IAEA Vienna I August 1997

SEISMIC RE-EVALUATION OF MOCHOVCENUCLEAR POWER PLANT

SLOVAK R E P U B L I C

Unit No. 1 and 2 - WWER 440 MW

Prepared by: Jin Podrouzek, M.Sc - SKODA PRAHA, a.s.

(1)

RE-EVALUATION OF THE SEISMIC HAZARD

(A) ORIGINAL DESIGN CRITERIA:

Maximal Calculation Earthquake (MCE) with an intensity 6° MSK-64 scaleTime history acceleration recorded at ,,Nis", zero period ground accelerationwas normalized to 0,06 g.

(B) RE-ASSESSMENT OF THE SEISMIC HAZARD:

Review Level Earthquake Peak Ground Acceletation of 0,10 g ,(for the zero period of the design response spectra),

PGARLE is 0,10 g in horizontal and 0,06 g in vertical directions,GRSRLE is Newmark's ground spectrum of absolute acceleration

(median + 1 sigma) for rock side.

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(2)

SEISMIC RE-EVALUATION INPUTS

(A) TECHNICAL GUIDELINES FOR THE SEISMIC RE-EVALUATIONPROGRAMME OF MOCHOVCE NPP

IAEA Vienna, IAEA/RU-5342, August 1995.

(B) TECHNICAL SPECIFICATION OF SAFETY PROGRAMMES - EH 01NPP Mochovce and VUJE Trnava, 1995.

(C) SYNTHETIC ACCELEROGRAMS FOR SEISMIC UPGRADINGCALCULATIONS

Stevenson and Associates, rep.09-95.egp, Pilsen, 8/1995.

(D) SEISMIC RE-EVALUATION GUIDE OF MOCHOVCE NPP STRUCTURESAND EQUIPMENT

Stevenson and Associates + SKODA PRAHA, rep.15-95.sph + Jc 43 075 Zp Revision 4,. • Pilsen, 4/1997.

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IAEA Vienna 3 August 1997

(3)

WORKPLAN PHASES AND TASKS

Task

The re-assessment of the seismic capacity of existing facilites andcorresponding upgradings to a level generally accepted by the internationalcommunity, i.e. Seismic Margin Assessment method.

Principal re-evaluation participated solvers

Responsibility for the complete seismic safety upgrading project has SKODAPRAHA.

~ Civil structure part = Energoprojekt Praha + subcontractors,— Technological part = Skoda Praha + subcontractors,~ Electrical part = VUJE Trnava + subcontractors.

Basic re-evaluation steps

(1) Safe shut-down structures and equipment list, incl. the seismicclassification and priorization.

(2) Seismic capacity of civil structures evaluation and realistic floorresponse spectra definition.

(3) Plant's walkdown timetable:

a) preliminary screening walkdowns(April 1996 - May 1996),

b) detailed screening walkdowns(November 1996 - August 1997),

c) evaluation of seismic margin capacity(November 1996 - November 1997),

d) seismic upgradings realization(September 1997 - September 1998 (plan)).

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(4)

SUMMARY OF MOCHOVCE SEISMIC RE-EVALUATIONAND UPGRADING PROGRAMME

(1) Seismic upgrading of civil structure part

REACTOR BUILDING COMPLET (reactor hall (800/1), controlroom and electrical building (805/1, 806/1), turbine hall (490/1)

Local strengthenings are necessary.

SUPEREMERGENCY FEED-WATER TANKS BUILDING (810/1)

Exact capacity calculation are necessary (the strengthening may berealized for roof structure and for storage tank anchoring points).

SERVICE BUILDING (840/1) - seismic category 2a (interaction)

Very problematic civil structure is analysed exactly now - more precisemargin HCLPF calculations are necessary for reduction of eventual

structure reconstruction upgradings (analyses are now provided).

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IAEA Vienna August 1997

(2) Seismic upgrading of technological part

(a) COLD SAFE SHUT-DOWN CONDITIONS BLOCK DIAGRAM

For first 72 hours following the occurence of the RLE (Review Level Earthquake) andseismic interaction prediction.

SEISMIC INICIATIONIMPULS

REACTOR SHUT-DOWN

\

HOT SAFE SHUT-DOWN CONDITIONS OF REACTOR FIXATIONLiquid boric acid shut-down concentration before unit

decay heat removal

\

UNIT DECAY HEAT REMOVAL1 stage

AUNIT DECAY HEAT REMOVAL

2 stage

ASAFE COLD SHUT-DOWN CONDITIONS OF PRIMARY CIRCUIT

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(b) SAFE SHUT-DOWN OPERATION BLOCK DIAGRAM

Iniciationof HOI

Operation of DPS03.02. system(main way)

Operation of DPS14.01. system(spare way)

Steam generatorfeeding from

superemergencysystem

(main way-6 SG)

Steam generatorfeeding from

superemergencysystem

(spare way-4SG)

1

T and P tracingof primary

circuit( T - p falling )(dependence)

Removing of PCresidual heat

through SG byBRUA block

away(main way)

Removing of PCresidual heat

through part ofSG by BRUA

block away(spare way)

Removing ofresidual heat

through SAOZmain system

PS 14(main way)

Removing ofresidual heat

through SAOZspare system

PS 14( spare way)

Removing ofresidual heatto technical

important feedwater 2 systems

(main way)

Removing ofresidual heatto technical

important feedwater spare system

(spare way)

Primary and secondary circuit operationparameters control and tracing

(according operation prescriptions and data)

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IAEA Vienna August 1997

(c) SEISMIC RE-EVALUATION SUMMARY OF TECHNOLOGICALSYSTEMS AND COMPONENTS

PS 20.HVAC systems in reactor building

PS 25.HVAC systems in electrobuildings

DPS 17.01.insert cooling

system of mainpumps

DPS 03.01.continual PC

cleaning system

DPS 17.03.insert coollingsystem of SAOZ

pumps

DPS 03.02.acid boric filling

andregeneration

IDPS 11.01.

cleaning waterstation

DPS 11.11.regeneration

stationI

PS 06. ..connecting pipeline systems ofprimary circuit

PS 34.dieselgenerator

station

DPS 03.04.hydrogen

combustionsystem

DPS 11.13.cleaning stationof technological

gases

PS 01.primary circuit

(PC)

PS 14emergency

crashsystems

PS 07.connecting pipes

of secondarycircuit

DPS 03.03.cooling systemof fuel storage

basin

DPS 26.03.support demi-water

storage tank

DPS 05.03.superemergency

water pumps

508/01-03

fan cooling towers

PS 32.connecting

outside pipelinesystems

PS 31.pumping station

of technicalimportant water

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IAEA Vienna

(d) APPLICATION OF

8

ORIGINAL SEISMIC DESIGN

August 1997

RESULTS

Viscodamper GERB dynamic strengthening of the systems and components

Seismic design calculations in original basic documents and realisation

Seismic tests of used equipment and components

Used the seismic tested equipment and components (I & C, HVAC, electro)

Simplify verification methodologies for seismic re-evaluation programme

(e) RESULTS OF SEISMIC RE-EVALUATION AND WALKDOWNS

1. Piping systems (hot) = Seismic margin CDFM calculations (realisedreality) - very low number of the additionalnecessary namely horizontal supports, in afew places additional damper instalation isnecessary.

2. Piping systems (cold) = Local upgradings with horizontal supports,local corrosion problems (pipe ducts namely),(no more 10% of all pipelines hot and cold).

3. Fittings and valves = Additional supportings of some electrovalvesare necessary - extend of masses betweenvalve body and electropower is higher thanlimit values (no more 10% of valves)

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IAEA Vienna * August 1997

4. Tanks and heaters = Pressure tanks or heaters without problems,local anchoring reconstruction is necessary.

= Additional anchoring of liquid storage tankswill be necessary (no more than 15%).

= For the acid-boric storage tanks more exactcalculations are necessary for local strengt-henings are supposed (in tank roof part).

= Superemergency feed-water tanks supportsmust be reconstructed - more anchoring fixpoints is necessary.

= Anchoring of feed water tank (2a/ category)will be reconstructed, spring-damper GERBunits are used.

5. Pumps and fans = Without great problems (local connectedpipings fixation may be applied for nozzleload cases reduction).

6. HVAC systems = Without great problems (some local fixationreconstruction is necessary).

= New anchoring of cooling units in building801/1 must be designed and realised.

7. Transport units = Some of the heavy transport-technologicalcomponents located on reactor room (501/1)must be additionaly fixed to prevent the freeseismic motion possibility (max. 30% of 2a/category components).

8. Seismic interactions = Additional fixation of some non-seismic 2a/category components - individualy solved.

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IAEA Vienna 1 ^ August 1997

(3) Seismic upgrading of electrical part

(1) Assembly works finishing and completation must be done.

(2) Existed and realised seismic tests verification and control.

(3) Additional seismic capacity equipment tests realisation.

(4) Verification of instaled cabinets and cable traces anchoringcapacity (local increasing if necessary).

(5) The new seismic tested and verified equipment and unitsreconstruction.

(6) Results of VARTA batteries re-evaluation is positive.

308

November 19-21,1997, Levice, Slovakia

Mochovce NPP Safety Improvement and Completion

ENERGOPROJEKT Praha a.s.Czech Republic

Prague, October 199730?

Abstract

In this contribution, an overview of seismic design proceduresused for reassessment of seismic safety of civil structures at theMochovce NPP in Slovak Republic is presented. As an introduction,the objectives, history, and current status of seismic design ofthe NPP have been explained. General philosophy of design methods,seismic classification of buildings, seismic data, calculationmethods, assumptions on structural behavior under seismic loadingand reliability assessment were described in detail in thesubsequent section. Examples of calculation models used fordynamic calculations of seismic response are given in the lastsection.

1. Objectives, History and Current Status of the SeismicEvaluation of the Mochovce NPP

1.1 Objectives of the Assessment of Civil StructuresAs postulated by Slovak authorities, the main objective of the seismic reevaluation of the

Mochovce NPP is to enhance the seismic safety of the plant to the level generally accepted for theinternational community and in compliance with the valid standards and recognized practice. Seismicsafety of civil structures should be understood in the context of the safety of the plant as a whole,which in principle depends on reliable function of the systems important from the viewpoint ofnuclear safety.

The reassessment procedure involves the following three components:

• evaluation of seismic hazard for plant site as an external event, specific to the site seismo-tectonicconditions,

• assessment of plant specific seismic capacity to withstand seismic loads generated by such event,

• upgrading of civil structures and components, if necessary.

1.2 Original Seismic Design of the Mochovce NPP

1.2.1 Site selection

The site for Mochovce 4 units of 440 MW with W E R 213 type reactors was selected on thebasis of a preliminary geological investigation report, released by IGP Zilina in 1974, and a meetingwith Soviet specialists held in June 1978 in Moscow at the Ministry of Energy, on which the issue ofsite selection for the nuclear power plant in Slovakia was discussed.

Supplementary discussion on adequate requirements in terms of seismic safety of the plant tookplace at the 46-th meeting of the board of Czechoslovak Atomic Energy Committee (CSKAE) inMarch 1979.

The site of Mochovce NPP was first investigated in accordance with the contemporary Sovietstandards VSN-15-78 and SNIP-2-7-81 for siting and design of NPPs in seismic regions.

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1.2.2 Original Seismic Input

The following levels of seismic input for seismic design of the category I buildings were acceptedat the early stage of design work:

- Design Earthquake DE (corresponds to SL-1 earthquake) with the intensity of 5 according toMSK-64 scale,

- Maximum Calculation Earthquake MCE (corresponds to SL-2 earthquake or SSE) with theintensity of 6° according to MSK-64 scale, for which the value of peak ground acceleration of0.05 g (50 cm/s2) was defined. Acceleration history of 1977 Vrancea (Romania) earthquake,scaled to 0.05 g and its response spectrum was accepted as seismic input. In following years, thevalue of peak ground acceleration was increased up to 0.06 g on the basis of contemporarydesign practice of the Soviet party.

In 1992, Mochovce NPP made an order for revaluation of the tectonic and seismic hazard at theplant site to be done by ENERGOPROJEKT Praha. The conclusion reports confirmed the originalseismic input, however, it was suggested to adopt the minimum level of 0.1 g as a peak groundacceleration, as recommended by IAEANUSS Safety Guide 50-SG-S1 ([2]).

1.3 Reassessment of Seismic Hazard for the site

Reevaluation of the seismic hazard specific to the seismo-tectonic conditions at the site, requestedby Mochovce NPP from the Slovak Academy of Science, was based on:

- IAEANUSS 50-SG-S1 ([2]) and S8,

- American standards, such as US NRC-RG 1.60 or ASCE-486 ([6])

Since evaluation of structures and components if still being finalized, IAEA is providing technicalassistance to Slovak regulatory authorities for reviewing the work results.

Regarding the fact, that construction of Mochovce NPP was finished before the reevaluationstarted, different methods of the reliability assessment than those used for newly constructed plant scould be adopted on the basis of the recommendation of IAEA materials [1] and [4]. According tothis philosophy, explanation of which is given at sec. 2.1, the seismic input is defined as ,,ReviewLevel Earthquake" (RLE). The recommended minimum level of peak ground acceleration has beendetermined as 0.1 g (1 m/s2), which corresponds with the peak acceleration of seismic event withprobability of occurrence of 10"4 (i.e. mean return period of such an event is 104 years) at the siteaccording to recent studies. The level of RLE has been chosen the same as that of SL-2 (SSE)earthquake. Technical detailed information on seismic input data are given at sec. 2.3.

Energoprojekl Praha a.s. *> 1 1 PaSe: 2'22

2. Information on the Seismic Reevaluation

2.1 General Principles of Seismic Reevaluation

General principals of seismic evaluation have been postulated in guidelines Requirements on Re-assessment of Seismic Capacity of Structures and Components of Mochovce NPP" (see [5]).

In the guidelines mentioned above, the following topics are covered:

• explanation of reassessment concept, definitions

• determination of seismic input data (RLE)

• seismic classification of structures, systems and components

• documentation to be used for preparation of calculation models

• requirements on dynamic calculations, basic assumptions on structural behavior and relevantdesign codes

• requirements on upgrading procedures for structures and components

• quality assurance requirements

As mentioned at sec. 1.3, since construction of the plant was finished before seismic reevaluationstarted, different approach than that for a newly constructed plant could be adopted. For seismicreevaluation of structures, systems and components, the Seismic Margin Assessment method (SMA),modified to NPPs with W E R reactor types has been used, as recommended by IAEA documents[1] and [4].

The main difference between the SMA method and conventional design methods is that ratherthan fulfillment of conventional design criteria, the actual margin of seismic safety of structures isevaluated. The reason for this is the fact, that a considerable level of conservatism in conventionaldesign criteria used for newly constructed plants, could hardly be attained and for an existing NPP.In the SMA method, the seismic capacity of the structure, usually in terms HCLPF (High Confidenceof Low Probability of Failure) values of the peak ground acceleration in g's is estimated. This appliesalso to evaluation of civil structures, that has been done using the CDFM method, as explained indetail at sec 2.4.4.

2.2 S e i s m i c C l a s s i f i c a t i o n

The plant civil structures were divided into two seismic categories as described at sec. 2.2.1 and2.2.2.

2.2.1 Definitions of Seismic Categories

2.2.1.1 Seismic Category I

Items within this category were the structures housing important systems and components, forwhich the following safety function is requested:

• safe shutdown of the reactor and maintenance of the reactor and primary loop system in safeshutdown conditions for a specified time,

Energoprojekt Praha a.s. -3 1 0 P a 8 e : 3 ' 2 2

• protection of surrounding environment from accidents involving an impermissible leakage ofradioactive materials.

Seismic Category I structures were subdivided into three sub-categories in dependence on theirsafety function, as described at Tab. 2.2-1.

Tab. 2.2-1: Seismic Category I subdivisions

sub-category

I A

I B

I C

items and definition of their safety function

plant civil structures housing systems or components that must perform their safetyfunction during or after the seismic event

plant structures that must retain their structural or leak-tight integrity after thespecified earthquake

plant structures that must maintain stability after the specified seismic event in ordernot to cause failure of important structures, systems or components included at sub-categories 1A and IB

It should be noted, that the designations I A, I B or I C may vary within the same SeismicCategory I building, depending on the safety function required at different parts or levels.

Furthermore, structures designed to support or connect Seismic Category I structures, systems orcomponents, were also included in this category, however, only I C designation for these structureshas been prescribed, as shown at Tab. 2.2-1.

2.2.1.2 Seismic Category II

Items included in this category were those structures, that were not included in Seismic CategoryI. In order to take into consideration possible seismic interaction between Seismic Category Istructures, the Seismic Category II was subdivided into two sub-categories, as shown at Tab. 2.2-2.

Tab. 2.2-2: Seismic Category II subdivisions

sub-category

nA

nB

items and definition of their safety function

Seismic Category II structures, that might damage or cause loss of function of aSeismic Category I structures, systems or components. The seismic event couldcause this to happen in a number of ways, such as falling impact due to proximity,spray or flooding due to release of liquids resulting from failure of a component,pounding effect due to vibrations induced by seismic event at the interface of twoadjacent buildings, etc.

All other structures, failure of which will have no effect on safety of SeismicCategory I items, i.e. no seismic design is required

2.2.2 Buildings and S t ruc tu res

The buildings and civil structures were classified in two categories as specified at 2.2.1. In Tab.2.2-3 and Tab. 2.2-4, buildings of Seismic Category I and II A are listed, respectively.

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313Page: 4/22

Tab. 2.2-3: Buildings included in Seismic Category I

building no.

800

801

805

806

810

350,352

372

401

442

568

580

584

656, 780

purpose

Reactor building

Auxiliary building (radioactive waste management and storage)

Longitudinal intermediate building

Transversal intermediate building (for electrical equipment)

Emergency feedwater tanks

Ducts for power supply cables

Ducts for safety water piping

Ducts for piping (technical service water)

Diesel generator building

Fuel storage facility for Dieselgenerator building

Ventilation cooling towers

Pump station for technical service water

Fire protection center (Firehouse - on Investor's demand)

Tab. 2.2-4: Buildings included in Seismic Category IIA

building no.

490

581

803

840

purpose

Turbine hall

Natural draught cooling towers

Ventilation chimney

Service building

2.3 Seismic Input

In accordance with IAEA safety guides 50-SG-S1 ([2]), 50-SG-D15 ([3]) and other IAEAmaterials, the Review Level Earthquake is defined as follows:

- peak ground acceleration values are 0.1 g (i.e. 1 m/s2) for horizontal and 0.067 g (i.e. 0.67 m/s2)for vertical component of seismic ground motion in the free field - this level corresponds toIAEA SL-2 (SSE) earthquake and 7° MSK-64,

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- elastic response spectra of absolute acceleration according to NUREG/CR-0098 for free fieldconditions, scaled to acceleration levels of zero period acceleration (ZPA) as mentioned above,defined for non-exceeding probability of 84 % and for rock or soil site conditions.

The method of scaling normalized response spectra has been found more appropriate thandetermination of input response spectra on the basis of natural earthquake records, since insufficientnumber of acceleration records from the region were available due to low seismic activity.

Since seismic input in terms of acceleration time history was needed for certain types of dynamiccalculations, artificial acceleration records have been generated. These artificial time histories withoverall time duration of 25 s were constructed under the assumption of the duration of the initialphase (built-up to quasi-steady state) of 5 s, the quasi-steady state of 15 s and the decay portion of 5s. The accelerograms are displayed at Fig. 2.3-3, Fig. 2.3-4 and Fig. 2.3-5. Artificial accelerationhistories (two for horizontal and one for vertical direction) have been generated in such a way, thattheir elastic response spectra at each frequency point are equal or higher than correspondingcomponent of NUREG/CR-0098 spectrum, taken as input response spectrum (see definition of RLEabove). The comparison between the spectra of the artificial time histories with input NUREG/CR-0098 spectra is shown at Fig. 2.3-6 and Fig. 2.3-7.

Components of seismic input ground motion are considered as simultaneous, i.e. in case ofmethods of dynamic response analysis for which response spectra are needed, both (horizontal andvertical) components have been applied in the same computation run. Similarly, when time historyresponse analysis was carried out, all three components were applied simultaneously. Therefore, twocomputation runs were needed in such cases, because both horizontal components might act in eachglobal horizontal principal direction. Correlation coefficients, describing the degree of statisticalindependence between two random processes, calculated for each combination of accelerationcomponents associated with corresponding principal direction to be applied in response analysis assimultaneous is smaller than 0.1 in all cases, which indicates that time histories are sufficientlyindependent of each other.

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315

Response spectrum NUREG/CR-0098 - component horizontal, rock

4.50-

4.00-

3.50-

3.00-

r 2.50-

^ 2.00-

1.50-

1.00-

0.50-

n rr\ -

c

1

::£IT: \

ill

T"" J

) E

_

5 10

i

ii

i

v •

_ L _1

1

_ L11|

15

F[Hz]

j

ii

j1i1

20

IBS

25

11I

" ~ rii

ii

ii

ii

ii

i

iiL11

30

percentof criticaldamping

— 1

3

4

5•j

10

35

Fig. 2.3-1: Input response spectra - horizontal component

Response spectrum NUREG/CR-0098 - component vertical, rock

2.50 -

2 00 -

l.oO-

1.00-

0.50-

0 00-

i

I \ \ii ' ^ill ' ^^

•

iir

L

-

L _ _ .

L _ -

»*-

' 1 1 110 15 20

F[Hz]

25 30

percentof criticaldamping

——— 1

2

3

4

5

7

10

35

Fig. 2.3-2: Input response spectra - vertical component

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Accelerogram EMO_HOR1.ROC

t[s]

Fig. 2.3-3: Artificial time history EMO JiORLROC

Accelerogram EMO_HOR2.ROC

Fig. 2.3-4: Artificial time history EMO_HOR2.ROC

Energoprojekt Praha a.s. 317 Page: 8/22

Accelerogram EMO_VERT.ROC

0.80

t[s]

Fig. 2.3-5: Artificial time history EMO_VERT.ROC

Mochovce NPP : response spectra for artificial time histories compared to input spectrumNUREG/CR-0098 - component horizontal, damping 5 %

3-r

2.5 -

2 - -

-NUREG/CR-0098

EM0_H0R1

EMO_HOR2

10 100

F[Hz]

Fig. 2.3-6: Response spectra of the artificial time histories - component horizontal

318

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Mochovce NPP : response spectra for artificial time histories compared to Input spectrumNUREG/CR-0098 - component vertical, clamping 5 %

NUREG/CR-0098

EMO VERT

100F[Hz]

Fig. 2.3-7: Response spectra of the artificial time histories - component vertical

2.4 Evaluation of Civil StructuresReliability of structural elements or equipment from the viewpoint of their strength or function

could be seriously affected by deformations, vibrations or forces resulting from dynamic response ofmechanical systems, such as civil structures, equipment supports or components. Therefore, dynamicresponse of these elements, subjected to various seismic effects was evaluated.

2.4.1 Assumptions on Behavior of Mechanical Systems

The assumptions on behavior of various mechanical systems, described by guidelines [5] andrecommended by ASCE-486 (see [6]), cover the following:

2.4.1.1 Modeling of Mechanical Systems

Modeling of mechanical systems were done using finite element models or simplified lumped massmodels. Calculation models of important civil structures, such as reactor building, have been mostlycreated using finite element method (FEM), in which plates, beams or trusses were used. Mass ofstructure and service loading was mostly introduced by values of mass density specified for materials.More significant masses, representing heavy components were modeled by lumped masses positionedapproximately at the center of gravity of such an element.

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2.4.1.2 Material Characteristics

Assumptions on material characteristics used for calculations, esp. stiffness characteristics,damping coefficients, mass densities, etc. are recommended by guidelines [5].

2.4.1.3 Soil-Structure Interaction

Regarding the fact, that most of the buildings, classified as Seismic Category I structures arefounded on solid bedrock, these effects need not to be considered, since shear wave velocities of thefoundation are mostly greater than 1100 m/s. This is consistent with the definition of rock foundationgiven at [6]. However, in some cases soil-structure interaction effects were taken into consideration,usually by means of introducing springs between structural models and its foundation (springconstants were evaluated with the aid of Table 3300-2 at [6]). This applies mostly to smallerstructures founded on made-up grounds or basins. Newmark spectra (NUREG/CR-0098) for a soilsite, recalculated to a surface layer using 1-D deconvolution theory, have been used in such cases.

2.4.1.4 Methods of Dynamic Response Analysis

At principle, the following methods of analysis were considered as acceptable for dynamiccalculations, assuming elastic behavior of structures subject to ground motion:

• modal transient time history analysis,

• direct transient time history analysis,

• response (shock) spectrum analysis,

• equivalent static analysis (this applies only to simple mechanical systems, which could be modeledas a one-degree-of-freedom system).

In most instances, modal transient time history calculations of Seismic Category I buildings havebeen carried out.

Requirements on certain calculation parameters, such as time step size for transient analysis, orrules for summing of modal and directional responses at response spectrum calculation were alsoprovided by guidelines [5].

2.4.1.5 Ductility Effects

Values of ductility coefficients to be used for adjustment of calculated response values (see sec.2.4.4 for usage of ductility coefficients) for various types of structures and components wereprovided by guidelines [5] to consider nonlinear effects and ability of certain types of structuraldetails to adsorb energy. It should be noted, that the values of ductility coefficients are considerablysmaller than those for newly constructed plants, since most of the details in NPPs with W E R typesof reactors were constructed as less ductile than for new plants.

2.4.2 Input data used for dynamic calculations

Depending on the mechanical system under investigation, the following input data were used fordynamic response analysis:

• Review Level Earthquake (RLE), as described at sec. 2.3 - these data were taken as input forresponse analysis of civil structures, assuming that the building is founded on rock. Input data,

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320

used for coupled soil-structure models, when soil-structure interaction effects had to beconsidered, were modified as described at sec. 2.4.1.3.

• Floor response spectra at the corresponding level (floor) were used as input for dynamic responseanalysis of built-in structures, such as equipment support systems.

2.4.3 Floor Response Spectra Calculations

Since seismic response of buildings classified as Seismic Category I has been investigated usingcomplex three-dimensional finite elements models, floor response spectra have been calculated at thelevel of each floor, where important systems and components are located.

Since behavior of buildings classified as Seismic Category I has been investigated using complexthree-dimensional finite elements models, floor response spectra have been calculated at the level ofeach floor, where important systems and components are located.

Frequency sampling of calculated spectra has been chosen similar, as recommended at Table2300-1 at [6], The minimum frequency for which spectra were calculated was 0.2 Hz, the highest 33Hz. The spectra have been calculated for the following values of damping ratio (i.e. percent ofcritical damping /100): 0.005, 0.01, 0.02, 0.03, 0.04, 0.04, 0.07 and 0.10.

For each floor, up to 20 nodal points were selected as representative points, i.e. nodes for whichacceleration time histories were recovered. Since two computation runs for a time history analysiswere needed (as mentioned at sec. 2.3), two acceleration histories for each representative nodal pointwere obtained. Subsequently, response spectra of each time history were computed for allrepresentative nodal points at a particular level and for each global direction (X, Y - horizontal, Z -vertical). Values of resulting response spectra for given global direction at each frequency point,calculated for various values of damping ratios, were obtained using all response spectra calculatedat the specified floor for the given direction as median plus one standard deviation, using log-normaldistribution for 84 % non-exceeding probability.

2.4.4 Reliability Assessment

Reliability of civil structures subjected to seismic loading, has been evaluated in the followingsteps:

1) Calculation of dynamic response in structural elements, esp. in those important from theviewpoint of the reliability criteria, defined by the seismic category, in which the building underinvestigation has been included,

2) Evaluation of design combinations involving seismic loading, as defined at sec. 2.4.4.1

3) Assessment of structural elements by CDFM method, as explained at sec. 2.4.4.2

2.4.4.1 Design Combinations

Combinations to be used for reliability evaluation of civil structures under the conditions ofReview Level Earthquake were considered on the basis of IAEA NUSS standards and IAEAdocuments [1] and [4]. RLE responses were superimposed to other loads, which are assumed to bepresent at the time of seismic event.

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2.4,4.2 Assessment of Structural Elements

Reliable function of a structural member in conditioned by fulfillment of design criteria, given bydesign standards for civil structures, or by special service criteria, defining for instance requirementson occurrence of cracking, crack width limits or maximum deformations or deflections, velocities oraccelerations at certain locations. Design criteria in the essence of the Ultimate Limit Statephilosophy, provided by Czech, Slovak or European Codes, define the capacity of structuralelements and joints in terms of their ultimate strength (expressed usually by allowable stress levels ormaximum internal forces) or limits of deformations or/and cracking, but the criteria for theServiceability Limit States, provided by those codes are for conventional buildings only. Specialservice criteria have been considered only in some instances of certain Seismic Category I buildings,when excessive values of deformations or accelerations might lead to malfunction of SeismicCategory I equipment, or when leak-tightness of the structure must be ensured. Since the reliabilityof components, installed at Seismic Category I buildings depends on peak accelerations, floorresponse spectra of absolute accelerations at various levels of Seismic Category IA buildings werecalculated, as described at sec. 2.4.3.

Reliability of structural elements from the viewpoint of the quantity under consideration, were inthe essence of SMA methodology (see sec. 2.1) assessed as a HCLPF value, i.e. member's boundingseismic capacity. The CDFM (Conservative Deterministic Failure Margin) method for evaluation ofHCLPF values of bounding seismic capacity of structural elements in terms of their mechanicalintegrity (strength) has been used, as explained in detail at [1].

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3. Examples of a Structural Model used for Evaluation ofSeismic Response and Floor Response SpectraCalculations

3.1 Reactor Building and Intermediate Buildings

The model shown at Fig. 3.1-1, Fig. 3.1-2 and Fig. 3.1-3 has been used for dynamic analysis ofReactor building (no. 800) and Intermediate buildings (no. 805, 806). It was essential to build acomplex FEM model, since structures of buildings no. 800, 805, 806 and 490 are interconnected.Only one half of the structure, associated with Unit 1 has been modeled, since both units 1 and 2 aresymmetrical and separated by a dilation gap.

This model was used for structural reliability checking of steel structures at Intermediate buildings(no. 805, 806), concrete structures of Reactor Buildings (no. 800) and for floor response spectracalculations.

Reinforced concrete (RC) structures were modeled using shell elements, for steel members(columns, girders, frames, bracing) beam and truss elements were used. Translational degrees offreedom were constrained for all nodes at the level of the foundation mat, i.e. soil-structureinteraction effect was neglected, because all buildings are founded on solid bedrock. Young'smodulus for concrete was 26.5 x 103 MPa, for structural steel 210 x 103 MPa. Model's parametersare given at Tab. 3.1-1.

Tab. 3.1-1: Parameters of the FEM model of buildings no. 800, 805, 806 and 490

number of nodal points

number of elements (including lumped masses)

number of materials

number of unconstrained degrees of freedom

5560

10541

56

28977

In terms of structural dynamics the reinforced concrete structure of the Reactor Building (No.800) has been found very stiff, as indicated by floor response spectra shapes at the floors of ReactorBuilding, which are close to the spectra of input ground motion histories. However, response of steelstructures, esp. those of Intermediate buildings (no. 805 and 806) is significant and detailedevaluation of steel columns, beams and their joints was carried out. The analysis results have shownthat there is a need for an upgrading of the connections between steel structures of Intermediatebuildings (no. 805 and 806) and adjoining peripheral RC walls of the Reactor building, since thoseconnections were not capable to carry the horizontal forces due to seismic event (RLE).Furthermore, detailed walkdowns indicated that it is also essential to strengthen some partitions inthe Intermediate buildings, failure of which could cause seismic interactions with important SeismicCategory I equipment.

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Fig. 3.1-1: Common FEM model for dynamic analysis of Reactor Building (no. 800), LongitudinalIntermediate Building (no. 805), Intermediate Transversal Building (no. 806) and Machine(Turbine) Hall (no. 490) - overall view

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\ \

Ml

Vj

\

\\

\

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X

Fig. 3.1-2: FEM model for dynamic analysis of buildings no. 800, 805, 806 and 490 -part of themodel detached by section parallel to global Y axis (from the left: Turbine hall, LongitudinalIntermediate building. Reactor htilding, bubbler condenser tower)

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Fig. 3.1-3: FEM model for dynamic analysis of buildings no. 800, 805, 806 and 490 - part of themodel detached by section across parallel to the global X axis (from the left: Reactor building,Transversal Intermediate building; at the rear bubbler condenser tower)

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3.2 Diesel Generator Building

Diesel generator building is one of the most important Seismic Category I structures, since itsdiesel generators render emergency power supply, needed to ensure safe function of the systemsimportant from the viewpoint of reactor shutdown and emergency cooling.

There are two diesel generator buildings associated with the Unit 1 and Unit 2, respectively.Therefore, only one calculation model has been created, as shown at Fig. 3.2-1. The undergroundstructure is made up of concrete - the diesel generators are mounted on solid concrete blocks belowthe ground level. The upper structure above ground has been designed as a steel skeleton withtransverse and longitudinal bracing. The building consists of three sections divided by transversalmasonry walls 0.25 m thick, which are built-in between steel columns. The masonry contributes tothe horizontal stiffness and this effect has been taken into account in the calculation model, intowhich shell elements with reasonable in-plane stiffness, overlapping steel columns and bracing wereincorporated.

The summary of model parameters is given at Tab. 3.2-1. The time histories obtained at selectednodes by dynamic calculations of seismic response have been used for floor response spectracalculations.

Tab. 3.2-1: Parameters of the calculation model for the Diesel Generator Building

number of nodes

number of elements (including lumped masses)

number of materials

number of unconstrained degrees of freedom

1001

2490

20

4491

The analysis have shown that the building will resist to seismic-induced forces safely and that peakacceleration values will not endanger safe function of the electrical equipment located in the building.Concrete blocks of diesel generators are located directly on the ground (rock) and are separatedfrom the surrounding structures.

The evaluation of seismic interactions of partitions with safety-important equipment according tothe SMA procedure is now being completed.

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Fig. 3.2-1: Diesel Generator building no. 442 - overall view with the roof and cladding removed

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3.3 Ventilation Cooling Towers

Another example of a seismic Category I structure, analyzed within the framework of seismicreevaluation of the Mochovce NPP, are Ventilation Cooling Towers (building no. 580). The facilityis one of the most important elements of the emergency cooling system, used for residual heatdisposal.

Under the conditions of the RLE, structural integrity as well as flawless function of ventilators isrequired. In order to check the reliability of ventilators, floor response spectra at correspondinglevels were calculated.

As depicted at Fig. 3.3-1, the resistance to horizontal forces acting in the transverse direction isprovided by RC walls, while in the longitudinal direction there are peripheral walls spanning acrossthe transversal walls, which together with the roof slab make up a rigid box-like structure. Therefore,amplifications of the acceleration response at the upper level are acceptable. RC walls and beamswere checked and forces due to earthquake have been found insignificant. Model parameters aresummarized at Tab. 3.3-1.

Tab. 3.3-1: Parameters of the Ventilation Cooling Towers Model

number of nodes

number of elements (including lumped masses)

number of materials

number of unconstrained degrees of freedom

1842

3194

5

10152

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Fig. 3.3-1: Ventilation Cooling Towers - overall view

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References[1] Technical Guidelines for the Re-Evaluation Programme of Mochovce NPP (Units 1-4). IAEA,

Vienna, August 1995.

[2] IAEA Safety Series 50-SG-S1 Earthquake and associated topics in Relation to Nuclear PowerPlant Siting. IAEA, Vienna, 1991.

[3] IAEA Safety Series 50-SG-D15 Seismic Design and Qualification for Nuclear Power Plants.IAEA, Vienna, 1992.

[4] Criteria for Seismic Evaluation and Potential Design Fixes for W E R Type Nuclear PowerPlants. Prepared for IAEA by Stevenson and Associates, Cleveland, 1994.

[5] Requirements on Re-assessment of Seismic Capacity of Structures and Components atMochovce NPP. Prepared by Stevenson and Associates for the Nuclear Regulatory Authorityof Slovak Republic, Rev. 3, 06/1996.

[6] ASCE 4-86 Seismic Analysis of Safety-Related Nuclear Structures and Commentary onStandard for Seismic Analysis of Safety-Related Nuclear Structures. American Society of CivilEngineers, 1987.

Institutions and engineers participating at seismicreevaluation of the Mochovce NPP, units 1,2Chief Designer: ENERGOPROJEKT Praha - Civil Engineering Division

I. Holub, Ing. - Head of Civil Engineering Division

J. Zitova, Ing. - NPP Mochovce Coordinator

J. Michalek, Ing. - Project Manager

J. Maly, Ing. - Structural Mechanics Specialist

M. Lukavec, Ing., V. Lerl, Ing. - Structural Engineers

Subcontractors:

David Consulting - M. David, Ing.

Energopruzkum Praha, ltd. - P. Simunek, RNDr.

Czech Technical University of Prague, Faculty of Civil Engineering - F. Cihak, Ing., CSc

Technical University of Bratislava - J. Kralik, Ing., CSc

Energoprojekt Praha as. 3 3 2 **age: