances into the environment. This need hasbeen reflected in Europe by the decision toperform the stress tests in all NPPs. There is alsoa tendency that evaluation should be aimed notonly at the external hazards, but generally at allextreme events, i.e. including internal scenariosand man induced ones. The stress tests alsoshowed the need to clarify and harmonise themethodologies, the objectives and the criteriaadopted both for design and safety assessment.Moreover, it has been pointed out that theassessment should be based on a comprehensiveevaluation of the capacity and reliability of thedefence in depth levels, in particular for thethird and fourth level of defence, taking intoaccount the need for a quantification of thesafety margins beyond the design basis.

4.1.2. Objectives of the activities in this area

The main objective of the activity is thedevelopment of the methodology for

comprehensive assessment of the defence indepth engineering measures, with specialemphasis on those pertaining to the third andfourth level, and the analysis of adequacy ofexisting methods or development of a new

methodology for a repeated execution of the"stress tests" for both existing and new NPPswith the modelling of the likelihood and extentof damage to components and systems.

4.1.3. Examples of specific tasks in the given area

� Development and validation of methods forcomprehensive evaluation of defence in depthmeasures, in particular for the third and fourthlevel of defence.

� Determination of the possible effects of extremeevents with respect to interrelation between theplant system operation and personnelintervention.

� Development of acknowledged procedures for theassessment of the ultimate resistance of thebarriers against releases of radioactive materialsto the extreme loads.

� Development of suitable methodology for theidentification of threshold phenomena (cliff edges),

� Determination of the sensitivity of the main safetyfunctions, to extreme loads.

� Development of comprehensive models forassessing the behaviour of NPPs under theconditions of the beyond design basis extreme loads.

� 4.2.Human/organizational factors under high stressand harmful conditions


The complexity of the decision-makingprocess in managing the accident in

Fukushima had a strong impact on the efficiencyand timeliness of necessary interventions andwas the source of misunderstandings and delays.

FT

G


4. Identification of research areas associatedwith the lessons learned

33

It is generally accepted that the need forresponses to serious accidents at the same timeon several reactors and spent fuel pools exceededacceptable limits for the workers of Fukushimaplant already stressed by the extreme workingconditions and long-term impossibility ofcommunication with their families. Some of theoperational procedures in place, appropriate fornormal operation (top-down decision making)turned to be ineffective for managing accidents.As confirmed by a recent report by the JapaneseGovernment, the human factor has proved to bea critical issue and a weak point in dealing withthe accident. Among possible ways to improveaccident management, specific training ofskilled intervention groups has been identified.The use of more sophisticated instruments forevaluation during the accident could alsoprovide faster guidance for the implementationof possible corrective strategies by the operator.


The analysis of the importance of the humanand organisational factors for managing

emergencies in the conditions of high stress andharsh working conditions, with the objective ofidentifying possibilities for improvement and todevelop efficient assessment tools andprocedures.


� Assessment of the importance of human factors inthe efficiency of accident management.

� Evaluation of the applicability of standardmanagement methods on the capability ofpersonnel to respond in emergency conditions(influence of routine behaviour on managementof extraordinary situations).

� Analysis of the functionality of the various meansof communication under conditions of heavilydamaged infrastructure.

� Analysis of habitability of working places neededfor management of emergencies on the site andbeyond.

� Development and updating of training materialsfor the preparation of emergency response teams,especially for the workers of the technical supportcentre.

� Evaluation of the effectiveness of interventionsunder extraordinary conditions (damaged

infrastructure, limited availability of the site,limited human resources).

� Assessment of the needs of redundant technicalmeans and emergency teams for individual unitson the given site or a region.

� Assessment of the conditions for the gradualtransfer of the responsibility for accidentmanagement from the level of operational staffto higher levels of management.

� Assessment of safety culture issues and of theirimportance for the management of severe accidents,

� Development of methodologies for safety cultureoversight.

� 4.3.Improved methods for external event hazard evaluation


Hazard evaluation has to be continuouslyreviewed on the basis of up to date

knowledge and expertise. Despite the currenttrend supporting the use of advanced methodsfor hazard evaluation, they are still affected bylarge uncertainties and they are not widely andgenerally adopted.

4.3.2. Objectives of activities in this area

Development of methodologies for hazardevaluation for low probability events of

both natural and man induced (accidental)scenarios. The research will aim at theminimisation of the uncertainties affecting thehazard and to the identification of suitable levelsfor the design basis values to be used for thedesign/re-evaluation.


� Identification of very rare, extreme internal andexternal events, potentially leading to commoncause failures simultaneously arising on severalunits at the same site.

� Implementation of suitable methodologies fordetermining the frequency of occurrence of extremephenomena with very long period of return,


including the combination of extreme events, evenin case of limited availability of historic data.

� Development of methods for hazard evaluation inthe case of rare events (i.e. tsunami), withinsufficient historic records available.

� Updating of methodologies for site selection in rela-tion to the likelihood of extreme external events.

� Development of methods for expert elicitation9 ofrare events.

� 4.4. Use of the probabilistic methods to assess plant safety in relation to extreme events


Despite the developed level of probabilitymethods and their wide use in design and

generally in safety assessments, the Fukushimaaccident has shown that there are a number offactors, which have not been sufficiently coveredin the current PSA studies. It is therefore neces-sary to further develop more effective modellingtools for wider use of the PSA. Examples of therequired improvements include the extension ofthe reporting period over 24hours, improvedassessment of the impact of the human factorsand common cause failures (harmonisation ofapproaches), consideration of the long-term lossof electric power and heat removal, considera-tion of potential staff recovery actions andmodified configurations and consideration ofthe occurrence of the accident simultaneously onseveral units. At the same time, it is necessary todevelop integrated assessment methods usingboth deterministic and probabilistic methods.

Moreover, it could be worth considering a classof probabilistically negligible, but potentiallyextremely dangerous events that could engenderdetrimental consequences. That could be done,e.g., adopting specific approaches to address theresidual-risk management instead of relyingupon the conventional PSA which could drive tomisleading conclusions, due to low probability .


Improvement of the methodologies, harmoni-sation of the criteria and extended range of

applicability of the methods with emphasis on amore comprehensive consideration of extremeexternal hazards, their combinations and theconsequences of these hazards with prolongedduration and simultaneous occurrence on sever-al units.


� Probabilistic approach for external events(flooding, climatic events), use of screeningmethods, assessment on the maturity andapplicability of existing methods, or developmentof new ones.

� Assessment of restrictions in the use of the resultsof the PSA.

� Rules for practical elimination of mechanisms,leading to large damage to fuel and to largerelease of radioactive materials.

� Methods for the evaluation of component failuremodes in relation to extreme, low probabilityscenarios, in case of lack of experimental data, byanalysis and similarity.

� Development of probabilistic models aimed at theanalysis of the reliability of human factors,analysis of common cause failures, behaviour ofthe spent fuel pool, consideration of prolongedloss of the ultimate heat sink, long term loss ofelectrical supply, availability off-site for theexternal help.

� Quantification within PSA: extended periods ofthe accident (more than 24 hours), interactionsbetween the interventions of the operators andthe automatic operation of systems, theconsequences of the presence of several units onthe same site and the sharing of equipment andpersonnel (common equipment, the links betweenthe units, the availability of operating personnel),evaluation of the reliability of recovery actionsunder harsh conditions, the consideration ofcorrective actions in the case of extreme externalhazards.

� Consideration of the status and availability ofreliable information on the infrastructure actualstatus in the PSA studies.

� Assessment of the reliability of innovative designsolutions, in particular the passive systems.

� Methodology for probabilistic assessment ofsecurity threats to NPPs.

� Assessment of the PSA predictions vs. thereliability of the safety and safeguard systems andthe efficiency of the detection, repair andreplacement of components.

9 - Expert elicitation is anestablished term: Inscience, engineering, andresearch, expert elicitationis the synthesis of opinionsof experts of a subjectwhere there is uncertaintydue to insufficient data orwhen such data isunattainable because ofphysical constraints or lackof resources. Expertelicitation is essentially ascientific consensusmethodology. It is oftenused in the study of rareevents. Expert elicitationallows for parameterization,an "educated guess", for therespective topic under study.Expert elicitation generallyquantifies uncertainty.

- Expert elicitation tendsto be multidisciplinary aswell as interdisciplinary,with practically universalapplicability, and is used ina broad range of fields.Prominent recent expertelicitation applications areto climate change,modelling seismic hazardand damage, association oftornado damage to windspeed in developing theEnhanced Fujita Scale, andrisk analysis for nuclearwaste storage.


� consideration of possible conflicting interactionsbetween different safety systems (or functions),

� development of specific approaches derived fromother fields of endeavour (finance, assurance) toaddress the residual-risk management .

� 4.5.Advanced deterministic methods to assess plant safety in relation to extreme events


Quantification of the safety margins (in par-ticular in the case of extreme external

hazards) in the stress tests initiated by theFukushima accident, in addition to drawing upsystematic procedures of evaluation of defencein depth protection also requires sufficientlydetailed methods of evaluation of the level ofdamage to structures, systems and componentsunder beyond design basis loads. The evaluationof such scope of damage was not a normal partof the safety analysis within the licensing of theplants and in many cases in the stress tests it wastherefore necessary to use simplified engineeringestimates. In connection with a new interpreta-tion of the safety margins (above the licensinglimits) for the anticipated future analyses it isnecessary to develop, harmonise and apply themethods of assessment of damage to equipment.


Improvement and harmonisation of methodsfor the assessment of the extent of the dam-age to structures, components and systems ofNPP under various extreme loads beyond thedesign basis, their combinations and measuresfor the elimination or mitigation of the damage.


� Analysis of operating experience from the eventsinitiated by unpredicted external events.

� Updating of methods for the assessment of theconsequences of hazards caused both by geophysicalfactors and extreme weather conditions.

� Methodology for determination of the secondaryeffects of earthquakes, including the loss of theultimate heat sink, floods, fires, loss of coolant,destruction of infrastructure, disruption of theaccess of personnel to the site, the dynamic effectsof the destruction of the civil structures, hydrogenexplosions.

� Modelling the effects of individual beyond designbasis extreme events and their combinations onthe damage of structures, components andsystems, the identification and potential foravoidance of threshold phenomena (cliff edges),

� Reassessment of the criteria for the design andevaluation of the effectiveness of protectionagainst external hazards.

� Analysis of the operability of safety importantequipment at the beyond design basis extremeevents.

� Analysis of extreme events initiated by potentialterrorist attacks, including those induced byexternal missiles, fires, blasts and explosions andspecific issues with cyber attack, hacking andsystem vulnerability (through the definition of acomprehensive envelope load that the plantshould be able to resist)10.

� 4.6.Advanced safety systems for nuclear power plants


The main cause of the destruction of theFukushima plant was a long-term interrup-

tion of electric power and consequently the lossof possibility of cooling the core and the con-tainment by active systems. It is acknowledgedthat a more extensive use of passive systems, i.e.systems able to perform their function withoutexternal energy supply, could have contributedto prevent the catastrophic consequences fromsuch an accident. Nevertheless, it is to beremembered that the isolation condenser (a pas-sive safety system) in Fukushima did not preventthe core from melting. Suggestion could also bemade to extend the investigation to the self-operated systems which could show-up bothmore effective and practical than the equivalentpassive ones. Moreover they could provide awider diversity.

In some new reactor designs, passive systems aremore extensively adopted. Nevertheless, theiruse is not fully care-less. Examples of potential

10 - Even thought this topiccannot be considered as adirect consequence of theFukushima’s events,nevertheless it was broadlydiscussed during the stress-test. Actually, its extremesensitivity andconfidentiality, can limit thespace for internationalcollaboration. Nevertheless,it is worth to be mentionedbecause it contributessignificantly to thedefinitions of the loads thenuclear installation isintended to resist,independently of theirnature and origin.


problems include the quantification of systemreliability, the application of the single failurecriterion, the so-called phenomenologicalfailures. Questions arise in particular for systemsusing small driving forces, based on the naturalcirculation. For convincing confirmation ofreliable functioning of these systems, furthertheoretical and experimental research is needed.


Acquisition of the necessary knowledge andmethods, demonstration of the reliability

and functional capabilities of the innovativesafety systems and components, in particularsystems based on passive features.


Modelling the behaviour and assessing theefficiency and reliability of passive equip-

ment and components, e.g.

� Improved battery autonomy.� Pre-pressurized core flooding tanks

(accumulators).� Elevated tank, natural circulation loops (core

make-up tanks).� Gravity drain tanks,� Passive coolers for secondary side of steam

generators based on natural circulation.� Passive cooling circuits for containment heat

removal.� Passively cooled core isolation condensers.� Passive filtered containment venting systems.� sump natural circulation.� Passive hydrogen recombiners.

� 4.7.Advanced materials for nuclear power


Asignificant way to mitigate the conse-quences of the multiple accidents, as in the

case in Fukushima, would be the adoption ofinnovative materials, able to better withstand the

loads and more resistant to severe accident con-ditions, with smaller production of hydrogenand lower staff radiation doses, etc. An exampleof a more long-term activity is to replace zirco-nium based claddings by silicon-carbide basedones to eliminate hydrogen production. Toassess the safety margins of key components(fuel, fuel claddings, reactor vessel and contain-ment) for present and future reactors, it isneeded to know the material properties in end-of-life conditions and during transients arisingfrom accidents. It is also important to determinethe variation of properties for inclusion in prob-abilistic assessments.


Development of new structural materials fornuclear components with increased resist-

ance to harsh operating conditions (corrosion,high radiation doses, high-frequency thermaland mechanical fatigue) and improvement ofknowledge existing and also new ones, throughtesting and modelling. International co-opera-tion is needed for the development of newmaterials for current (e.g. cladding materials)but especially for new reactors.


� Further assessment of existing fuel and claddingmaterials in conditions of severe accidents anddevelopment of new fuel and cladding materialswith increased resistance and improved coolabilityduring severe accidents.

� Assessment of existing and development of newstructural materials for spent fuel storage(including dry storage) aimed at increasing theirresistance to hazards.

� Further assessing the degradation mechanism andageing of materials.

� Developing simulation and monitoring ofdegradation, including non-metallic materials,cables and civil structures.

� Modelling and testing of the behaviour ofcomponents under severe accident conditions.

� Conduction of test programmes to determinematerial properties under severe accidentconditions (e.g. irradiation, strain rate, hightemperature) and development of data base tosupport analysis.


� Evaluation of how the use of non-standard coolingmedia (e.g. polluted water) affect the integrityand functioning of components (e.g. blocking ofvalves, dampers, creation of deposits on cladding,effects on material of the reactor vessel, etc).

� 4.8.Advanced methods for the analysis of severe accidents


The accident has shown that despite preven-tive measures, some accidental sequences

(though for very unlikely conditions) may devel-op into a severe accident. They can eventuallylead to melting the core, damaging the safetybarriers and resulting in the dispersion ofradioactive materials from the containment,with consequential risk to the surroundings ofthe plant. While the Fukushima accident didnot reveal any fundamentally new, unknownphenomena, it did show that the ability to pre-dict the course and consequences of the accidentand the results of the analyses are limitedbecause of large uncertainties, especially in thecase of modelling of long-term processes.Although the analysis already undertaken hashelped significantly in the interpretation of thecourse of the accident, there are still gaps inknowledge of the sources and the distribution ofhydrogen, the quantitative scale of the coremelting and the mechanism of relocation of thecore to the containment, about absorption of fis-sion products in the pressure suppression tankand the behaviour of iodine and other areas.


Critical assessment of the state of the art,improvement and validation of robust

computational tools for the comprehensiveevaluation of the course of accidents in thereactor cooling system and in the containment,from the initiation up to its transition to thesevere accident phase, with a view of reducingthe uncertainty in the results. Fulfilment ofthat objective will also require experimentalresearch and must be implemented by a broadinternational co-operation at least within theEU.

4.8.3. Examples of specific tasks in the given area11

� Critical assessment of the status of knowledge(state of the art) in the modelling of severeaccidents.

� Development and validation of models andsimulation platforms for the analysis of severeaccidents with emphasis on phenomena that arecurrently regarded as insufficiently understood.

� Evaluation and validation of the computer codesand assessment of the uncertainties associatedwith severe accidents.

� Modelling of core heating, chemical reactions,degradation of the core, post-accident heatremoval, coolability of overheated and partiallydestroyed core.

� Detailed examination of the interaction ofmaterials under conditions of severe accidents,including the interaction of molten corium withconcrete.

� Detailed examination of the possibilities ofstabilisation of molten corium in the reactor vesselor in the containment.

� Detailed examination of efficiency of filteredcontainment venting.

� Analysis of the spectrum of scenarios inprogression of core melting in the reactor vessel orcorium in the containment (not only the finalstage).

� Production, distribution and accumulation ofhydrogen, the mechanisms of flame accelerationand the transition from deflagration to detonation.

� Issues of recriticality in complex geometries causedby severe damage of the core, coolant injectionand boron dilution.

� 4.9.Improved procedures for management of severe accidents


The development and the consequences ofthe Fukushima accident have demonstrat-

ed that despite the large volume of existingknowledge and the efforts of the staff it was notpossible either to prevent the accident or tomitigate it without endangering the environ-ment. Although the actual cause of the failureto manage the accident is not entirely clear, the

11 - The topics presentedhere are in general termsaddressed by SARNETactivities with the prioritiesbeing updated consideringthe impact of Fukushima



underestimation of the combined effect of theearthquake and the tsunami wave led to thesequential disabling of safety barriers at severallevels, with the loss of the integrity of the fuelcladding, the fuel matrix, the reactor coolingsystem, and to some extent also of the contain-ment, with the subsequent dissemination ofradioactive substances into the surroundings.For a long time it was impossible to restore elec-trical power supply to important systems andalso to recover heat removal from nuclear fuelfrom the containment. The staff failed to pre-vent the hydrogen explosions, which causedgreat damage to 4 units. A particular problemwas the need to deal with the accident in theabsence of information from the instrumenta-tion, while having simultaneous occurrence ofaccidents at several units on the site, with limit-ed number of staff, and extensive destruction ofinfrastructure.


Update of the basic knowledge about thecauses and consequences of severe acci-

dents, and development of simple computationalaids and alternative ways of obtaining informa-tion regarding the status and expecteddevelopment of the accident, with the possibili-ty to verify the effectiveness and extension of thescope of severe accident management strategies,determination of the rules for the timely deploy-ment and prioritisation of strategies.


� Update of possible accident managementstrategies, taking into account their positive andnegative effects.

� Optimisation and validation of strategies andguidelines for accident management, includingsimple but robust and effective passive measures.

� Investigation of new options for improvedstrategies for accident management and theirprioritisation.

� Improving the functional ability of instrumentationin conditions of severe accidents, increasing thereliability of the information from theinstrumentation relevant to the management ofaccidents, development of alternative ways ofobtaining information in cases where the loss of DC

power supply prevents the reading of standardinstruments.

� development of robust tools and computingdevices (prediction models) needed formanagement of accidents, including on line ICTtools operating in real time to aid decision makingprocess.

� use of advanced simulators to improve operatorspreparedness to avoid cliff edge effects.

� analysis of the possibilities for prevention ofcontainment bypass.

� analysis of the possibilities of containment over-pressurization, assessment of positive andnegative impacts of containment venting.

� accurate monitoring and classifying all possibleon-site and off-site intervention means.

� 4.10.Assessment of the radiological effects of the severe accidents

4.10.1.Lessons learned from the Fukushima accident

Nuclear safety is primarily associated with theability to prevent releases of radioactive

materials, which could endanger the surroundingsof the plant. In this connection Fukushimashowed a number of weaknesses. Data presentedon the radiological consequences consider only afew isotopes of iodine and caesium, while in thecase of a severe accident a substantially widerrange of radioisotopes is expected. In the model-ling it would be appropriate to refine thedescription of the links between weather condi-tions during an accident at a NPP (ventilation intothe surroundings, the explosions and fires, meas-ures for detention and fixation of radionuclides)and radiological consequences. A useful toolwould be a more accurate determination of therelation between the scale of the damage, compo-sition of the source term and measured dose rates.

The issue of radiological consequences of reactoraccidents deserves more attention regardless ofFukushima.

4.10.2.Objectives of the activities in this area

Improvement of both probabilistic and deter-ministic computational tools and enhanced

39


modelling of radiological consequences of reac-tor accidents, with higher flexibility inconsidering links to the processes and activitiesin the plant. In addition more accurate descrip-tion of the conditions for the transport ofradioactive materials in the vicinity of the plantshould be required and the development of pro-posals on the possibility of reduction of theradiological source term to the environment.

4.10.3.Examples of specific tasks in the given area

� Determination of the release of individualradioisotopes from heavily damaged fuel, theirphysical and chemical forms.

� Improved modelling of chemical and physicalforms of released radioactive materials fromheavily damaged fuel.

� Improvement and validation of models fordissemination of radioactive materials in theatmosphere and in aquatic systems, includingpossible effects of explosions and fires in thevicinity of sources of radioactive materials.

� Development of probabilistic methods forcontamination simulation (PSA level 3).

� The use of high-quality meteorological data andweather forecasts in the analysis of radiologicalconsequences.

� Harmonisation of the methods of transfer of datasuitable for prediction of radiological consequences,

� Prediction of the effects of radioactive releases tothe environment.

� Assessment of the current state of knowledge inthe field of the health effects of low doses ofradiation.

� 4.11.Improved modelling of fuel degradation in the spent fuel pool


Although the latest observations have shownrelatively limited damage to the fuel and

spent fuel pool itself in the 4th unit ofFukushima Dai-ichi, interim evaluation drewattention to the potential severity of the accidentin the spent fuel pool as a significant source ofradioactive materials. There are several impor-

tant differences between accidents (includingsevere accidents) taking place in the reactors andin the SFPs. For example, the long duration ofthe processes in the SFP has an effect on thequantitative release fractions of various radioiso-topes. In addition, all processes of claddingoxidation, fuel relocation and release of fissionproducts take place in high temperature airatmosphere instead of in steam.

Despite the large thermal inertia of the pool inFukushima, there were several negative factorsfor the management of the accident, includingthe location of the pool in the upper part of thebuilding, which made it less accessible forreplenishment of the coolant inventory, andoutside the containment. Although, in this caseit was the explosion of hydrogen, which causedthe destruction of the reactor building, probablydue to penetration from the adjacent Unit. 3, ina similar manner this could have occured afterthe uncovering and overheating of the fuel in thepool. Another way of fuel overheating could bethe loss of coolant from in the pool as a result ofcracks caused by the earthquake. Underunfavourable circumstances the pool couldbecome a source of release of radioactivematerials, the effect of which would be furtherboosted by explosion of hydrogen and fire in thebuilding. Doubts about the status of the poolforced the operating personnel in the long-termto refill the pool by different means (helicopters,fire trucks).


Improvement of the status of knowledge,development and verification of adequatecomputational means for the assessment ofprocesses with failure of heat removal from thespent fuel pools (possibly also from dry storageof spent fuel) including transition into a severeaccident with fuel melting, and systematicreassessment of all the possibilities of occur-rence and development of accidents in spentfuel pools.

4.11.3.Examples of specific tasks in the given area12

� Re-assessment of accidents in spent fuel poolscaused by loss of cooling and boiling of the coolant.

40

� Identification of potential pathways leading tosevere accidents with fuel melting.

� Mechanisms of fuel degradation in the spent fuelpool, focusing on late phase phenomena and 3Deffects.

� Creation of chemical forms of the substancesproduced in heavily damaged fuel in the pool,taking into account the presence of an highlyoxidizing atmosphere.

� Evaluation of the possibility of the re-criticality inthe spent fuel pool (including situations withoutfuel damage).

� Possibility of production of hydrogen and theconsequences of deflagration and detonation ofhydrogen.

� Specific conditions for the source term and fordissemination of radioactive materials.

� Measures for the prevention of the degradation ofnuclear fuel.

� Measures for mitigating the consequences of caseswhere prevention failed.

� Methodology of analysis of the integrity of barriersin relation to the spent fuel pool.

� 4.12.Methods for minimization of contamination in the NPP surroundings and for treatment of large volume of radioactive waste


Assessment of the radiological conditions ina wide area around the NPP and mitiga-

tion of the effects of contamination on thepopulation and the environment have been keychallenges not only since the beginning of theaccident to the present day, but such tasksremain relevant for the next few years. Theultimate goal is the complete renovation andrestoration of contaminated territory. It wasalso shown that attention should be paid to thevarious possibilities for reducing the dose rateson the site and in the surrounding as well as tothe isolation and fixation of the radioactivematerials released outside the NPP.

4.12.2. Objectives of the activitiesin this area

Evaluation of the effectiveness of existingmethods of radiological environmental

monitoring as well as the effectiveness and avail-ability of the technical means for therehabilitation of potentially contaminated terri-tory.


� Evaluation of the potential extent of thecontamination of the environment and its impactin the case of a radiation accident.

� Methods of identification and assessment of theeffects of the released radioactive materials.

� Methods of fixation of radioactive substances onthe contaminated territory.

� Evaluation of methods and means of radiologicalenvironmental monitoring, assessment of theresistance of the off-site monitoring networkagainst extreme external hazards.

� Availability and efficiency of technical means forthe elimination of the consequences of accidentsand the rehabilitation of the territory.

� 4.13.Accident management in the framework of theintegrated rescue system


A separate problem in Fukushima was theimplementation of measures to protect the

population up to a distance of a few tens of kilo-metres from the plant, including theinformation to the public, implementation ofthe rescue and evacuation plans and coordina-tion of decision-making on all relevant levels ofmanagement.

A significant weakness in the course of theaccident was the way of communicating theradiological data to the public. Variousmeasurement units were used; presented datadid not permit their easy interpretation from theperspective of the seriousness of the threat;different technical values were alternatelypresented (doses, dose rates); a link between the

12 - The topics addressed inthis section are intended tospan a wide field of R&Dissues, which are intendedto be effectively eliminatedin current designs, such asthe fuel degradationmechanism and therecriticality under degradedconditions in the spent fuelpools. Despite the post-accident investigations inFukushima, eventuallyreducing the importance ofsuch phenomena, thesephenomena are consideredas relevant for R&Dactivities,


presented data and possible health effects wasnot clear. The shortcomings of the INES scale asa tool of communication with the public wasalso demonstrated, assigning to the Fukushimaaccident the same degree of severity as to theChernobyl accident, at a significantly differentlevel of risk.

Evacuation of the population within a radius of30 km around the power plant was needed,which concerned approximately 185,000 people.The decision on how to deal with the accidentwas not made in a sufficiently short time for thechosen method to be effective. It was notentirely clear how to determine evacuation zonesdepending on the anticipated doses, thereforetheir reassessment is planned.


Evaluation of the current status and proposalfor improvements of the integrated rescue

systems at the national level, with emphasis onthe management of accidents of NPPs.


� Evaluation of the effectiveness of the currentrescue system for management of accidents atNPPs in view of the experience of the Fukushimaplant, including the assessment of its applicabilityfor other hazards (e.g. chemical accidents), usinginternational experience.

� Improvement of the methods of risk assessment inthe neighbourhood depending on the size andother characteristics of the source term for varioustypes of hazardous substances.

� assessment of needs and feasibility of externalsupport to the NPP at corporate, national andinternational level.

� Detailed development of methodology documentsand instructions for radiation protection inradiation emergencies.

� Identification of the necessary technical resourcesand the development of training programmes forintervention teams.

� Detailed development of training programmes foremergency drills at the national level.

� Detailed development of scenarios for emergencydrills, including evaluation criteria for success,

� Assessment of the size of evacuation zones in caseof natural disasters.

� Evaluation and proposal for improvements of thesystem for informing the public, thecommunication of risk to the public, including thecoordination of the preparation andimplementation of the evacuation of thepopulation.

� Review of the suitability of the INES scale as a toolof communication with the public regarding theseverity of the accident.

� The harmonisation of criteria of acceptability andintervention levels for radiation accidents.

� Proposal for technical and organisationalmeasures for the improvement of integratedrescue systems.



� 5.1 Overview

The NUGENIA association has definedeight topic areas to identify future R& Dneeds, as listed below:

1. Plant Safety and Risk Assessment.2. Severe Accidents.3. Core and Reactor Operation.4. Integrity Assessment and Ageing of System

Structures and Components.5. Fuel Development, Waste and Spent Fuel

Management and Decommissioning.6. Innovative Gen III Design.7. Harmonisation.8. Inspection and Qualification.

This section of the report therefore the basicinsights for the research needs identified by theTask Group and wonders how they may impacton the scope defined by the road maps for eachof these topic areas. It also takes into account theresearch topics identified in section 3.7 above.

It has been generally concluded to date, that theFukushima incident has not led to the requirementfor any new key R&D target areas, but rather anenhancement of the existing ones to accommodateany additional considerations. The implications ofthe incident might also lead to a different emphasison the scope of particular R&D areas. Thisremains true for the eight NUGENIA topic areasand those identified in section 3.7.

� 5.2 Research areas

In adopting a complementary approach tothat employed to identify the NUGENIAR&D target areas, the Task Group defined

thirteen research areas following a review of theDai-ichi accident, as defined in chapter 4 of thisreport. To recap, these are:

1. Systematic assessment of vulnerabilities todefence-in-depth and safety margins for beyonddesign basis loads.

2. Human/organisational factors under high stressand harmful conditions.

3. Improved methods for external event hazardevaluation.

4. Use of the probabilistic methods to assess plantsafety in relation to extreme events.

5. Advanced deterministic methods to assess plantsafety in relation to extreme events.

6. Advanced safety systems for nuclear power plants.7. Material behaviour during severe accident.8. Advanced methods for the analysis of severe

accidents.9. Improved procedures for management of severe

accidents.10. Assessment of the radiological effects of the severe

accidents.11. Improved modelling of fuel degradation in the

spent fuel pool.12. Methods for minimisation of contamination in the

NPP surroundings and for treatment of largevolume of radioactive waste.

13. Accident management in the framework of theintegrated rescue system.

Not surprisingly there has been a tendency tofocus on severe external hazards and theirimplications, but other areas must not beneglected as well. In the following section anattempt is made to map the implications arisingfrom the above thirteen areas onto the eightNUGENIA Technical Areas. In most cases thisleads only to the identification of additionalaspects or potentially a change of emphasis onthe existing ones.

A mapping of interaction between the researchareas identified in this report and the NUGENIATechnical Areas is provided in table 2 below.

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5. Basic insights related to research needs for Gen II and III reactors

43

The number of + indicatesthe level of correlation ofeach NUGENIA‘s Topic Areain the field of endeavour, asdefined in the roadmaps. Italso allows viewing thesharing of different topicsamong NUGENIA’s TopicalArea

Table 2 – Correlation among the proposed research areas and the NUGENIA Technical Areas

NUGENIA TECHNICAL AREAS

PROPOSED RESEARCH AREAS

1 - P

lant s

afety

& r

isk a

ssessm

ent

2 - Se

vere

acci

dent

s

3 - Co

re &

reac

tor o

pera

tion

4 - In

tegrit

y asse

ssmen

t & a

geing

of

syste

m st

ructu

res &

com

pone

nts

5 - Fu

el de

velop

men

t, wa

ste &

spen

tfu

el m

anag

emen

t & d

ecom

miss

ioning

6 - In

nova

tive

Gen

-III d

esign

7 - H

arm

onisa

tion

8 - In

spec

tion

& qu

alific

ation

1 - Systematic assessment of vulnerabil-ities to defence-in-depth and safetymargins for beyond design basisloads

++ + + +

2 - Human/organisational factors underhigh stress and harmful conditions

+ + ++ +

3 – Improved methods for externalevent hazard evaluation ++ + + +

4 – Use of the probabilistic methods to assess plant safety in relation toextreme events

++ + + +

5 – Advanced deterministic methods to assess plant safety in relation to extreme events

++ + + +

6 – Advanced safety systems for nuclear power plants ++ + + + + +

7 – Material behaviour during severeaccidents + + ++ + + + +

8 – Advanced methods for the analysisof severe accidents + ++

9 – Improved procedures for management of severe accidents + ++ + + +

10 – Assessment of the radiologicaleffects of the severe accidents + ++ + +

11 – Improved modelling of fuel degradation in the spent fuel pool + + + ++

12 – Methods for minimisation of con-tamination in the NPP surroundingsand for treatment of large volume of radioactive waste

+ +

13 – Accident management in the framework of the integratedrescue system

+ + + + +


� 5.3 Implicationsfor NUGENIA Technical Areas

Table 2 summarises the correlation amongthe proposed research areas and theNUGENIA’s Topics. To facilitate the

understanding, the following chapter recalls thecurrent topics roadmap content. EachNUGENIA topic ach has been sub-divided intoindividual research fields. The relevance andimplications for scope and prioritisation of theresearch needs for each topic area is discussed.The ETSON position paper [10] produced byETSON has provided valuable input to thisexercise. As the paper rightly points outhowever, it should be noted that the prioritiesfor research subjects may be different to thoseidentified by system designers and utilities. Asthe NUGENIA road maps have not yet beenfully developed or endorsed by the utilities thismust be recognised.

5.3.1 Plant Safety and Risk

The road map for this topic area has beendivided into seven fields:

1. Data, methods and tools for risk assessments.2. Plant transients.3. External loads (environmental impact on NPPs)

and hazards.4. Electrical disturbances from the grid to the plant.5. Human performance and safety culture.6. Advanced safety assessment methodologies.7. Design of reactor safety system.

This is a comprehensive set of research fieldsand encompasses many of the areas highlightedin [10]. It is however important that field 5includes the aspects associated with humanperformance under situations of extreme stress,which has been highlighted following theFukushima incident.

5.3.2 Severe Accidents

The priority topics identified for this targetarea under SARNET were: in-vessel core

coolability, molten core-concrete interaction,fuel-coolant interaction, hydrogen mixing andcombustion in containment, impact of oxidising

conditions on source term, and iodine andruthenium chemistry. It can be deduced fromthis list that the R&D identified for severe acci-dents encompasses the issues arising from theFukushima incident. The only addition to scopeshould be the implications of prolonged stationblack out.

The road map for this area has been split into 6fields:

1. Corium/debris coolability (in-vessel, ex-vessel).2. Molten core-concrete interaction.3. Steam explosion and hydrogen combustion in

containment.4. Source term (as released from the NPP to the

environment).5. Modelling of severe accident scenarios (integral

codes, PSA level 2 and 3, emergency situations).6. Impact of severe accidents on the environment

(near field out of the NPP).

These research fields cover the main issuesidentified in [10] and chapter 4 of this report.

5.3.3 Core and reactor operation

The road map for this area has been split into5 fields:

1. Human and organizational factors.2. Integration of digital technologies.3. Numerical modelling and core optimization.4. Water chemistry and LLW management.5. Radioprotection.

These align closely with a number of areasidentified in [10] and chapter 4; consequently nonew R & D needs are identified.

5.3.4 Integrity assessment of systems, structures and components

This topic area has been divided into fivefields:

1. Integrity assessment.2. Materials performance.3. Ageing.4. Functionality.5. Qualification of methods, equipment and

materials.


These fields cover all of the areas identified inchapter 4, but their scope must be extended toinclude performance under more extremeconditions that originally envisaged

One of the major items was R&D work to seek for optimisation of materialsperformance/selection. It is recognised thatnuclear grade material assessment is a longduration process that requires an early start. Thefour critical areas for material selection areconsidered to be:

� Fuel cladding.� Reactor pressure vessel.� Primary circuit components and pipe-work.� Concrete containment structures.

The reactor vessel is a key component for safetyand requires continuous R&D. The primarycontainment barrier has to be maintained for allloadings and over the entire lifetime. Major lifelimiters are the degradation mechanisms thatcause embrittlement, and it is important tocontinue to address them in the near term andespecially as the design life is extended. In theaftermath of the Fukushima accident there willbe even stricter requirements to demonstrate theintegrity of the reactor vessel and fuel claddingunder severe accident scenarios. With regard tothe third point, existing materials and coatingsthat are not yet used in the nuclear environmentwere considered to have potential benefits. Itwill be necessary to ensure that the dutiesspecified for such new materials take intoaccount a suitably extended parameter range.Component manufacturing methods andassembly technology were identified as topicsand the latter in particular must not lead toreduction of the robustness of construction. Inparticular the response to seismic, other dynamicenvironmental loadings and blast damage mustbe considered. This must not only cover thecommon components but also cables, electronicsand support structures.

Reinforced concrete containment structuresprovide the third barrier to radioactive releasesand there have been significant improvements inthe development of materials that can withstandhigh temperatures whilst maintaining highstrength. This is particularly relevant as thedecision to depressurise the Dai-ichicontainment was a consequence of the designpressure being exceeded due to the combinationof hydrogen, steam and normal inert gas filling.

With regard to ageing, industrial obsolescence

was an important factor in the Dai-ichi plantresponse to the tsunami. Thus in consideringthis topic the important consequences for futuredesigns must be recognised. Comparativelyspeaking the nuclear industry is a small playerwith the market being driven by otherindustries. Products become obsolete in atimescale which is short in comparison to thepotential 60 – 80 year life projected for futurereactors. In addition if more “off the shelf ” itemsare used technologies and components maybecome harder to replace and may not be so easyto fulfil nuclear requirements. This R&D topicmust therefore encompass this possibility andmust investigate the development of obsolescentresistant technologies. The vulnerability of suchdigital and wireless devices must be assessedagainst more challenging environments thanthose considered to date.

5.3.5 Fuel development, waste and spent fuel management and decommissioning

The road map for this topic area has beendivided into five fields:

1. Fuel development for existing, advanced andinnovative core designs.

2. Fuel behaviour mechanisms and computationalcodes.

3. Fuel transportation, interim storage andtreatment.

4. Waste and spent fuel management.5. Decommissioning/dismantling.

This NUGENIA topic area potentially coversan extremely wide range of research areas. Fueldesign, behaviour, transportation and interimstorage are all areas that have been identified asbeing extremely important following theFukushima incident. As there is some potentialoverlap with topic area 3, care must be taken thatno significant areas are missed by either topicarea road map.

The condition of the fuel is a majorconsideration in the evolution of any NPPaccident scenario as it is the major contributor tothe ultimate severity of the consequences.

The original R&D topics identified for fuelfocussed on four main areas:

� Improved understanding of fuel assemblybehaviour.


� Development and extension of two phase flow CFDcomputational capacity and coupling methods.

� Advanced fuel design to increase integrity andreliability while achieving high performance.

� Innovative fuel designs with higher enrichment.

By simple extension, these four areas stillencompass the issues that have arisen due toFukushima.

Improved understanding of fuel assemblybehaviour includes improved understanding offuel resistance in environmental conditions,predictive modelling of fission product releaseand development of advanced numericalsimulation capabilities for modelling materialmicrostructure evolution in relation to corrosionand hydrogen embrittlement. Pellet-cladmechanical interaction and stress corrosioncracking are also included in this area. All ofthese topics are relevant; it is only the range ofinvestigation that may need to be extended tocover more extreme environmental conditions.

Development and extension of two phase flowCFD may need to consider potentially evenmore complex geometries than those originallyenvisaged, to accommodate partially damagedcores during accident evolution. Theconfiguration and condition of fuel in dried outcooling ponds should also be considered in therange of such calculations.

Advanced fuel design will not only have toconsider increases in integrity and reliabilityduring normal operation, but also morerobustness to accommodate harsherenvironmental conditions. One of the majorconcerns is rapid oxidation of the cladding whenexposed to high temperature in a steamenvironment, leading to brittle failure. In thiscontext the focus may be on alternative advancedcladding materials such as ceramics or carbonbased composites. Another option to beconsidered could be the use of coatings.

Interaction of fuel cladding with alternativecoolants, such as sea water that could ariseduring accident scenarios may be another areathat should be considered for study.

The topic of innovative fuel design with higher(>5%) enrichment also encompasses theprevious item, as it may require advancedmaterials and cladding integrity. It will also needto consider the implications of fission productretention at high burn up and the potentialimpact on spent fuel storage.

An additional overarching requirement wasimprovement and advances in instrumentationand diagnostics to acquire data for the aboveareas. This requirement for advanced, reliableand robust instrumentation for assessment ofreactor performance is consistent with needsidentified from the Fukushima investigations.

Fuel behaviour and in particular clad ballooningand fuel relocation during operation have alsobeen highlighted as focus areas.

The Fukushima incident highlighted thedependence on forced water circulation forcooling of spent fuel in interim water pools. It isclear that improvements in the design andmodelling of spent fuel ponds and passive heatremoval systems will become increasinglyimportant.

Acceptable and safe solutions for radioactivewaste produced during the various stages of thenuclear fuel cycle was identified as one of the 8key R&D target areas at the SNETP GoverningBoard of 31 March 2011. Management ofnuclear waste needs further significant researchefforts.

Operation of nuclear reactors generatesradioactivity that has to be managed to achievean optimised approach to minimize workerdose, public exposure and radioactive waste. Asa result five main topics can be identified:

� Control of mobilisation, transport and depositionof source material within primary circuit.

� Use of simple remediation technologies toclean/decontaminate primary circuits and/or fuel.

� Application of major remedial technologies toreplace large components and permit lifeextension.

� Treatment of radioactive waste to reduce volume,improve its stability and recover materials forreuse.

� Minimisation of source term in case of a severeaccident.

In considering this target area in relation to thebasic insights arising as a consequence of theFukushima incident, minimisation of the sourceterm is obviously a key item in relation toradioactivity release. Transport and deposition ofsource material within the primary circuit andits possible subsequent release needs to beconsidered both during normal operation andaccident scenarios. The design of circuits andcomponents was already identified as an itemthat should address the potential for reduction


of erosion/corrosion, which in turn leads tomore robust plants able to accommodate moreonerous fault conditions. This should beextended to include overall plant layout,proximity of safety critical potentially vulnerablesystems and issues associated with sharedservices on multiple reactor sites. This shouldalso feed into design manuals and guidelinesusing lessons learned from experience and goodpractice.

Development of improved remediationtechnologies both for decontamination andreplacement of large components are areas thatare directly relevant. If plant life extension is tobe considered, the design criteria for suchreplacements should take into account revisedoperational limits to accommodate moreonerous duty and resistance to external hazards.

Improved methods for dealing with radioactivewaste, both in terms of volume reduction andtreatment methods is an area that already had ahigh profile in terms of public acceptance ofnuclear power generation. The Fukushimaevents have resulted in increased scrutiny of themanner in which large volumes of both liquidand solid waste should be dealt with.

The implications of long term storage of wasteand potentially dry storage of fuel at NPP needto be reviewed in the light of Fukushimaparticularly in relation to the ageingcharacteristics of such stores and their ability towithstand external hazards.

In the light of Fukushima, research intodecommissioning, dismantling anddecontamination methods should be extendedto consider the additional difficulties andtechniques required to deal with potentiallydamaged nuclear facilities.

5.3.6 Innovative Gen III Design

The road map for this topic area is dividedinto 5 fields:

1. Innovative technologies for reactor componentsdesign and construction.

2. Innovative LWR concepts such as: High ConversionRatio LWR/Small Modular Reactor.

3. Innovative Gen III specific operation and safetyapproach.

4. Key success factors for innovative Gen IIIdeployment.

5. Public acceptance drivers for new builds.

This research area is somewhat different to theothers as it incorporates some elements that arealso covered in other topic area road maps. Thetask is focussed on Gen III reactorsencompassing both pressurised water andboiling water cooled technologies. Consequentlyits primary aim is to consider the R&D needsfor optimisation of reactor construction costsand time as well as the ratio of reactorperformance to safety and reliability for longterm operation of 60 to 80 years.

Within this context the implications of havingmultiple reactor units on one site with sharedservices will need to be reconsidered.

Fields 4 & 5 will take on additional emphasis asa consequence of the Fukushima incident.

No additional aspects are required for this targetarea as they are covered in the other sections.

5.3.7 Harmonisation

The Harmonisation topic area roadmap inNUGENIA identified four fields of

research:

1. Pre-normative research.2. Plant design, construction and operation

methodologies and practices.3. Harmonisation of safety objectives and practices.4. Harmonisation of standards.

� The pre-normative research, is defined as both thepreliminary phase of research aimed at bettercharacterising new technologies and evaluatingthe related safety aspects by applying wellestablished procedures and methodologies and astudy of the results which are to be used to developregulations, standards and technical codes.

As an outcome of the Fukushima events, thefollowing topics can be targeted, among others,for pre-normative research activities:

� Structures and components (metallic, concrete),aiming at characterizing their capabilities andlimits following standardized definitions, alsoaccounting for the ageing management whichneeds anticipated assessment of long termphenomena related to corrosion, thermo-mechanics, irradiation, fluid–structure interaction.

� Instrumentation, to increase its robustness andreliability, in extreme high temperatures and highpressure conditions.


� High performance computing, to enhance thecapacity to carry out numerous calculations in avery short time, for uncertainty and sensitivityevaluation.

� The plant design, construction and operationmethodologies and practices.

The current trend to accounting for whatever isunknown and uncertain in nuclear safety isadopting the defence-in-depth approach, whichallows accounting for the uncertainties - whichare known - relying on study rules of thedeterministic demonstration and of probabilisticapproach, as well.

Fukushima events confirmed that the mainweakness in the defence in depth is to besearched for in the independence of the defencelevels from each other: each device (system,structure, component, operation procedure, etc.)belonging to one level of defence is to be asindependent as reasonably possible from anyother. Improvements are to be searched for,mainly to put the current GEN II reactorrequirements in conformity with those adoptedfor GEN III and GEN III+.

To complement the current approach, thefollowing should be improved and extended:

� The safety margins approach - which is intendedto account for all major potential contributions tothe risk, independently from their ranking andexpected issues – and is still too impractical in use.

� The risk informed approach which still needsextensive validation through a comprehensiveapplication, which could be carried-out takingadvantage from a plant operation change, such asthe prolonged operation.

A great number of the existing standards andguidelines are not always applied worldwide.The Fukushima events demonstrated that thiswide dispersion deserves attention. A first effortshould be done to promote dissemination and

acceptance, suggesting the identification in theR&D reports of pre-normative entry dataformatted to be easily integrated in the standardactivities, supporting the participation in theresearch projects of experts having an experienceof standard development in particular to preparethe formatted pre-normative entry data,

Ref [10] identifies that harmonisation ofstandards and practices is an issue that appearsin several different fields, for example in safetyassessment methods, safety marginmethodology, component/material ageingpredictions.

It is important that the lessons learned from theFukushima accident contribute to thepromotion of common practices.Standardisation of safety assessment andlicensing will facilitate deployment of LWRtechnology all over Europe and abroad. Acommon set of qualified tools for safety analysisand advanced methodologies for the predictionof the consequences of extreme hazards shouldbe developed. Use of data from Fukushima willprovide a valuable source of validation materialfor these developments.

5.3.8 Inspection and Qualification

This technical area derives from the integra-tion of the ENIQ Network and was added

to NUGENIA recently. There is therefore not aspecific NUGENIA Roadmap yet. The ENIQNetwork dealt primarily with inspection tech-niques for defects and ageing of componentswith the two technical areas Qualification andRisk Assessment. Although the classical ENIQwork so far has not been directly linked to severeaccidents, it is expected that technologies andmethods can also be applied to areas relevant toFukushima. This is particularly true for qualifi-cation methodologies for instrumentation andlinking instrumentation and inspections to riskmitigation. The use of expert elicitation isanother relevant area.



The objective of this report was to identifyand outline research areas that need tobe further strengthened in view of the

lessons learned from the Fukushima accident.First of all it is important to stress that theFukushima accident does not radically changethe SRA of SNETP due to the fact that nocompletely new phenomena have beenidentified. Nevertheless it is clear that muchstronger attention should be paid to researchassociated with safety issues. In particular moreemphasis needs to be given to the assessment ofrobustness of the plant against extreme hazards,prevention and mitigation of severe accidentsincluding organizational and societal factors.

The SNETP Fukushima Task Group hasidentified a series of “lessons learned” from theFukushima accident, which can be grouped intofour main areas:

� Nuclear system organization, safety infrastructureand safety culture.

� Assessment of risk from external events andprevention measures.

� Phenomenology, management and mitigation ofsevere accidents.

� Emergency preparedness and management.

Based on the assessment of the lessons learned,in chapter 4, R&D needs for thirteen specificresearch areas are presented with identificationof more specific tasks, as follows:

1. Systematic assessment of vulnerabilities todefence-in-depth and safety margins for beyonddesign basis loads.

2. Human/organizational factors under high stressand harmful conditions.

3. Improved methods for external event hazardevaluation.

4. Use of the probabilistic methods to assess plantsafety in relation to extreme events.

5. Advanced deterministic methods to assess plantsafety in relation to extreme events.

6. Advanced safety systems for nuclear power plants7. Advanced materials for nuclear power.8. Advanced methods for the analysis of severe

accidents.9. Improved procedures for management of severe

accidents.10. Assessment of the radiological effects of the severe

accidents.11. Improved modelling of fuel degradation in the

spent fuel pool.12. Methods for minimization of contamination in the

NPP surroundings and for treatment of largevolume of radioactive waste.

13. Accident management in the framework of theintegrated rescue system.

Examples of the specific tasks to be addressed inindividual areas are given. These tasks are listedwithout specific classification or prioritization; itis clear that there are tasks of quite differentnature. These include basic research, applied andpre-normative research, harmonisation ofmethods, investigation on organizational andsocietal issues, but also tasks focused onapplication of existing modelling tools.

The specific research needs andrecommendations in chapter 4 are thenincorporated into chapter 5 for a broader systemoverview, considering a number of horizontalareas as defined in the roadmap of NUGENIAassociation:

1. Plant Safety and Risk Assessment.2. Severe Accidents.3. Core and Reactor Operation.4. Integrity Assessment and Ageing of System

Structures and Components.5. Fuel Development, Waste and Spent Fuel

Management and Decommissioning.

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6. Conclusions

51

6. Innovative Gen III Design.7. Harmonisation.8. Inspection and Qualification.

Although this report primarily addresses LWRs,many general implications and areas foradditional research and development areapplicable for other nuclear installations andreactor designs, including Gen IV reactors.

The Task Group recommends that, in the futurerevisions of the SNETP documents, a centralposition should be attributed to the safetyproblem as a whole, so that safety-orientedR&D becomes the overarching driving forceand engine for the entire SNETP nuclearresearch agenda. It is also necessary that theresearch not only deals with understanding andmodelling phenomena, but is strongly orientedtowards deriving practical implications,

especially on prevention and mitigation deviceswhich can improve safety .

The Fukushima accident had a large impact onthe public view of nuclear energy and willimpact the design and operation of nuclearreactors in the future. In spite of this fact, theaccident didn’t change the reasons formaintaining the role of nuclear power in theenergy mix as sustainable, safe and low carbonsource.

The R&D activities identified in the currentdocument should continue playing an essentialrole in future utilization of nuclear power,supporting the life extension of the currentLWRs, which should be associated withcomprehensive safety assessment using up todate standards, the deployment of theGeneration III reactors and the development ofthe Generation IV technologies, always keepinghigh safety level as utmost priority.



[1] Isnard O., Raimond E., Corbin D. and Denis J.,Radioactive source term and release in theenvironment, Proceedings of the Eurosafe Forum2011, Conference, Paris, November 2011.

[2] Chino M., Nakayama H., Nagai H., Terada H.,Katata G., Yamazawa H. Preliminary Estimation ofRelease Amounts of I-131 and Cs-137 accidentallyDischarged from the Fukushima Daiichi NPP intothe Atmosphere. 2011. J. of Nuc. Science andTechnology. Vol 48, N°7, pp 1129-1134.

[3] Bannai, T. Regarding the Evaluation of theConditions on Reactor Cores of Unit 1, 2 and 3related to the Accident at Fukushima Dai-ichiNuclear Power Station, Tokyo Electric Power Co. Inc.Nuclear and Industrial Safety Agency, June 6, 2011

[4] Bruna G. B., Isnard O, Raimond E., Corbin D. and.Denis J. Fukushima Dai-Ichi’s Radioactive sourceterm and release in the environment, Proceedingsof the “One year after Fukushima: rethinking thefuture” Workshop, Italian National School forPublic Administration, Via Testoni 2 – BolognaItaly, March 15-16 2012

[5] Takashi Sato (TEPCO), “Status and perspectives ofplant restoration process”, ENEA Workshop “Oneyear after Fukushima: rethinking the future”,Bologna, March, 2012

[6] The Great East Japan Earthquake Expert Mission,“IAEA International Fact Finding Expert Mission ofthe Fukushima Dai-Ichi NPP Accident Followingthe Great East Japan Earthquake and Tsunami, 24May – 1 June 2011”, IAEA Mission Report

[7] “Report of the Japanese Government to the IAEAMinisterial Conference on Nuclear Safety - TheAccident at TEPCO’s Fukushima Nuclear PowerStations”, Nuclear Emergency ResponseHeadquarters, Government of Japan, June 2011

[8] “Recommendations for Enhancing Reactor Safetyin the 21st Century”, The Near-Term Task ForceReview of Insights from the Fukushima Dai-ichiAccident, U.S.NRC, July 12, 2011

[9] OECD/NEA Forum on the Fukushima Accident,Paris, June 8, 2011, http://www.oecd-nea.org/nsd/workshops/fukushima-forum/

[10] Position paper of the Technical SafetyOrganisations: Research needs in nuclear safetyfor GEN 2 and GEN 3 NPPs, October 2011

[11] IRSN newsmagazine “Repères”, January 2012

[12] IAEA International Experts Meeting, 19-22 March2012, documents available onhttp://www.scribd.com/collections/3548893/International-Experts%E2%80%99-Meeting-on-Reactor-and-Spent-Fuel-Safety-in-the-Light-of-the-Accident-at-the-Fukushima-Daiichi-Nuclear-Power-Plant?page=1

[13] The National Diet of Japan, “The official report ofthe Fukushima Nuclear Accident IndependentInvestigation Commission”

[14] “Special Report on the Nuclear Accident at theFukushima Daiichi Nuclear Power Station”, INPO11-005, November 2011

[15] Stress Tests Performed on European NuclearPower Plants as a Follow-up to the FukushimaAccident, Overview and Conclusions Presented toENSREG by the Peer Review Board, April 2012

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7. References

53

Acknowledgements

Contributors to drafting and review


G

� Giovanni Bruna, IRSN � Paolo Contri, ENEL� Gaudenzio Mariotti, ENEL� Benedikt Martens, SCK•CEN � Jozef Misak, UJV Rez (chair)� Karl-Fredrik Nilsson, JRC Petten� Elena Paffumi, JRC Petten � Mike Smith, AMEC � Hans-Georg Willschuetz, EON

We acknowledge the participants of the GenII/III Technology Working Group andSARNET who defined the original R&D targetareas, which formed the basis for this report.

We equally acknowledge the members ofSNETP Governing Board and ExecutiveCommittee and other experts who provided aseries of valuable contributions and comments.


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ISBN 978-2-919313-05-1