module 08 fukushima dai-ichi accident 11.3 - oektg.at 08 fukushima dai-ichi accident 11.3.2011 ......

Module 08 Fukushima

Dai-ichi Accident 11.3.2011 1.10.2016

Prof.Dr. Böck Technical University Vienna Atominstitut Stadionallee 2, 1020 Vienna, Austria ph: ++43-1-58801 141368 [email protected]

Contribution of Dai-ichi NPPs to electric power production in Japan

• ~30% of Japanese electrical grid supplied by 54 reactor units: 30 BWRs / 24 Pressurized Water Reactors, (PWRs)

• ~17% of electrical grid supplied by the 30 BWRs • ~2% of electrical grid was supplied by the 4 damaged BWRs at Fukushima Dai-ichi

Location of the NPPs near Earthquake Epicenter

(Tokai (I) is decommissioned)

Dai-ni

Dai-ichi

Fukushima Dai-ichi npp Status prior to Earthquake / Tsunami Event

Six Boiling Water Reactors (BWRs) at Fukushima Dai-ichi NPP

• Unit 1: 439 MWe, 1971 (in operation prior to event)



• Unit 4: 760 MWe, 1978 (shutdown prior to event)



Scram Levels

• Normal earthquake scram level: 0,15 g

• Design basis acceleration level: 0,45g, in this case all safety functions must stay operational

• Actual ground acceleration at site: 0,56 g - 20% above design basis

Cross Section and Photo of GE Mark I (32 units world-wide, 23 in USA, 1 in Switzerland)

Fukushima Dai-ichi NPPs

turbine buildings

cooling water discharge

14.3.2011 9

Loss of fuel tanks for emergency diesel generators

before tsunami

after tsunami

Status upon Earthquake: March 11, 2011 11 operating reactors automatically shut‐down due to ground acceleration exceeding the reactor seismic trip settings • Onagawa Units 1,2,3 • Fukushima (I) Dai-ichi Units 1, 2, 3 • Fukushima (II) Dai-ni Units 1, 2, 3, 4 • Tokai (II) (Tokai (I) is decommissioned)

3 reactors were shutdown prior to earthquake for periodic inspection • Fukushima Dai‐ichi Unit 4 (reactor was defuelled), Unit 5

and Unit 6

Tsunami hits coast in Fukushima I

Sea starts to overflow Tsunami wall at Dai-ni, (10 km from Dai-ichi site)

1 minute later

15

Non-nuclear facility damage: oil refinery fire, Ichihara, Chiba Prefecture

Relative elevation of NPP systems

Total loss of power

Simplified view of BWR reactor vessel and containment design

DW: Drywell

WW: Wetwell

SFP: Spent Fuel Pool

SCSW: Secondary Concrete Sheild Wall

RPV: Reactor Pressure Vessel

Drywell: Bulb-shaped : 30 mm steel,

during operation filled with N2

Wetwell: Torus with about 3000 m3 cold

water

Normal Operation Emergency Cooling

Fuel elements partially uncovered and start to heat up

Containement designed for 4 bars pressure exceeds 8 bars, pressure relief through relief valves

Fuel storage pool damaged, water level decreases, fuel elements overheat are damaged and release fisson products

H2 explosion below reactor roof, release of fission products to the

environment

Accident progression 1

• Emergency battery power provided core cooling for about 8 hours (Unit 1), after which the batteries were discharged. Decay heat in the reactors and spent fuel ponds could no longer be removed

• Offsite power could not be restored, delays were encountered in obtaining and connecting portable generators

• Reactor temperatures increased, water levels in the reactor vessel decreased, eventually uncovering and overheating the Unit 1 core.

• Hydrogen was produced by oxidation of zirconium metal fuel cladding, zirconium-water reactions in core start above 1000ºC


• Spent fuel pool temperature rises in Unit 4 (containing all the reactor fuel), as water is lost from possible pool wall damage, 150 tons seawater pumped in, then fresh water pumped in.

• Unit 4 fire breaks out, April 11, that later extinguishes, with subsequent significant fire damage.

• April 2011 core damage status (subject to revision): • Unit 1: 55% Unit 2: 35% Unit 3: 30%.


• Operators vented the reactor pressure vessel to relieve pressure, steam and hydrogen were discharged to primary containment (drywell), causing primary containment temperatures and pressures to rise

• Hydrogen explosions occurred (March 12, Unit 1: March 13, Unit 3: and March 15, Unit 2) while venting secondary containment, likely ignited by a sparks and hydrogen encountering free oxygen

• Hydrogen re-combiners, designed to burn vented hydrogen, did not operate probably because of power requirements

Unit 1 Unit 2 Unit 3 Loss of AC power + 51 min + 54 min + 52 min Loss of cooling + 1 hour + 70 hours + 36 hours Water level down to top of fuel* + 3 hours + 74 hours + 42 hours

Core damage starts* + 4 hours + 77 hours + 44 hours

Reactor pressure vessel damage* +11 hours uncertain uncertain

Fire pumps with fresh water + 15 hours + 43 hours

Hydrogen explosion (not confirmed for unit 2)

+ 25 hours service floor

+ 87 hours suppression chamber

+ 68 hours service floor

Fire pumps with seawater + 28 hours + 77 hours + 46 hours

Off-site electrical supply + 11-15 days

Fresh water cooling + 14-15 days

Event sequence following earthquake (timing from it: 14:46, 11 March)

Fuel assemblies: 4m long, About 60 rods per assembly Block 1: ca 24 000 fuel rods (70 t Uran) Block 2-4: ca 33 000 Fuel rods (95 t Uran)

Spent Fuel Pools Dimensions in m

Num,ber of Fuel Assemblies

Heat input if not cooled

Depth in m

Unit 1 12 x 7 900

Unit 2 12 x 10 1240

Unit 3 12 x 10 1220

Unit 4 12 x 10 1590 appr. 3 MW

12*

* Above fuel top: 7m, after accident : - 5m ** Evaporation: 100 m3 per day Normal temperature: 30 C, max: 85 C

GE-BWR Reactor Hall with Fuel Pool

GE-BWR during revision

Decay Heat

33 33

Nachzerfallswärme der Spaltprodukte

Reactor decay heat problem

Decay heat after shutdown (Dai-ichi Units 2 and 3)

Unit 4 with over 1000 fuel rods in the spent fuel pool, the decay heat

could boil off about 100m3 of water per day, if the water was at

boiling point

Time Percentage of Full power Decay Heat MW_thermal 1 sec 7 % 167 1 day 1 – 2 % 47 1 year 0.2 % 5

Reactor decay heat problem

• Radioactive isotopes (fission products) in the fuel produce radiation as they decay (gamma, beta, and alpha radiation)

• This decay radiation deposits most of its energy in the fuel (decay heat), about 7% of thermal power, immediately after a reactor is shutdown

• The decay heat must be removed at the same rate it is produced

or the reactor fuel core will heat up and in the absence of any cooling the fuel may melt

• The removal of decay heat is performed by various cooling systems that provide water flow through the reactor core with the heat being transferred to heat exchangers and the ultimate heat sink of the sea.

• At Fukushima the integrity of the cooling systems was compromised by the tsunami and made it difficult for the operators to sustain decay heat removal

Unit 1 Hydrogen explosion of outer secondary containment building

March 12, 2011

March 14, 2011 Unit 3 Hydrogen explosion

Hydrogen Explosion Unit 1 to 3, especially Unit 4

Fukushima Unit 4 January 2016

Emergency Actions 1

• Operating and emergency support staff used portable

generators and portable pumps to inject seawater into the reactor vessels of and primary containments of Units 1, 2, and 3 via mobile fire trucks

• This had the effect of flooding the primary containment

to cool the reactor vessel and any core debris that may have been released into primary containment.

• The seawater was the only cooling possibility but

seawater effectively would render reactors unusable due to corrosion of fuel, coolant piping and reactor vessel, as well as leading to salt accumulation and clogging

Emergency Actions 2 • A general emergency was declared in response to the initial

events at Unit 1, with evacuation of public within 20-30 km of the plant: about 200,000 people evacuated

• Electrical power and cooling functions eventually restored,

but much equipment is damaged and status unknown • Offsite radiation and contamination, as a result of venting

and the uncontrolled releases from hydrogen explosions and fires

• Potassium iodide pills distributed but not administered to public to minimize I-131 uptake in the thyroid

• Measures implemented to control affected contaminated food

Summary of a Complex Accident 1

• All 13 effected Japanese NPP units withstood a massive earthquake, with likely minimal damage, but 4 units were overwhelmed by the tsunami

• Seawall was 5.7 m but the tsunami was 14 m (the design basis was underestimated)

• Significant reactor core fuel damage to 3 units (full meltdown predicted at Unit 1), damage to fuel in some fuel ponds (Unit 4 undamaged, Unit 3 damage due to explosions)

• Loss of site emergency diesel generators on site was key.

• 3 staff fatalities (explosion and heart attack), 15 injured by hydrogen explosions and tsunami


• 1 indirect death, reportedly from suicide, of a local farmer after farming restrictions

• 4 hydrogen explosions in reactor buildings caused very significant structural and reactor equipment damage, fires in Unit 4

• At plant site: About 20 000 involved during accident 135 workers 100-150 mSv 23 workers 150-200 mSv 3 workers 200-250 mSv 6 workers up to max 678 mSv (500 mSv is the internationa allowable short term dose for emergency workers taking life- saving actions)

• Beyond plant site: UNSCEAR Report April 2014: People of the

Fukushima area are expected to receive less than 10 mSv due to the accident over their whole lifetime compared with 170 mSv lifetime does from natural background radiation


• Over 200,000 people evacuated from the area, ground contamination up to 50 km from site

• Huge Japanese economic and financial consequences

• Nuclear industry worldwide: future increased safety vigilance and costs; international scrutiny (IAEA)

• Nuclear ‘renaissance’ projects worldwide now on hold or being reviewed

• TEPCO has said it may take the rest of the year to bring the plant back under control

• The world’s worst industrial accident (Bhopal) far exceeded Fukushima in human toll, but likely not in economic consequences

Major Accident Contributors

• Underestimation of maximum earthquake

• Tsunami height • Location of safety related systems in

buildings (Diesel) • No backfitting of pressure relief pipes • No filters in pressure relief pipe • No H2 recombinators

International Nuclear Event Scale INES

Stress Tests: Participation

All 14 EU Member States that operate nuclear power plants, plus Lithuania, Switzerland, and Ukraine

Stresstests for NPPs initiated in Europe

• Total loss of all power supply (extern, emergency diesel, batteries)

• Total loss of cooling capacity including spent fuel ponds

• Effects of earthquake on water environment (ocean, river, dams, hydrostations upstreams, mud slides)

• Human deficiencies • Aircraft crash and terrorist attacks considered

through other national security actions

Stresstest Time Schedule

• Until Sept. 2011 all operators had to check their NPP according to the stresstest conditions

• NPP operators reported to the National Regulatory Body which evaluated the results and reported to the EU commission by December 2011

• The EU Commission then recommended further steps to improve the safety of NPP‘s

• Web pages dedicated to public engagement: www.ensreg.eu/EU-Stress-Tests/Public-

engagement

Stress Tests: Follow-up • On October 4th 2012 the EC published its position on the stress

tests • Totally 145 NPPs have been analysed • The risk analysis for earthquake and flooding should be based for

all NPPs on the 10 000 year time frame • On-site seismic instruments • Containment filtered venting systems • Equipment to fight severe accidents • Backup emergency control room • Nuclear insurance and liability should be harmonized in Europe

New enhanced safety requirements Japan

Conclusion • Acivity release to the environment especially during

explosions • Depending on meteorology local short term increase of

activity without reaching international emergency radiation limits outside NPP site

• Iodine pills distributed but not administered • Activity continuously measured by radiation network

both in air and seawater • No radiological short- or long term effects exspected • Cs-137 and I-131 were measured also in Austria with

high precision equipment (€ 150 000)

What you should remember

• All operating NPPs were shut down by earthquake sensors without any damage

• The tsunami destroyed all possible power supply systems

• In this case core temperature increases rapidly and fuel elements were destroyed both in the core and in the spent fuel pool as no cooling was possible

• Mainly Iodine-131 and Cs-137 were released • Hydrogen explosion took place due to Zr-H2O reaction • No excessive overexposure of public • Four staff members were killed by mechanical accidents

References

55

• https://www.iaea.org/newscenter/focus/fukushima • http://online.itp.ucsb.edu/online/plecture/bmonreal11/ • www.meti.go.jp/english/index.html • www.tepco.co.jp/en/index-e.html • http://www.grs.de/node/2460 • http://fukushima.jaea.go.jp/english

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