Comparison of the Methods of Seismic Analysis Applicable to Fast Reactors in the EEC Countries

8/9/2019 Comparison of the Methods of Seismic Analysis Applicable to Fast Reactors in the EEC Countries

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Commiss ion of the European Communi t ies

n u c l e a r s c i e n c e a n d t e c h n o l o g y

C O M P A R I S O N O F T H E M E T H O D S

O F S E I S M I C A N A L Y S I S

A P P L I C A B L E T O F A S T R E A C T O R S

IN T H E E E C C O U N T R I E S

R e p o r t

EUR 10586 EN

Blow-up f rom mic ro f iche or ig ina l


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Commission of the European Communities

n u c l e r s c i e n c e n d t e c h n o l o g y

COMP RISON OF THE METHODS

OF SEISMIC N LYSIS

PPLIC BLE TO F ST RE CTORS

IN THE EEC COUNTRIES

M. DEFALQUE, P. KUNSCH, A. PREUMONT

BELGON U C LEAIR E

Place du Champs de Mars, 25

- 1050 Bruxelles

Contract No. RAP-020.B.

FINAL REPORT

This work was performed under the aegis of the

Commiss ion of the European Communit ies

fo r the : WORKING GROUP CODES AND STANDARDS

Activ i ty Group 2 Structural Analys is

w i th in the FAST REACTOR COORDINATING COMMITTEE

Directorate-General Sc ience, Research and Development

1986 EUR 10586 EN


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P u b li s h e d b y th e

C O M M I S S I O N O F T H E E U R O PE A N C O M M U N I T I E S

D i r e c t o r a t e - G e n e r a l

T e l e c o m m u n i c a t i o n In f o r m a t i o n In d u s t r ie s a n d I n n o v a t io n

B t i m e n t J e a n M o n n e t

L U X E M B O U R G

L E G A L N O T I C E

Neither the Com miss ion of the European Com mun it ies nor any person act ing on behal f

of the Commiss ion is responsib le for the use which might be made of the fo l lowing

information

ECSC EEC EAEC Brussels-Luxe mbo urg 1986


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III

Resum

COMPARAISON DES METHODES D'ANALYSE SISMIQUE APPLI

CABLES AUX REACTEURS RAPIDES DANS LES PAYS DE LA CCE.

Les pays de la Communaut concerns sont ceux qui parti

cipent actu ellem ent l'exploitation ou la mise au point

des racteurs rapides savo ir:

- FRANCE (F) : Phnix - Supe rph nix

- RFA - BELGIQUE - PAYS BAS associs au sein du

DeBe Ne : SNR - 300

- Le ROYAU ME UNI (UK) : PFR-CDFR

- I TALI E (I) : PEC

Le premier object if de cette tude est de mettre en vi

dence les points communs et les divergences existant entre

les rgles nationales pour l'analyse sismique de Racteurs

Neutrons Rapides

(RNA).

Ces diffrences peuvent survenir diffrentes tapes de la

concep tion sav oir : dans la dfin ition s des donnes sismi-

ques d'entre, dans le choix des limites admissibles et dans

le conservatisme associ aux mthodes de calculs.

Pour chacunes de ces trois tapes, il convient d'identifier

les points pouvant influen cer les rsultats de l'analyse et

par consquent la marge de scurit globale vis--vis de

l'vnement concern.

Summary

COMPARISON OF THE METHODS OF SEISMIC ANALYSIS APPLI

CABLE TO FAST REACTORS IN THE EEC COUNTRIES.

The countries in the Community which are concerned by this

study are those currently involved in the operation or deve

lopment of fast reactors, namely:

- FRANCE (F) : Phnix - Sup erph nix

- FRG - BELGI UM - THE NETH ERLA NDS associated within

DeBeNe : SNR - 300

- UNITED KI NGDOM (UK) : PFR-CDFR

- I TALY (I ): PEC

The first aim of the study is to enumerate the common points

and differences in the national rules and regulations for

the seismic analysis of fast breeder reactors (FBR).

Such divergences may be encountered at different design

stages, namely: in the definition of the seismic input data,

in the choice of design limits and in the degree of conser

vatism applied to the calculation methods employed.

For every one of these three stages, it is necessary to

identify the points likely to influence the results of the

analysis and consequently the over-all safety margin with

regard to the event concerned.


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TABLE OF CONTENTS

I. Introduction. j

1. Subject of study . j

2.

Framework of the study. ]

3. Methodology. 2

4.

Execu tion of the study. 5

II.Seismic analysis in EEC .

0. Preliminary remark.

1. Reference ground motion. 7

2.

Seismic classification of components - Safety prescriptions - Design

criteria. g

3. Methods for analysis of seismic systems and subsyst ems.

\

III.Synthesis of national answe rs. 25

25

26

0. Introduction.

1. Ground motio n.

2.

Seismic classification of components - Safety prescriptions - Dimensional

criteria. 29

3. Seismic analysis meth ods. 49

IV.

Prospects and further developments . 55

1. Part common to all types of react ors. 55

2.Fast reactors characteris tics. 56

Bibliography. eg


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I. INTRODUCTION

1.1. Subject of study

The Commission of the European Communities has awarded BELGONUCLEAIRE a

study contract (No RAP-020-B) entitled: Comparison of the methods of seismic

analysis applicable to the fast reactor components in the EEC countries.

This study is being monitored by activity group AG2 of the working

group Codes and standards (WGCS) which itself is under the aegis of the Fast

Reactor Coordinating Committee.

The countries of the Community which are concerned by this study are

those currently involved in the operation or development of fast reactors, name

ly:

- FRANCE (F ): Phnix - Superphnix

- FRG - BELGIUM - THE NETHERLANDS associated within DeBeNe: SNR - 300

- UNITED KI NGDOM (UK ): PFR-CDFR

- ITALY (I ): PEC

The first aim of the study is to enumerate the common points and diffe

rences in the national rules and regulations for the seismic analysis of

fast

breeder reactors (FBR).

Such divergences may be encountered at different design stages,

namely:

in the definition of the seismic input data, in the choice of design limits

and

in the degree of conservatism applied to the calculation methods employed.

For every one of these three stages, it is necessary to identify the

points likely to influence the results of the analysis and consequently the over

all safety margin with regard to the event concerned.

1.2. Framework of the study

Since fast breeder reactors are still in the development stage and,

except for France, far from the stage of commercial operation, practices and

regulations are still changing and are mainly based on practices for light water

reactors and, in particular, on American rules and regulations such as Regula

tory Guides (RG) , Standard Review Plan (SRP) , ASME Code Section III and its

Code Cases .


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A fruitful comparison of aspects not yet dealt with in that type of

document is not possible at the present time.

Hence we feel it would be desirable to limit the study to the following

aspects:

(1) ground motion;

(2) classification of components;

(3) methods of analysis.

With regard to point (2), the present study will be limited to mechani

cal components; experimental methods will be excluded from point (3).

This approach deliberately does not take into account certain fundamen

tal aspects which are specific to fast breeder reactors and result from their

operating conditions:

- large masses of liquid sodium, especially in the pool concept;

- low pressures entailing thin walls;

- high temperatures and irradiations entailing problems of material behaviour;

- severe thermal gradients and temperature fluctuations.

Problems arising as a result of these conditions will include the

fol

lowing:

(1) fluid/structure interactions;

(2) instabilities (elastic or plastic

buckling);

(3) creep and plasticity problems.

At the moment, these would seem to belong more to the field of research

than to that of established practices.

1.3. Methodology

In order to specify the different factors which influence the result of

an overall seismic analysis and the associated safety margin, each of the three

aspects that we have identified in 1.2. has been included in a questionnaire (see

para.

I I ).


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(1) Ground_mot ion

This questionnaire aims at comparing the definitions of the two refe

rence earthquakes, the associated probabilities, the corresponding ground accele

rations and response spectra.

(2) Safe_ty_provisions_ -_Classj^fica_tion of_cmonent

s

- Dein_c,iteri

The principle of the classification procedure has been described in a

working document prepared by activity group No 4 (WGCS-AG4) under the title So

dium cooled fast reactors - Classification of the mechanical systems and compo

nents .

The questionnaire proposed here aims at applying that procedure to the

specific framework of earthquakes, considering the following steps :

A. definition of functional requirements for the reference earthquakes;

B. classification of reference earthquakes in relation to the various categories

of operating conditions (normal, upset,

. . . ;

combination with other types of loads, and

definition of the resulting categories of operating conditions;

C. criteria allowing the classification of components into safety classes (e.f.

RG 1.16) and seismic classes (e.g. RG

1.29).

Starting from these classes, the

component function, the consequences of its failure and the normal loading

conditions, definition of its quality level and the corresponding ASME code

subsection;

D.specification of the design rules for mechanical components based on:

1. quality level;

2.functional requirements.

These steps are described in the table given below.

A double entry table is appended to the questionnaire; this enables a

definition to be made for each mechanical component, of its safety class and the

operating condition category corresponding to the reference earthquakes. This

table must be adapted to meet national technologies, in particular for pool and

loop concepts.


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DFINITION

A N D

CLASSIFICATION

; O F

EARTHQUAKE-

:

INDUCED S ITUATIONS

k,

k.

r

W^

SELECTIONO F DESIGN

CATEGORYO F

OPERATING

CONDITIONS

NORMAL

UPSET

EMERGENCY

Tr n

FUNCTIONAL

REQUIREMENTS

1

w

SELECTIONOF A

SET

O F

CRITERIA

A^

RULES (SCHEMATIC DI AGRAM)

w

^\C0DE

CRITERIA

LEVELA

LEVEL

LEVELC

LEVELD

SAFETY CLASSIFICATIONO F EQUIPMENTS

SELECTIONO F ADESIGNAN D FABRICATION COD E ^

@

CLASS

1

CLASS

2

\

\

CLASS

3

\

CONTAIN-

MENTS

SUPPORTS

V

A

I

J-

I

1. The rules of corresp ondence between category of operating conditions

and service level take into account

:

- the

type

of

functional r equirement (act ion, leakti ghtness, structur al

integrity)

- the

possibilities

fo r

inspection

a nd

repair (accessible components) .

2.

Example

of

rule

:

qua lit y level (class)

safety class.

For each block, there is a corresponding

set

o f

design

a n d

construction rules.


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(3) Methods^ for

analys

is_of_

seismic systems_ and subsystems

The questionnaire aims at comparing the analytical verification methods

and the associated degrees of conversatism.

It deals with:

1. rules for modal superposition;

2. decoupling criteria for subsystems;

3. determination of floor spectra;

use of artificially generated accelerograms;

4.

acceptability of approximate methods;

5. damping (reference values, composite structures, etc.).

It follows approximately sections 3.7.2. and 3.7.3. of the SRP.


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1.4. Execution of the study

The questionnaire was sent to members of the working group Codes and

Standards , who contacted the relevant bodies in each country. Various meetings

of experts took place.

The reports of these meetings were drafted by BELGONUCLEAIRE represent

atives, then revised and amended by the national experts.

The French position was sent to BELGONUCLEAIRE after an internal mee

ting held in France.

Contacts were established with the following organizations:

France: CEA - EdF - Novatome;

Italy: ANSALDO - ENEA - NIRA;

United Kingdom: CEGB - UKAEA - NNC;

FRG: IA.

Belgium and the Netherlands are associated with the SNR project in the

FRG. Their representatives (Belgium: BELGONUCLEAIRE; the Netherlands: TNO-Nera-

toom) have approved the document issued by the FRG.


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II.

SEISMIC ANALYSIS IN EEC

11.0.

Preliminary remark

As the present comparison must reflect the evolution of regulation with

time, answers to the questionnaire may consider separately several aspects: rules

and regulations applicable to reactors operating or under construction, rules and

regulation applicable in the future. Differences with LWR practices will be

indicated, if any.

11.1.

Reference ground motion

1. Safety levels

1.1. Could you explain the philosophy that led to setting up two safety levels

(OBE and SSE in the US Regulatory Guides terminology)?

1.2. What are the corresponding probabilities of occurrence?

2. Maximum ground acceleration

2.1. USNRC recommends that maximum ground acceleration be at least equal to

0.1 g for SSE and at least half the SSE value for OBE.

Is such a rule also applied in your country?

2.2.

Is maximum acceleration defined on a site dependent basis or is it

considered constant throughout the country? On what basis has its value

been chosen?

3. Response spectra

USNRC has defined standard shapes for horizontal and vertical spectra. They

must be normalized according to the maximum horizontal acceleration.

3.1. Is a similar rule applied in your country?

3.2. Are the design spectra site dependent or not?

3.3. If the design spectra differ from those of RG 1.60, could you make them

available to us?

4.

Duration

4.1. Is there any specification concerning the duration of the two reference

earthquake?


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I I.2.

Seismic classification of components - Safety prescriptions - Design crite

ria

0. DEFINITIONS AND REMARKS

51 level 1 earthquake: OBE, SB, SNA, AEB, TBE;

52 level 2 earthquake: SSE, SM, SMS, SEB, TSS;

Rl last reactor built or already in construction (Superphnix, SNR-300,

... );

R2 reactor* to follow to Rl (reactor in design phase, reactor in construc

tion).

Where applicable, a distinction should be made between criteria defined

for reactors Rl and R2.

Questions are purposedly redundant. They can be answered by referring

to an official document or an appended document and also by referring to an ans

wer given to another question.

*Fast neutron reactor, excluding research reactors.


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A. FUNCTIONAL CRITERIA.

AO - Are there any official documents defining functional requirements in case of

earthquakes? If so, what are these documents?

Al - What are the safety related functional requirements after an SI earthquake?

A2 - What are the safety related functional requirements after an S2 earthquake?

A3 - Which are (therefore) the circuits and systems that shall remain functional

in the case of an S2 earthquake?

A4 - If the concept of containment [or barriers] appears in the safety regula

tions,

which containments should remain tight after S2?

A5 - Is earthquake detection considered in the safety regulations? If so,what

are the prescribed actions and what are the thresholds triggering them?

A6 - What are the functional consequences of earthquakes that must be taken into

account (emergency shut-down, external electricity supply loss, water flow

failure,

leaks, ...)?


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.

CLASSIFICATION OF EARTHQUAKES WITH REGARD TO OPERATING CONDITIONS.

BO - Are there any official documents specifying earthquake classification? If

so,

which are these documents?

Bl - How are operating conditions classified in the safety regulations?

B2 - With which operati ng conditions should an SI earthquake be combined? In

whic h category of operating conditi ons should the so defined combination be

classified?

B3 - With which operating conditions should an S2 earthquake be combined? In

whic h category of operating condi tions should the so defined combination be

classified?

B4 - In particular , should the simultaneous occurrence of earthquake and the

followin g events be considered? If so , how should the combined situation be

classified?

- Normal shut-down

- Emergency shut-down

- Failure in the steam generator

water supply

- Secondary loop failure

- Loss of external power supply

- Normal handling operations

- Exceptional handling operations

CLASSIFICATION OF THE

COMBINED SITUATION

SI S2


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C . C L A S S I F I C A T I O N O F M E C H A N I C A L C O M P O N E N T S

C O - Are t h ere any o f f i c i a l d o c u m e n t s s p e c i f y i n g t h e c l a s s i f i c a t i on

c r i t e r i a

of

com ponents acc ording to their safet y relate d functions? If

0 ,

wha t a re

t h e s e d o c u m en t s ?

C I - Wh ic h are the safety clas ses of c omp onents and whic h ar the

c l a s s i f i c a t i o n

criteria?

C2 - Is there an additional component classification with regard to

e a r t h q u a k e

(" s e i s m i c cl a s s i f i c a t i on" ) ? I f s o:

- wh a t are t h e c l a s s i f i c a t i on cri t eri a ?

- wh a t are t he re l a t i o n s h i p s wi t h t h e g enera l s a f e t y c l a s s i f i c a t i on ( q u e s -

t i on C I ) ?

- wh i c h re l a t i on s h i p s wi t h t h e d e s i gn cri t eri a m u s t b e a p p l i e d t o t h i a c bw -

ponent with regard to earthquakes? 'M.>:


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D. MECHANICAL DESIGN CRITERIA

DO - Are there any official documents specifying the choise of design rules?

Dl - Which design codes* are used? How are the design and fabrication rules

applying to a specific component to be chosen? (example, in relation to the

safety class, the type of component, the temperature,

etc.).

D2 - Are those design criteria classified in a way comparable with the A, B, C

and D levels found in ASME III?

D3 - What is the relationship between the category of operating conditions and

the level of criteria to be associated with

it**

(with regard to equipment

type and functional requirements)?

D4 - On which basis is fatigue damage assessed (number of cycles per earthquake,

number of earthquakes to be considered)? Are the aftershocks taken into

account?

E. MISCELLANEOUS

El - How are the seismic load specifications officially transmitted to the compo

nent manufacturers? (equivalent of ASME Design Specification).

*A code is defined as a complete set of design and fabrication rules such as the

subsections of ASME III and some code cases.

**In the USA, NRC has defined this relationship for light water reactors in Regu

latory Guide 1.48.


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DESIGN CRITERIA APPLICABLE TO THE VARIOUS COMPONENTS

REACTOR :

Component

1. Reactor block

1 - Main tank

2 - Safety tank.

3 - Roof slab

. 4 - Large rotating plug

5 - Small rotating plug

6 - Core cover plug

7 - Control rod mechanism

8 - Core diagrid

9 - Core support plate

10 - Internal structures of

primary circuit

11 - Internal structures for

thermal shielding

12 - Dome

Safety

class of

component

TYPE :

POOL*

Design criteria level

Earthquake

SI

Earthquake

S2

*A loop version is presented in the appropriate national answers


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REACTOR :

TYPE : POOL

Component

Safety

class of

component


Earthquake

SI

Earthquake

S2

2. Heavy components

- Primary pumps

+ rotating parts

+ static parts

> -I HX (intermediate heat

exchangers,

normal and

emergency circuits)

+ exchange tubes

+ secondary sodium pipework

+ protective shell (sup

ports and cover gas ple

num seals)

- Secondary pumps

+ rotating parts

+ static parts

- Steam generators

+ exchange tubes

+ protective shells

- Integrate purification

circuits


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15


REACTOR :

Component

3. Handling

- Fuel transfer machine

- Transfer lock

+ cover-gas plenum seals

+ handling mechanism

+ rotating transfer lock

+ charge/discharge ramps

- Storage drum for new and

irradiated fuel

+ vessel(s)

+ drum

+ cover plug

- Handling flasks

- Secondary handling lines

Safety

class of

component

TYPE : POOL


Earthquake

SI

Earthquake

S2


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REACTOR : TYPE : POOL

Component

Safety

class of

component


Earthquake

SI

Earthquake

S2

4.

Circuits

- Secondary circuits

+ main pipework

+ sodium storage tanks

+ auxiliary circuits

+ double jacket in dome

+ expansion tank

- Decay heat removal circuits

(in reactor and in storage

drum)

+ main pipework

+ pumps

+ sodium/air exchangers

+ auxiliary circuits

- Primary argon gas circuits

+ piping and vapor traps

+ primary storage tanks

+ argon purification

- Storage drum auxiliary

circuits

- Water/steam circuits

+ up to safety valves

+ beyond safety valves


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I I.3.

METHODS FOR ANALYSIS OF SEISMIC SYSTEMS AND SUBSYSTEMS

1. RULES APPLIED IN CONNECTION WITH THE MODAL SUPERPOSITION METHOD

1.1. Combination of modal responses - Closely spaced modes

The most popular rule for the combination of the modal responses is the

so-called square root of the sum of the squares (SRSS). This can be justified

theoretically if it is accepted that the modal components are statistically inde

pendent. For closely spaced modes, the combination rule must be modified in

order to allow for the correlation between the modal components of the response.

A conservative rule generally accepted for these closely spaced 'modes is the rule

of the absolute sum. The following combination rule is proposed by the USNRC

(SRP, Section

3.7.2.):

- N

R =

k=l

R,R

1 m

1/2

(3.1)

where N is the total number of modes and the second sum includes all modes whose

frequencies are within 10% of each other (of the lowest frequency of the

pair).

A similar rule is given in R.G. 1.92.

Q_.l_.l_.

Is this combination rule applicable in your country?

If not, what is the rule used?

1.2. Combination of three spatial components

In order to estimate the maximum response R of the structure subjected

to a three dimensional excitation (2 horizontals + 1 vertical) from the maxima

R, (i = 1, 2, 3) obtained separately for each of the components of the excita

tion,the USNRC (R.G.1.92) recommends the use of the SRSS rule:

A

2 2

+ R^ + R3

(3.2)

(see Chu, Amin & Singh, NED 21 (1972),

126-136).

This approach has been critici

zed as too conservative whenever R^ are obtained by a modal superposition me

thod, because of the statistical independence of the various components of the

seismic accelerogram (C.W. Lin, NED 24 (1973), 239- 241).


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Q_.K2. Is rule (3.2) applicable in your country?

What other rule is applied, if any?

1.3. Significant Modes

Usually, the combination rule for the modal contributions applies to

those modes whose frequencies are below the excitation cut-off frequency (fre

quency at which the acceleration spectrum reaches its asymptote - 33 Hz in the

case of NRC spectra).

This can sometimes cause certain modes of important effective mass to be ignored

and can lead to substantial errors, especially concerning the support reactions

and the stresses. This approach can, however, be improved by introducing a resi

dual mode which takes into account the rigid part of the response (see for exam

ple, G.H. Powell, SMI RT-5, paper K 10/3, 1979). This mode is then combined with

the others by the SRSS rule.

(}.1_.3_.1_.

What rules are applied in your country, concerning the modes to be con

sidered? (Criteria on frequencies? Criteria on effective masses?).

>1_.3_.2^

What procedures allow the high frequency modes to be taken approximately

into account?

2.

DECOUPLING CRITERIA FOR SUB-SYSTEMS

According to Section 3.7.2. of the USNRC's SRP, the decoupling criteria

are based on the mass R^ and frequency Rf ratios:

Total mass of supported subsystem

m Mass which supports the subsystem

Fundamental frequency of subsystem

f Dominant frequency of support motion

The decoupling can be carried out under the conditions:

(1) R

m

< 0.01

(2) 0.01 < R

m

< 0. and R

f

>

1.25 or R

f

< 0.8

(3.3)

(__.2_. Are these criteria applied in your country? If not, what other criteria are

used?


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3. DETERMINATION OF FLOOR SPECTRA - CONSIDERATIONS ON THE USE OF ARTIFICIALLY

GENERATED ACCELEROGRAMS

3.1. Calcula tion methods

Several procedures have been proposed for determining the floor spec-

tra:

- app roxi ma te m ethod of t he Biggs'.type (J. Bigg s, SMIRT-1, pap er K 4/7,

1971).

- time-history analysis.

- pr obabil ist ic methods (Singh & Ang, SMIRT-2, pa per K 6/1, 1973 or Scania n &

Sachs, Keswick 1978, for ex a m p le) .

C__._3_.l_.

Which of these are r egarded a s a ccepta ble in your country?

3.2. Combination rule for non-symmetric structures

For a non-symmetric structure, the motion in each direction will con-

tain a contri but ion from each of the three components of the seism ic excita tion

(2 horizontal + 1

v e r t i c a l ) .

R.G.1.12 2 st ipu lat es that, if the effect of each of

these components is analy sed s epara tely, the correspondin g ordinates of the floor

spectra should be combined acc ording to the SRSS ru le. A three-dimensional an a-

lysis of the structure subjected to a simultaneous excitation in the three direc-

tions will use statis ticall y independent time-histories ( C. Chen, proc. ASCE,

ST2,

pp . 449-551, 1975).

__.3^__.

Is a similar rule applied in your country?

3.3. Number of ti me-histories - Duration

_._3.3_.__.

- Is there a recommendat ion concerning the min imu m num ber of. st at is ti -

cally independent tim e-histories (of a spectru m enveloping the design

spect rum ) to be used for g enerating floor spectra?

0_3_3___ - Is there a rec omm end at ion con cer ni ng the min im um du ra ti on of t he a c c e-

lerograms to be taken into account in a time-history analysis (C.W.

Lin, SMIR-4, paper 1/11, 1977)?

3.4.

Spectrum broadening

In order to take into account the uncertainties in the properties of

the mat erial and in the models (see for examp le, B.J. Benda et a l. , NED 67, pp .

109-123 (1981)), the computed spectra are smoothed and broadened.


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The USNRC imposes the following broadening (R.G.1.122)

A f .

=

J

r

(0.05 f.)

2

+ ) (. )'

n=l

/2

(3.4)

wit h a minimum of 0.1 f.. In this formul a, Lt . is the amount of broadening

to be applied (on both sides of f.) ; A f j

n

represents the variation of the

j-th natural frequency resulting from the uncertainty on the n-th parameter; the

sum extends over all possible parameters affecting the structural response. The

foregoing procedure can be avoided providing a peak broadening of + 0 . 1 5 f .is

applied.

0;.3_.4_. Is a simi la r r ule appl ied in your c ount ry? If no t, wha t is the curr ent

rule?

3.5. Account of uncertai nties in a time-history analysis

In the case of a syst em analysis by the time-history metho d, the SRP,

Section 3.7.2.

(II.9),

rec ommends that account be taken of the uncertainty in the

properties of the mat erial and in the structure model by using the same values of

acceleration but for several values of the time step (N.C.

TSAI,

Transformation

of Time Axes of Accelerog rams, Proc. ASCE , Vol. 95, EM3, pp. 807-812,

(1969)).

At lea st, the follow ing three values of the time step shall be considered: At and

At(l + ./f . , where f. is the dominant frequency of structural response

Hoc

r

or the floor concerned and

represents, as in the foregoing section, a

measure of the uncertainty on f.. If, in addition, one of the frequencies of

the equipmen t, f lies withi n the range f. + f., the time step

At[l - (f -

AltA will also be considered.

An alternative to this method consists in generating artificially an

accelerogram which would be consistent with the broadened spectrum mentioned in

the preceding section.

Q.3.5. What procedures are permitted in your country?

4. APPROXIMATE METHODS

4.1. Analysis method for multiply-supported equipments

As an alternative to the time time-history analysis [see, for example,

Leimbach, NED 51, pp. 245-252, (1979) ; NED 5 7, pp. 295-307 (1980) ; C.W. Lin &

F. Loceff, NED 60, pp. 347-352 (1980)], Section 3.7.3.(11.9) of the SRP recom

mends the following conservative approach for the response spectrum analysis of

multiply supported equipments with distinct inputs.


29/76

- 21 -

(a) Use a response spectrum which is the envelope of the individual spectra at

the various supports and analyse the structure assuming that the motion is

identical at all supports. This gives an estimate of the dynamic response.

(b) Analyse the structure statically, under the effect of the support maximum

relative displacements. These will either result from the response of the

supporting structure or will be conservatively computed from the floor res

ponse spectra. In the latter case, the maximum support displacement is

evaluated by means of the relationship:

S. - S / w

2

(3.5)

d a

where S is the high frequency asymptote of the acceleration spectrum (i.e.

the maximum absolute acceleration for the floor under consideration) and is

the fundamental frequency of the supporting structure. The relative displa

cements are combined in the most unfavourable manner.

The dynamic and static responses are then combined using the absolute

sum method. Stresses associated with the differential support displacements are

to be considered as secondary in the ASME sense.

.4_.__.

Is a similar rule applicable in your country?

4.2. Equivalent static load method

The dynamic response of systems can be estimated in an approximate and

generally conservative way (see, for example, J.D. Stevenson & W.S. Lapay, ASME

paper 74-NE--9) by a static analysis performed with an acceleration of 1.5 times,

the maximum ordinate of the acceleration spectrum for frequencies larger than the

system's first natural frequency.

The combination of the dynamic response with the contribution from the

support differential motions has to be done as indicated in the previous section.

Q_.4_.2_.

Is a similar procedure accepted in your country? Which one?

4.3. Use of a static factor for the vertical direction.

According to Section 3.7.2.(11.10) of the SRP, an equivalent static

analysis is acceptable [in the vertical direction] if it can be proved that the

structure is rigid in this direction; that is if the first natural frequency of

the structure in the vertical direction is larger than the cut-off frequency of

the excitation (33 Hz in the US A) .


30/76

C_.__.3_.__.Is a similar rule applicable in your country?

Q_.4_.3_.__.Wha t is the corresponding c ut-off frequency?

5. DAMPING

5.1. Reference values for the modal damping

Maximum damping values to be considered in the dynamic analysis of

structu res are recommended by the US NRC . These values depend on the type of

struc ture, the material and the types of joint. Two sets of values have been

defi ned, one for the SS E, one for the O BE , thus reflecting the fact that damping

increase s wit h deforma tio n ampl itu des . Thes e values are given in R.G. 1.61 (see

also Newmark B lume , Kapp ur, Proc. AS M E V ol. 99.P02, November

1973).

Damping

val ues larger tha n those giv en in the R.G. 1.61 may be used in the des ign,

pro-

vidi ng they are justifie d by experimental data .

C_.__.l_.l_. Are su ch st and ard va lu es use d in your co untry?

Q_.5_.l_.2 .

If t hey are d ifferent from those give n in the R.G. 1.61, what are they?

5.2.Damp ing val ues to use in a diiect integration method

The US NR C rec omm ends the us. of the R.G. 1.61 st andard d ampi ng valu es

for all modes considered in the dynamic analy sis . These values cannot be direct-

ly used in case of a di rect integration meth od where a full damping matrix is to

be used . It is common pratice to assume a Rayleigh damping (see, for example,

Bathe & Wilso n, Prentice

H a l l ,

197b, paragraph

8.3.3.

: in this cas e, the dam-

ping ma trix is a l inear combination of the mass and stiffness matrices :

C = + K (3.6)

The resulti ng matrix C can be diagonalized simultaneously with M and K.

Coe ffic ients and

ca n be determined in order to fit two modal danping val ues .

The major drawback connected with this procedure is that it leads to high

fre-

quency mode s considerably more damped than the low frequency modes for which the

constants were chose n. The refore, this leads to non conservative resul ts.

Q.._5.__. Is there a regulation in your country, concerning the use of Rayleigh

damping?


31/76

23

5.3. Composite structures - C ombination of various modal d amping.

The systems involved in the seismic analysis of nuclear power plants

are often composed of substructures having different modal dampi ng. This is

particularly true for models considering soil- structure interaction. The fol low-

ing formula are recommended by the U SN RC (S RP , sect ion 3.7.2. (11.15)) for the

determination of the modal damping values of a composite structure. They result

from the use of the mass or stiffness matrices of the various subst ructures, as

weighing functions for the damping :

_, - i j

5

i i

( 3

7

>

d_ K d

.

~ (3.8)

where _ is the i-th modal damping of the composite structure;

dj is the i-th M- normali zed eige nmode (eig enmode normalized with

regard to mass

matrix);

and M are the modified mas s and stiffness matrices constructed f rom

the substructure matrices by multi plying them by the co rresponding

modal damping;

is the assembled stiffness matrix.

In the case of a direct integration method with Rayleigh damping, the

damping matrices of the various parts of the structure can be calculated from

(3.6),

comp uting the and

coefficients in order to fit two of the modal da m-

ping values for the corresponding subst ructure. The assembled damping matrix is

no longer simultaneously diagonal with the mass matrix. As already mentioned,

this method has the drawback of overdamping the high frequency mo de s.

Of all the approximate methods, equation (3.8) leads to results that

are the closest to those of a more sophisticated method based on the use of sub-

system modal properties to evaluate the damping matrix of the complete structure

(see K.

Koss,

Element Associated Damping by Modal Synthesis, Water Reactor Sa fe-

ty Conference, Salt Lake City, 1973). The damp ing mat rix obtained by t h latter

method is also not simultaneously d iagonal with the mass matrix.


32/76

- 24

Q.5.3. What are the procedures accepted in your country for treating structures

composed of substructures having different modal damping?

C_._3.___.J_. In case of a modal superposition method?

Q.5.3._2_.

In case of a direct integration method?


33/76

25

I I I .

SYNTHESIS OF NATIONAL ANSWERS

1II.O.

Introduction

Based on the national answers to the questionnaire, gathered in the

appendix, a tentative synthesis has been made.

The subdivisions of the questionnaire and the various national.repprts

have been adopted.

Proposals are also made to continue and complete the present study.

The following abbreviations are used:

F France

GB Great Britain

D Federal Republic of Germany

I Italy


34/76

- 26 -

III.1.

Ground mot ion

1. Sa_fej_y_l_2vels

1.1. Philosophy leading to the establishm ent of two safety levels :

51 = OBE in American terminology

52 = SSE in American terminology

General ly spea king , the reference earthquake S2 is the only one to be

defined by safety conside rati ons. In all the countries considered, it is defined

in agreement wi th the American SSE philosophy ; it is the maximum hypothetical

eart hquake, taking into account the geological site conditions. For this earth

quake,

it must be possi ble to shut down the reactor and cool it in order to keep

it in a safe shutdown c ondition. This earthquake may not entail any significant

release of fission gas outside the plant.

Earthquake SI represents the normally acceptable earthquake (D,F,I),

that is to say the one that can be borne by the plant without any significant

dam age . It can be defined as the historical earthquake of the highest intensity

(D).

It is frequently defined as being 1/2 S2 (F,I). In Great Britain, an

eart hquake of very low intensity is defined (0.05 g ) ; it is not used at all in

the desig n. Shutdown and a new analysis be fore restart would required if it were

to be exceeded.

1.2. The reference earthquakes are generally defined on a deterministic

basis. Probab ilitic meth ods are generally only accepted as back-up to a dete rmi

nistic analysis (exception : D ) . The probabilities per annum of it being exceed

ed have been quoted as follows :

D

GB

S2

I O

4

( SN R : 3 I O

- 4

) *

1

S I

1 0 - 3

( SN R = 8 I O

- 4

) *

-

*A posteriori calculations.

2 .Maximum __ro_und__acjce_lej:a_t__o__.

2.1 . In some cou ntr ies , a lower bound is specified for the maximum ground

acceleration at the time of an earthquake S2 (see

Table).

2 . 2 .

The maximum acceleration of earthquake S2 is , in principle, defined on

a site-dependent basi s. In some countr ies, however, for the sake of simplicity

and sta ndardization, a single acceleration is defined (GB).


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- 27

3. Re__p__n__e_spe__trum

I The R.G. 1.60 spect ra are appl icab le for grounds whose natural freq uen-

cy verifies 3 f 9 H z. A proposal is being studied to modify the lowfre-

quencies spectra for soft ground.

F In principle, the spectra are site dependent. In practice, an envelope

spectrum is used for several sites . This spectrum is different from that

defi ned in R.G. 1.60. The vertic al spe ctrum = 2/3 horizontal sp ect rum.

GB Standard spect ra have been defined for three types of ground. The

vertical spectrum is equal to 2/3 of the horizontal spectrum.

D For SNR , Housner's average spectrum has been used ; the questi on re-

mains open for the future : site-depe ndent shapes or standard shapes which

may or may not be those of the R.G. 1.60.

4.

Du__a__ion

GB The following durations are used for articially generated acce lero-

grams :

soft ground 13 s

medium 12 s

hard 11 s

The minimum duration for q ualific ation tests is 6 s for AGR and 10 s

for PWR.

I Not specified by the safety auth orities . For the mechanical calc ula-

tions,

it is comp rised between 15 s and 30 s.

For PE C, the following numbers of cycles are used:

- 10 cycles corresponding to the S2 peak val ues ;

50 cycles corresponding to the SI = 1/2 S2 peak valu es .

F No formal rule. For SP X1, the durati on has been fixed at 20 s.

D For SNR , the strong motio n period is set at 8 s. In the futu re,

the

duration will be shorter and site- dependent.


36/76

DEFINITION OF GROUND MOTION FOR SI AND S2 EARTHQUAKES

Definition of SI

Minimum value of

a

ma x

f o r s 2

Way of defining

the maximum

acceleration for S2

Response spectrum

Relation between the

vertical spectrum and

the horizontal spectrum

Duration

GB

a

max.=

5

g

not used in design

-

Standard value*: 0.25 g

Standard shapes defined

as a function of the

soil conditions

2/3

11 - 13 s

Ground dependent

F

1/2 S2

o.i g

Site-dependent

So far, envelope

standard values are used:

0.15 g - 0.2 g **

Site-dependent

So far, envelope

spectra have been defined

for several sites

2/3

SPX1 : 20 s

I

1/2 S2

0.18 g

Site-dependent

PEC = 0.3 g

PEC : Housner. Future :

RG 1.60 for the grounds

whose natural frequency

verifies

3 < f < 9 Hz

PEC : 2/3

Future : RG 1.60

15 - 30 s

D

SNR = 0.5 m/s

2

0.5 m/s

2

Site dependent

SNR =1.2 m/s

2

SNR: Housner

In the future, site-

dependent or standard

shapes

1/2

SNR : 8 s

00

*Could become site dependent (0.20 + 0.05) g.

**Two standard shapes are used: one for Superphnix and the 900 MWe PWR's, with a corresponding maximum acceleration of 0.2 g

one for the 1300 MWe PWR's,with a maximum acceleration of 0.15 g.


37/76

- 29 -

I I I .2.

Seismic classification of components - Safety provisions - Dimensional

criteria

A. FUNCTIONAL CRITERIA

A.O.

Official documents

General_

__emark

No official regulations applicable to fast reactors in general exist

at the present time. Safety prescriptions relating to earthquakes are defined

for every reactor, generally on the basis of the operator's proposal. They are

usually included in the safety reports issued for the reactor.

In some countries

(F,D),

there exist official regulations applicable to

pressurized water reactors.

A.l.

Functional requirements after a SI earthquake

_lj_ssi cal _~riteria

Subsequent operation of the plant must be possible without any inspec

tion of the safety related components.

Emissions of radioactive products must remain below the limits imposed

during normal operation.

__xcet__o__s_v__r__an^s_:

GB There is no SI earthquake in the standard sense. There exists a low

intensity earthquake (OSE) beyond which the reactor must be shut down. It

must be inspected before any new start-up.

D The possibility of a restart without inspection is not required. The

criterion failure, which must be foreseen is specified

(SNR-300).

Other

minor differences.

I An inspection is required before restart. The radiological risks in

curred by the operating staff cannot exceed the normally acceptable limits.

F No damage is tolerated to parts which cannot be inspected or repaired.

For other parts', damage must remain extremely low.


38/76

30

. 2 . Functional requirements after an earthquake S2

Classical reguiremen_ts:

Devic es with a safety function must be designed to withstand earthquake

S2 and must continue to function.

The fo llowing systems have a safety function :

1. devices necessary for reactor shutdown and maintenance of safe shutdown condi

tion (including equipment ensuring core cooling and residual heat

evacuation);

2 . devices designed to prevent or limit releases of radioactive material, which

could result in an accident or would be dangerous for the population.

Ad ditio_n__l_r__q__i__ement__ :

G B :

Add to the list of sy stems having a safety function :

3. devices ensuring containment of radioactive material.

D: Additional requirement s:

. to prevent radio activ e releas es which would prohibit access to reactor

building;

. to fulfil the above mentioned conditions without manual intervention for 10

hours;

. to foresee fai lure of an active componen t and the unavailability of compo

nents which undergo maintenance during reactor operation.

I: Add to the list of systems with a safety function :

3. devices ensuring containment of highly radioactive material;

4 . the sodium envelope.

F: Safe shutdown conditio ns imply :

. no leaks in active circuits (including inside the reactor

b uilding);

. no water sodium reaction;

. no out of control sodium fire.


39/76

- 31 -

. 3 .

Equipment which must remain functional after an S2 earthquake

The comparison of answers is made difficult because of the fundamental

design differences between the fast breeders developed in the various countries.

We will limit ourselves to indicating the equipment which cannot be

immediately associated with a fundamental functional requirement and wh ich , n e

vertheless, is designed to continue to function or to remain leaktight after S2 .

F(R1): The primary pumps are designed to operate after an S2 earthquake, the

secondary pumps are not .

The handling system is designed to withstand an S2 earthquake.

The secondary circuits are designed to withstand an S2 earthquake.

The steam generators are designed to withstand an S2 earthquake.

D(R1): Primary pumps are designed to operate after an S2 earthquake.

The secondary circuit parts external to the reactor building are not d i

mensioned- for earthquake S2 _they are designed to withstand an SI ear th

quake

.

The part of the handling system inside the reactor building is designed to

withstand earthquake S2 (the part of the handling system outside the reac

tor building is designed to withstan d an SI

earthquake).

GB:

Primary pumps are designed to operate after an S2 earthquake.

The handling system is designed to operate after an S2 earthquake.

The secondary sodium envelope is designed to withstand an S2 earthquake

(the aim is to avoid sodium fires and sodium-water

reactions).

I: Primary pumps are designed to operate at reduced rate after an S2 ear th

quake.

The fuel element transfer machine is not dimensioned for earthquake S2 .

However, its collapse must not damage the core and its replacement must be

possible.

These results are summarized in Table Al .


40/76

- 32 -

Remark:

It seems that, in general, three types of earthquake-related require

ments may be distinguished:

- a system or component may be required to remain functional (during and) after

the earthquake;

- a system or component may be required to retain its leaktightness (during and)

after the earthquake;

- finally, a system or component may be required to resist collapse (because of

the consequences of this collapse on equipment having a safety

function).

However, the consequences of this distinction on the mechanical design

are not always clear. This subject is covered in paragraph 3.8. of the KTA

2201.4 standard, as well as in the various countries'answers to question C.2.4.

A.4. Containments that must remain tight after S2

The only containment barriers considered here are those of radioactive

core material. Comparison between the various reactors is difficult (see table

A2).

Nevertheless, the following conclusions can be drawn:

1) The first barrier (except fuel rod cladding) is always the envelope of the

primary circuits. It is always designed to remain leaktight after an S2

earthquake.

2) There is always a second barrier remaining leaktight after an S2 earthquake.

This barrier is not always metallic.

A.5. Earthquake detection - Planned actions

Earthquake detection is planned in all countries.

There is a German standard which defines the detection system in de

tail (KTA 2201.5).

Exceeding a threshold always entails reactor shutdown. According to

the country, the shutdown type is either an automatically triggered emergency

shutdown or a normal shutdown controlled by the operator as a response to an

alarm triggered by the earthquake detection system.

A more complete comparison is given in Table A3.


41/76

- 33 -

. 6 .

Functional consequences of an earthquake to be considered

The principl es seem clear : it is necessary to consider:

- emergency reactor shut down (triggered by the earthquake detection system or by

a condition resulting from the earthquake detected by the reactor safety

system);

- loss of external electricity supplies;

- collapse of component whi ch has not been shown to withstand earth quakes;

- unavailabilit y of systems whose functi oning (during an d) after the earthquake

has not been demonstrated.

Application of these principles in the various countries is compared in

T a b le A 4 .


42/76

- 34

TABLE Al - EQUIPMENT WHICH MUST WITHSTAND EARTHQUAKE S2 *

Primary pumps designed to

operate after S2

Secondary pumps designed to

operate after S2

Secondary circuits and steam

generators designed to

remain leaktight after S2

Fuel element handling system

designed to operate after

S2

D (1)

X

0

(2)

(2) (4)

F (1)

0

GB

0

(3)

(5)

(5)

(3)

0 (6)

* Only components for which a doubt may exist are mentioned in

this table.

(1) Answers relating to reactor under construction Rl (SUPERPHENIX,

SNR-300).

(2) Only the part insid e the reactor building is dimensioned for S2 ;

the part outsid e the reactor bui lding is designed to withstand SI,

(3) Only the sodium envelope (not the argon

circuits).

(4) Operation not required.

(5) Operation at reduced rate.

(6) Replacement of the fuel handling system must be possible.

X = yes .

0 = no.


43/76

- 35 -

TABLE A2 - RADIOACTIVE MATERIAL CONTAINMENT BARRIERS

Primary circuit envelope

(sodium + gas)

Double walled primary

circuit (+ dome)

Wall of primary cells

(metal clad)

Reactor building

Safety metallic shell

D(l)

S2

S2

SI

F(l)

S2

S2

GB

S2

S2

I

S2

S2 (2)

S2

(1) Answers relating to reactor Rl (SNR-300,SUPERPHENIX).

(2) Reduced leaktighness is accepted after S2:

51 = dimensioned to remain leaktight after SI.

52 = dimensioned to remain leaktight after S2.

? = answer not supplied.


44/76

- 36

TABLE A3 - DETECTION OF EARTHQUAKES AND ASSOCIATED ACTIONS

Earthquake detection requi-

red

Shutdown (A: automa tic,

M: manual)

Shutdown (E: emergency,

N:normal)

Threshold

Required inspection

D

X

M

N

s 0.25 S2

X ( D

F

X

A (1)

E (1)

X

GB

M

(1)

s 0.25 S2

X (2)

I

X

A (1)

E

S 0.5 S2

X

(1) Interpretation of answers supplied.

(2) An instrumentation is planned in order to assess the state of the

plant before restart.

X yes.



45/76

37 -

TABLE A4 - CONSEQUENCES OF EARTHQUAKE S2 TO BE TAKEN INTO ACCOUNT

Loss of external electricity

supplies

Emergency shut down of

reactors

Loss of water flow to steam

generators

Leakages of slightly radio

active products

Sodium circuit leaks

Water/steam leaks

D

X

X

X

X (D

X (D

F

0 (2)

0 (3)

(1)(4)

GB

0

0

(1)

-

0

0

-

(1) External to reactor building .

(2) The design of the nuclear boiler system is such that no radio active

leak must result from the earthquake.

(3) Rl : the design of the boiler system is such that no sodium leak

must result from the earthquake;

R2 : not yet decided (small leaks in auxiliaries ? ) .

(4) Inside the reactor and steam generator build ing, steam and water

pipes are designed to withstand earthquake.

X = to be taken into account,

0 = not taken into account.

- = not relevant.


46/76

- 38

. EARTHQUAKE CLASSIFICATION IN OPERATING CONDITIONS

B . l .

Categories of operating conditions

In all co unt rie s, four categories of operating conditions are defined;

they are designated:

- category 1: normal conditions;

- category 2: upset condition s;

- category 3: emergency conditio ns;

- category 4: faulted conditi ons.

These categories are not defined in a precise way:

- categor y 2 conditions ofte n correspond to transient states ;

- category 3 conditions correspond to exceptional circumstances which have to be

taken into consideration;

- category 4 conditions correspond to hypothetical failures of equipment.

. 2 .

Combination of SI and classification of combined conditions

Co__d__t__ons_to c_ombine with__S1_

The pri nciple s seem clear : it is necessary to combine:

- the initial conditions;

- the earthquake;

- the possible consequences of that earthquake (cf. A 6 ).

Usually, all conditions in categories 1 and 2 are considered as possi

ble initial co nditi ons. The conditions whose total duration is low are an excep

tion : such situations are not considered as possible initial conditio ns, or else

the corresponding combined conditions are classified in a different way (i.e.

analysed with less severe

criteria).

Remark : The same remark as in point B3 is applicable here.

l_is__ifica__ioji_ojf omb__ne_d_cc_nd_i__i__ns_

T he c o n d i t i o n s r e s u l t i n g fro m t h e c o m b i n a ti o n s a r e c l a s s i f i e d i n t h e

s e c o n d o r i n t h e t h i r d c a t e g o r y d e p e n d i n g on t h e c o u n t r y .


47/76

- 39 -

.3.

Combi nation of S2 and classifi cati on of combined conditions

C_ondi_t__or_s_to_ __omb__ne_

w

_iJ^h_S2^

Here as w e l l , it is necessary to combine:

- the initial conditions;

- the earthquake;

- the possibl e consequenc es of that earth quake (cf. A . 6 . .

Usually, all conditions in categories 1 and 2 are considered as possi

ble initial cond itions . The conditions for whic h total duration is low are an

excep tion: these conditions are either not considered as possible initial condi

t i o n s , or the corre spond ing combined co nditio ns are analysed off desi gn (fifth

c a t e g o r y) .

Remark :

An elegant s oluti on consist s in using integr ated du ration as a cr it e

rion distinguishing upset conditions from normal operating conditions and to

impose only, as initial con ditio ns, normal operating conditions.


48/76

- 40 -

TABLE Bl - CLASSIFICAT ION OF EARTHQUAKE SI

Category of operating condi

tions resulting from

earthquake SI

Category of operating condi

tions which must be com

bined with earthquake SI:

- categories

- exception for operating

conditions with a small

cumulated duration

- special cases:

.normal handling

.exceptional handling

D

3

1 + 2

X

X

X

F

3

1 + 2

(2)

GB (1)

-

-

-

-

I

2

1 + 2

(3)

(1) Earthquake SI has no influence on plant design.

(2) Rl : the classification of combined conditions depends on the total

duration of the handling operations considered.

R2 : these combined conditio ns are analysed off des ign .

(3) The classif icatio n of combined conditions depends on the total dura

tion of the handling operations considered.

X = yes .

- = not relevant.


49/76

41 -

TABLE B2 - CLASSIFICATION OF EARTHQUAKE S2

Classification of operating

conditions resulting from

earthquake S2

Categories of operating co n

ditions which must be

combined with earthquake

S2:

- categories

- exceptions for opera

ting conditions having

low total duration

- threshold (total dur a

tion limit)

- special cases:

.normal handling

.exceptional handling

D

4 (1)

1 + 2

?

X

0 (3)

F

4

1 + 2 (1)

? (2)

0 (2)

GB (1)

4

1

0

-

4

1 + 2

?

0

(1) Interpretation of answers received.

(2) On RI, the threshold is determined by probabilistic calculations

On R2 , exceptional conditions are not analysed off design .

X = yes .

0 = no.


- = not relevant..


50/76

- 42 -

C. CLASSIFICATION OF MECHANICAL COMPONENTS

C.l. Safety classes

There is no general rule :

F: Three safety classes exist in addition to unclassified equipment.

I and

D(R1):

the safety class concept is not used. On the other hand, a quality level

(equivalent to ASME code class) is attributed to components.

D (KTA) and perhaps GB:

The safety class concept is not used. Standards are established per compo

nent type rather than per quality level.

C.2.1.

Seismic classes

There is no general rule.

F(R1) and D(R1):

There exist three classes :

- equipment to be designed to withstand SI ;

- equipment to be designed to withstand S2;

- equipment not designed to withstand earthquake.

I: There exist three classes :

- equipment with a safety function;

- necessary equipment with in the long run a safety function (equipment ne

cessary for a long duration operation in safe conditions)(see Italian ans

wer for more details in appendix of the french version of this report).

- equipment not designed to withstand earthquake.


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GB: There exist four classes :

- systems which must function after earthquake S2;

- components not required to function after S2 but for which structural inte

grity and leaktightness must be ensured;

- components not designed for earthquakes;

- buildings and systems for handling and storing radioactive material.

D

(KTA):

There exist two classes :

- components with a safety function. These components must be capable of

operating after several SI occurrences. They must ensure their function

after one S2 earthquake;

- other components; it must be demonstrated that no component with a safety

function will be damaged by collapse or malfunction of the other compo

nents.

C.2.2. Relationship between component classification design criteria

The design criteria used during seismic stress analysis depend on the

requirements applicable to the component and on its accessibility.

Rules differ from one country to another: they are compared in Tables

CI and C2 and discussed hereunder.

Special

f_u__c__ions

In some countries, systems ensuring some specific safety functions are

subjected to more severe criteria during seismic analysis.

For example, in Great Britain, components ensuring reactor shut-down,

core cooling, residual power evacuation, and so on, are dimensioned for S2 with

special criteria. On the other hand, components ensuring a containment function,

are dimensioned with normal criteria (level D ) .

In other countries, there is no distinction between safety functions.


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__c__i__e_c__mp_onent_^

Active components are sometimes subjected to special criteria:

- Are called active , components which are not static in performing their safety

function (pumps for which an operation is required, valves which must change

state, etc.).

- A demonstration of the correct operation of such equipment after the earthquake

is often required . This demonstration can be experimental (tests or trials).

In some countries, this experimental demonstration can be avoided by

using more severe design criteria.

(_ompo ne n__s_f_ r_wl_i_;h_c_ ll_ap_se_ must be_ avoided

In some cases, less detailed analyses are permitted when it is only

the collapse of the components which must be avoided.


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TABLE Cl - COMPONENT CLASSIFICATION AND DESIGN CRITERIA FOR SI

Level of criteria relating

to SI (principle)

Distinction accessible/not

accessible:

- applicable

- overclassification not

accessible

Distinction active/passive:

- applicable

- overclassification

active

D

RI KTA

C

0

0

(2)

F

C (4)

(5)

C (4)

(5)

C (4)

GB (1)

-

I

(1) Earthquake SI does not influence the plant design.

(2) An additional analysis of the possible causes of malfunctioning

is required.

(3) Level criteria are imposed, except if it is demonstrated that the

function remains assured.

(4) Rl : C;

R2 : probably C.

(5) Distinctions accessible/not accessible or active/passive have no

consequences on the level of criteria, but affect the class (quality

level) of the component. As an example, inaccessible components are

always class 1.

X = yes.

0 = no.

- = not relevant.


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TABLEC2 -COMPONENT CLASSIFICATIONANDDESIGN CRITERIAFOR S2

Criteria level relatingto

S2

( withstand )

Distinction special func

tion :

- applicable


Distinction active/passive:

- applicable


Distinction( withstand /

collapse avoided):

- applicable

- underclassification

Rl

0

X

(3)

X

(5)

D

KTA

D

X

B/C

(1)

0

-

X

(6)

F

D

(7)

(7)

GB (1)

D

?

0

-

0

I

D

(2)

B/C (4)

0

(1) This overclassification

is not

required when

a

strain analysis

demonstrates correct functioning.

(2 ) A strain analysisisrequired.

(3)

An

additional analysis

of the

possible causes

of

operational failure

is required.

(4) Level

criteria

are

imposed, except

if it is

demonstrated that

the

function remains ensured.

(5) More simple criteria

are

used.

(6) Reduction

of

design effort

as a

function

of

risk

to be

taken.

The

designof thesupportsand thebucklin g analysisareunchanged.

(7) Anadditional analysisisrequired,inordertodemonstrate that

components subject

to

additi onal functional requirements after

S2

are abletomeet them. This analysisisspecifiedon acasetocase

basis. Design rulesdo notensureanyfunctional guaran tee.

X

= yes.

0

= no.

-

= not

relevant.

?= answernotsupplied.


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D. MECHANICAL DESIGN CRITERIA

D.I.

Design codes used

The ASME III code (subsections + code cases ) still is the joint refe

rence code.

In some countr ies, national codes have been drawn up for the design and

construction of pressurized water reactors .

D . 2 .

Classification of design criteria

Criteria levels A,B,C,D of ASME code section III are generally used for

the design of mechanical components with a safety function.

In France, levels A and are grouped together.

D.3.

Relationship between categories of operating conditions and levels of cr ite

ria

With regard to earthquakes, this topic has been analysed in paragraphs

C.l. and C.2 .

D.4. Fatigue analysis

Analysis of earthquake induced fatigue is required by some countries .

Table Dl compares the data gathered.

E .TRANSMISSION OF SEISMIC LOADINGS

Depending on the country, seismic loadings are part of the general

design specification or are dealt with in special specifications.


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TABLE Dl - ANALYSIS

Fatigue analysis required

Number of SI earthquakes

Number of cycles/Si earth

quake

Number of S2 earthquakes

Number of cycles/S2 earth

quake

Are the aftershocks taken

into account ?

OF EARTHQUAKE INDUCED FATIGUE

D

RI KTA

SI 0

1

10-15'

-

? -

? -

? -

F

RI R2

?

?

1

(D

0 ?

GB

0

-

-

-

-

-

I

5

10

1

10

(1) Fatigue analysis not required for S2,

X = ye s.

0 = no .


- = not relevant.


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III.3. Seismic analysis methods

With the exception of Germany (KTA

2201.4),

no written rules exist re

garding seismic analysis methods. Consequently, answers refer to current practi

ces rather than to rules in the real meaning of the word. The analysis procedu

res are in principle the result of an agreement between equipment supplier and

safety authorities; the trend Is towards using more refined analysis methods for

more sensitive equipment, for which the excessive degree of conservatism of the

seismic analysis methods may constitute a functional hindrance (excessive rigidi

ty, too many snubbers,

. . . .

1. Rules used in connection with the modal superposition method

1.1. orab__ntion of modal __r eeoo ns es

F: SRSS (square root of sum of

squares),

without considering interaction of

closely spaced modes.

D:

SRSS below cut-off frequency, without special modification to tak int ac

count interaction closely spaced modes.

GB :

SRP 3.7.2 practices are acceptable (formula (3.1) of questionnaire).

I: RG 1.92 is used. The CQC* method (Complete Quadratic Combination) la also

used.

To the authors' knowledge, the CQC method represents the first attempt

to rationally take into account the correlation between closely spaced modes. It

is based on the hypothesis that the correlation coefficients of the various medal

responses to a wide band excitation may be approximated by those of the stationa

ry response to white noise**.

*E.L.

Wilson, A. Der Klureghian & E. Bayo, A Replacement of the SRRS Method in

Seismic Analysis , Earthquake Engineering and Structural Dynamics, 9, 187-192

(1981).

**See also: A. Der Klureghian, Structural Responses to Stationary Excitation ,

Proc. ASCE, Vol.

6,

6, pp. 1195-1213, December 1980.

. Der Klureghian, A Response Spectrum Method for Random Vibration Analysis of

MDF Systems , Earthquake Engineering and Structural Dynamics, Vol. 9, 419-435

(1981).


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1.2. __omb__natio_i of_the_tliree_Sa_ti


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3. Determination of floor spectra, considerations on the use of artificially ge

nerated accelerograms

3.1. The time history method is the most popular of the methods in use . It

is the only method accepted in Great Britain. In other countries (D.F.I),

direct or probabilistic methods are accepted subject to appropriate valida

tion*.

3.2.

The spatial components combination rule was discussed In paragraph 1.2.

In the case of a time history analysis, accelerograms used in the ,,

directions must be statistically independent.

3.3. With regard to the use of artificially generated accelerograms, rules

exist in Germany on the agreement between the accelerogram spectrum and the

sig

spectrum. Most often, a single set (I) or two sets (GB ) of accelero

grams are used for the calculation**. The duration of artificially generat

ed accelerograms was discussed in the chapter on Ground motion .

3.4. A spectrum-broadening procedure similar to that described in RG 1.122

is most often used (+_

15Z)(F,I,GB).

It may or may not take Into account the

uncertainty of soil properties. The latter is particularly significant for

soft soils ( D) and was the only one to be taken Into account for SNR-300

3.5. When taken into account in a time history analysis, uncertainties on

structural properties are either treated by a procedure similar to the SRP

procedure described in the questionnaire (F ), or included in the accelero

gram by generating the latter on the basis of a broadened spectrum (G B ).

*It may be interesting to mention a recent study devoted to the direct determina

tion of floor spectra including interaction between equipment and supporting

structure: J.L. Sackman, A. Der Klureghian & B. Nour-Omid, Dynamic Analysis of

Light Equipment in Structures : Modal properties of the Combined System , Proc

ASCE, V ol. 109, EMI, February 1983, 73-89.

A. Der Klureghian, J.L. Sackman, B. Nour-Omid, Dynamic Analysis of Light Equip

ment in Structures : Response to Stochastic Input , Proc. ASCE, Vo l. 109, EMI,

February 1983, 90-110.

** A. Kurosakl and M. Kozekl : Statistical Uncertainty of Response Characteristic

of Building Appendage System for Spectrum Compatible Artificial Earthquake Motion.

SMIRT-6, Paris (1981),Paper K7/ 7.


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4 Approx ima te me thods

4.1. E_quij_ment wi__h_mulj_ip_le supports

The SRP practice described in the questionnaire is generally applicable

in the countries considered (GB,F,I,D) but with differences regarding treatment

of the stresses resulting from the relative displacements of the supports (D) .

Multiple support modal methods are also used in several countries (I,D,F)*.

4.2. Equivalent_s_tat_ic me__hod

The approximate procedure described in the questionnaire is generally

applicable (F,I,D) to small diameter circuits or to circuits of little importance

(cold

piping).

The coefficient changes from 1 to 1.5 as a function of the model-

lization (1 or several degrees of freedom)(I) or of the structure type (D ); it is

reduced to 1 if the first frequency is above the cut-off frequency.

4.3. _5_ta_ti_c_fc_to foj_ vert_Lcal_d___recti on

A static analysis is generally allowed for the directions in which the

structure can be considered as rigid (first natural frequency cut-off frequen-

cy)(F,GB,I,D).

The acceleration used is the spectrum asymptotic value, without

any increase factor.

*It may be of interest to mention recent studies: A. Der Kiureghian, A. Asfura,

J.L. Sackman & J.M. Kelly, Seismic Response of Multiple Supported Piping Sys

tems , SMIRT-7, paper K7/ 7, Chicago 1983.

M.C.

Lee & J. Penzien: Stochastic Analysis of Structures and Piping Systems

Subjected to Stationary Multiple Support Excitation , Earthquake Engineering

Structural Dy namics, Vol. 11, 91-110

(1983).


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5. Damping

5.1. Reference _va_lues__fo_r_m__d__l__d__mp_ing

Standard values of modal damping are generally applicable (F,GB,I,D).

They are mostly identical to those of RG 1.61 (F,GB,I). Higher values can be

used, subject to adequate experimental justification.

5.2. D_i__ect_integraj_i on

Direct integration is rarely used in seismic analysis. An analysis in

the modal basis is often preferred, because of the low frequency content of the

excitation. However, when Rayleigh damping is used, the and coefficients

must be chosen in such a way that all significant modes have a modal damping

lower than the limits fixed in RG 1.61 (GB,I).

5.3. Composi_te j truc.tu.r

s

The SRP procedure outlined in the questionnaire is generally applied

(F,GB,I,D). Formula (3.8) is often preferred to formula (3.7)(F,I).


62/76


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IV. PROSPECTS AND FURTHER DEVELOPMENTS

IV.1.

Part common to all types of reactors

It was found necessary to limit this study to the traditional aspects

of the seismic calculation methods for nuclear reactors not only because of the

broad scope of the subject, but also because these traditional aspects are the

only ones to be standardized in codes or official documents and, hence, the only

ones which may be systematically compared.

This report indicates a great similarity between the various European

countries: with regard to the methods, especially in the areas which are not

contested, and with regard to preoccupations as far as more controversai issues

are concerned.

Some of the topics that still remain open are, in our opinion:

- The SI earthquake definition (does earthquake SI have a safety function?).

Three functions can be attributed to it :

(i) to remedy the insufficiencies of level D criteria;

(ii) to define the earthquake beyond which it is necessary to shut down the

reactor;

(iii) to define the earthquake beyond which inspection Is required.

- The integration of seismic rules in design rules:

Is it necessary to add a seismic classification to the safety classifi

cation?

Does a basic difference exist between the earthquake and the other

reference accidents (possibility of common failure

modes?).

- The effect of functional requirements on the design criteria (active compo

nents, leaktightness assured, collapse avoided).

I s this distinction justified? What are the consequences for design?

- Conditions which must be combined with earthquakes:

What are the operating conditions (handling, for example) during which

the occurrence of an earthquake must be considered? The answer could result from

an overall risk analysis.

A substantial improvement of the calculation methods should result from

the application of random vibration theory. As examples, we shall quote:

- The combination rules for closely spaced modes, a particularly important pro

blem in thin shells*. The CQC method (Complete Quadratic Combination)**,based

on the theory of random vibration in conjunction with reasonable hypo

theses,

offers hope of improvement in this field.

*I .

Elishakoff, A.Th. Van Zantem, S.H. Crandall: Wide-Band Random Axisymmetric

Vibration of Cylindrical Shells, J. of Applied M echanics, Vol. 46, p. 417, June

1979.

**E.L.

Wilson et al, op.cit. p. 49.


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- The application of the theory of random vibrations to the direct calculation of

floor spectra

1

and to piping calculation. In addition to taking into account

the correlation between the modal responses of closely-spaced frequencies, in

the latter case the method offers a unique possibility of taking into account

the correlation between excitations at the various supports*.

Finally, the following suggestions from the British experts seem of

particular interest :

1. Methods of analysis of soil-structure interaction should have been included in

the survey. Although the methods are not unique to the fast reactor plants,

such a survey has not been previously conducted with European countries and is

fundamental to seismic assessment of all types of reactors.

2.

Damping. Although the questionnaire was quite exhaustive, the answers were

insufficiently detailed (including my own ). This is regrettable since the

damping values are crucial to the outcome of the seismic analyses. It would

be worthwhile to follow it up with further enquiries to find out what damping

values are used for individual components such as cranes, steam generators,

heat exchangers, sodium pumps, fuel transfer routes, rotating shields, etc.

3. Design criteria - stress limits. Once again the answers were often superfi

cial, e.g. level D , where many different ways exist to satisfy this crite

ria.

This item is of a particular interest to WGCS-2 and should be followed

up if a co-operation of WGCS-2 members can be obtained.

IV.2.

Fast reactor characteristics

The following specific characteristics of fast reactors:

- thin walls resulting from low pressures;

- high temperatures and neutron flux entailing problems of material behaviour;

- severe thermal gradients and temperature fluctuations;

- structures of very large dimensions (especially in the pool concept) containing

large masses of sodium;

- presence of water and sodium,

B.J. Sullivan : A Method for Generating Floor Response Spectra through Power

Spectra/Response Spectra Relationship . SMI RT-7, Chicago (1983),Paper Ml/9.

*M.C.Lee, J. Penzien, o p.cit., p.52.

Documents

Comparison of the Methods of Seismic Analysis Applicable to Fast Reactors in the EEC Countries