‘German leak-before-break concept’ (description of German LBB procedures, practices and applications)

Int. J. Pm. Ves. & Piping 71 (1997) 139-146 0 1997 Elsevier Science Limited. All rights reserved Printed in Northern Ireland 030x-0161/97/$17.00

PII:SO308-0161(96)00057-9

‘German leak-before-break concept’ (description of German LB.B procedures, practices and applications) Giinther Siemens/KWlJ

BartholomC Erlangen,

Germany

(Received 20 July 1996; accepted 12 September1996)

This paper gives a definition of the LBB-concept in Germany, as well as a general outline of the prerequisites and practices of the LBB-concept, including practices and prerequisitesfor break preclusion (e.g. basic safety, redundancies). An analysis of LBB-behaviour is also covered, including: defects to be considered;fatigue crack growth (FCG); demonstrationof LBB fatigue crack growth; analysisof surfacecracks for end of life (EOL): through wall crack (TWC) stability analysis under maximum loads: completenessof loads used for fatigue crack growth and crack stability: leak rate of stable TWC; and leak detection capabilities(LDS). Application of LBB-procedures is alsodiscussed.0 1997Elsevier ScienceLtd.

1 DEFINITION (LBB)-CONCEPT

OF LEAK-BEFORE-BREAK IN GERMANY

The leak-before-break (LBB)-concept (Figs 1 and 2) is part of the break preclusion (BP)-concept in Germany. This BP-concept has been used in Germany since 1979 and is based on the basic safety (BS)-concept.’ An important feature of the BSconcept is the LBB-behaviour. hence, the global BP-concept is often called (pars pro toto) the LBB-concept in international use.

2 GENERAL OUTLINE, PREREQUISITES AND PRACTICES OF LBB-CONCEPT 2.1 General

outline,

practices

The break preclusion concept according to German practice is detailed in a logic chart (Fig. 1) for the different steps: basic safety, independent redundancies, LBB-behaviour, break preclusion and the break postulates; derived from the BP-concept. The BS-concept and the BP-concept are synonyms. The two main prerequisites for the general procedure of BP2 (identical with the BS-concept) are (see Fig. 2): basic safety and independent

redundancies: LBB-behaviour being an important part. The safety against break was proven in Germany by research programs performed at MPA. Siemens, Interatom. These programs (FKS, Phenomenological Burst Tests, BVI, II, etc.) included tests on representative pipings under relevant loading conditions. Other international research programs gave similar results.‘-’

2.2 Prerequisites

for break preclusion

(BP)

The realization of BP (Fig. 3) is based on the KTA-rules (for nuclear technology in Germany) as cited in the German Reactor Safety Commission (RSC) guidelines. Part of these prerequisites is the application of fracture mechanics to prove either LBB-behaviour or equivalent safety margins (Fig. 4). The BP-analysis addressesall relevant items which contribute to minimization of piping failure probability such as: l l l l

piping technology (design, material and fabrication); operating loads and stresses: field experience: leak detection requirements: . transient monitoring;

German Practice (Basic Safety + Redundancies)

Basic

Safety

Break Preclusion Logic Chart

i Basic

(BS)

Safely

.-. __-----.I__ __..

(BS)

Independent .---.

Redundancies I-----._

(IR)

; -

I

Quality

through

inservice monitoring and documentation principle DWR RSK LL lOi Chapter 4 1 4

pKXJttCUOtl

principle DWR RSK- LL IO/81 Chapler 4 1 2 R&dancies OR)

4 Optimization COlltrOl Qualification l Design . Material

LBB - Behaviour

l

Break WV

Insetvice inspections Load monitoring . Leak detection l

KTA 3201.2 KTA 3201 .l KTA 3201.3

Manufacturfng

Preclusion of Leak - Before (Determinatfon

l

Evaluation - Break (LBB) - Behsviour of safety margins)

Break Postulates

Fig. 3. Realisation of break preclusion (BP)-concept. Po8bdated Breaks and Effects on l ECC-System - Contalnmanf and RPV intemak l MaIn components

Fig. 1. Break preclusion (BP)-concept practice.

crack growth and stability

l l

I

I

2.2.1 Basic safety (BS) To preclude the catastrophic failure of a pressureretaining component due to defects in manufacture the BS-procedure (Figs 2 and 3) was developed.2*3 The BS of plane components depends on:

according to German

requirements:

in service inspections (ISI): l

and states to what extent each item is relied upon to preclude catastrophic failure of the piping systems. Details of the application of the safety principles to the primary side are discussed in detail in the RX.’

Optimization Control Qualification *Design *Material

Basic

Safety

l

l l

high-quality material characteristics, in particular toughness; a conservative restriction of stresses; the prevention of peak stresses through optimal design;

Independent

Independent Redundancies 1

(ES)

J, Preclusion

I

I

of Catastrophic

Failure

Fig. 2. Summary of the BS-concept.

(IR)

German LBB procedures Evaluation

3 ANALYSIS

of LBB-Behaviour

Fracture

Mechanics

crack

. Crack growth (LBB-Behaviour

growth

in life time

behaviour “Beyond Design” or equivalent safety measures)

. Detectable leak size smaller than critical size

Fig. 4. Evaluation

OF LBB-BEHAVIOUR

Fracture mechanics methodology and criteria are used to demonstrate LBB-behaviour or equivalent safety margins (Figs l-4). The principle of LBB-behaviour (Fig. 5) is b ased on the fact that a crack will develop in such a manner that a detectable leak is produced (Fig. 6) which has sufficient safety margins against the critical through wall crack size. The calculational concepts used by Siemens are validated by numerous tests. The procedures using fracture mechanics6 are explained in detail in Figs 5, 7 and 8.

__i / - Negligible

141

of LBB-behaviour and determination safety margins.

of

3.1 Defects considered

The BS-criteria are specified as specific requirements for design, material and fabrication.

Initial defects (or reference defects) (Figs 5-8) are used in the fatigue crack growth (FCG) analysis and in the LBB fatigue crack growth demonstration. Reference defects are circumferential surface defects. They are postulated in the highly stressed welds. The geometry of a reference defect is semi-elliptical, defined by the depth (a) and the total length at the surface (2~). The size of the reference defect is an envelope of the allowable defects for preservice examination and in-service inspection. Performance of inspection technologies and accumulated experience are taken into account to define the reference defects.

2.2.2 Independent redundancies (ZR)

3.2 Fatigue crack growth (FCG)

the assurance of the application of optimized manufacturing and testing technologies (especially in respect to welding); the knowledge and assessment of possible defect conditions; consideration of the time dependent influences of especially with respect to the operation, corrosion.

l

l

l

The IR (Figs 2 and 3) are: a multiple parties testing principle; a worst case principle: a continuous monitoring and documentation principle; a validation principle, part of which is the proof of LBB-behaviour.

l l l

l

The independent redundancies required for break preclusion’.7 are in-service inspections (ISI), load monitoring (LM) and leak detection systems (LDS). The inspections during operation are defined in KTA 3201.4 and RSC guidelines. Essential items are l

l

ultrasonic testing guidelines): load monitoring.

(KTA

3201.4

and

FCG (Figs 5-7) computation performed under normal

of the surface defect is and upset transients.

no LB8 Behaviour

RSC

The general requirements for load monitoring are given in KTA 3201.4. All plant load informations influencing the component integrity (pressure, temperature, mass flow) have to be monitored continuously. 2.2.2.1 Leak detection system. The capability of the leak detection system has to be sufficient to detect leaks long before a critical leak (critical crack size) is reached (see section 4).

Crack

Fig. 5. Procedure of LBB-behaviour.

Length

2c

G. Bartholom~

0 , 0

I

I

I

I

I

I

I

I

2

4

6

6

10

12

14

16

Fig. 6. Leakage

crack length (2&) under tension

Paris’ law is the basic crack propagation model, the computation is performed simultaneously at the depth of the crack and at the surface. It is shown that an elliptical crack remains elliptical with growth determined by Paris’ law applied at the surface and at the deepest point. Thus a new elliptical ratio (c/a) is obtained after every increment of crack growth. A conservative crack growth curve is used, taking into account environmental effects. The criterion is to demonstrate small fatigue crack growth of the initial defects during one specified load collective of the plant (one plant life).

I

I

16 2clt

2o

loading.

3.3 Demonstration of LBB fatigue crack growth (Figs 5-8)

A similar analysis to the previous one is performed with unlimited specified plant load collectives in order to show the fundamental tendancy of the growth of the considered crack, with respect to its shape development. (a) If the crack grows through the wall by fatigue or the ligament breaks without instability in the circumferential direction (2,,, < 2,.,) then the LBB

Criteria allowable defects

md inservice) rtigue rack growth i. C. G.)

) LB9 Fatigue crack growth resulting in LBB-behaviour

Small F. C. G. in 1 spacif. load collectiie

2ct

4 I. ‘. Ic, computation tith qualified n&hods

NOrmal 8 upset trans. for unlimited multiple SpeCifM load collectives

at ’

BB-Behaviour

b’ ifZc,‘
_-3,2

) Fatigue crack growth of leaking crack

..

NOrmal IL upset trans.

) Fatigue crack gresulting in proof of integrity

Fig. 7. Break preclusion

dficient margin: :< / zc, *’

2c, ’ > Zc, , then luivalent safety e*S”WO !ClZSSary

(BP): LBB-behaviour.

German

of T . W. C.

LBB procedures

methods

operation

143

margin: 2c. I 2CLDs

Fig. 8. Break preclusion (BP): LBB-behaviour.

fatigue crack growth condition is demonstrated (‘leak-before-break’) (see Fig. 5). For condition (2,r < 2,.c) sufficient safety margins have to be reached. The criterion is to demonstrate LBB fatigue crack growth with unlimited specified plant load collectives. (b) If the crack becomes critical before it goes through the wall (2,,, > 2cc), then the condition of ‘break-before-leak’ exists (see Fig. 5). For condition (2,,. > 2,.,) equivalent safety measures (e.g. ISI) have to be taken into account to ensure BP (‘proof of integrity’). 3.4 Analysis of surface (Figs 5 and 8)

crack for end of life (EOL)

In this analysis, it is shown that the fatigue crack growth of the reference defect during the whole life (one life) of the plant is limited and that there is no risk of break of the remaining EOL crack size under normal operation + maximum accident load (e.g. SSE = safe shutdown earthquake). The flow stress concept and/or the plastic limit load concept generally are used for defect assessment. To perform this analysis, one needs the corresponding material data at the appropriate temperature, the loads and the geometry. The criterion is that no plastic instability act. to flow stress concept and/or plastic limit load can occur. 3.5 Through wall crack (TWC) stability under maximum loads (Figs 5 and 8)

analysis

The size of the critical TWC is computed under normal transients + maximum accident load (e.g. SSE). The critical TWC is the crack size which leads

to instability under normal operation + the maximum accident load. Types of loads are the pressure + thermal expansion loads + weight. Simplified fracture mechanics approaches are used: the flow stress concept4,6,7 and/or plastic limit load.’ Sufficient safety margins have to be reached. 3.6 Completeness of loads used for fatigue growth and crack stability

crack

The main principle is that all failure modes are identified and ruled out, either by previous experience or predictability of margins against such failure implies that the operating modes. The principle conditions are known with confidence at the design stage. Particular care is given to: 9 high external loads of erratic natures (such as seismic loads, water hammer, unstable flow conditions): operating thermal transients (in lines with stratification potential, two phase flow lines, lines with unstable flow conditions): vibrations (induced by high flow velocity, rotating machinery): lines with potential for corrosion or erosion (such as high fluid velocity lines). l

l

l

The operating conditions have to be well defined at the design stage by all the sophisticated analyses performed (transients, seismic) and with reference to operating plants. The utility experience of plants has to complete the plant design transient list with new transients

G. BartholomL;

!4-i

during the hrst years of operation, greater or smaller temperature variation (or gradients) for an identified transient and different number of cycles. The LBB demonstration, particularly fatigue crack growth in normal and upset conditions, based on the design transient list, has to be assessed with great confidence. Transient monitoring and book-keeping further reinforce this confidence.

nwnitorrd

3.7 Leak rate of stable TWC (Figs 5 and 8) The crack area computation is performed using crack length under full power operation (without SSE) load. Qualified and internationally recognized methods are used. Lower bounds of the leak areas are used to establish conservative predictions about leak rates. 3.7.1 Calcidatior2 description The leak flow through a TWQ is assimilated to the case of critical flow through a long pipe with a large hydraulic resistance. The utilization domain of the calculation covers thermohydraulic conditions, ranging from subcooled water up to saturated steam. The critical flow model uses the two-phase homogeneous model and a model for subcooled conditions. 3.7.2 Quali$cation Since the flow through a through wall crack is generally characterized by a length to hydraulic diameter ratio the flow pattern along the flow is considered to be homogeneous. Non-equilibrium critical flow may appear at the outlet for subcooled and very low flow conditions. The calculation assessment relative to LBB-applications was conducted, among others, by comparisons with different appropriate experimental test results. 3.7.3 Parameters influencing the mass flow rate All LBB-estimation schemes require a prediction of the size and shape of the crack in the pipe. For these reasons, it is important to understand the uncertainties in the leakage prediction that are due to uncertainties in the mechanical results. The most sensitive variables for both wavy and straight cracks are crack-opening displacement, crack shape (i.e. elliptical, diamond or rectangular), crack length, entrance discharge coefficient and crack-face waviness. The relevant parameters are taken into account. 4 LEAK DETECTION SYSTEM CAPABILITIES (Fig. 5) The necessary redundancies l l l l

(LDS)

(surveillance

in-service monitoring (load recording); leak detection system; water chemistry surveillance; in-service inspection.

and ISI) are:

As an example, the capabilities of the lI>N In Germany are described below. ‘..’ The primary circuir is monitored for potential leakage by means g!l ;I system of devices for measuring humidity. temperature, condensate flow and airborne activity as well as the level of water in the building sumps. The location of the instrumentation used for measuring the condensate flow. noble gas activity and sump water levels is such that the source of leakage can bc narrowed down to various large equipment compartments. Monitoring through measurement of humidity, temperature and condensate flow is based on changes in the following variables: rise in temperature of 10 K above normal temperature in compartment; rise in relative humidity of 10% above normal humidity in compartment; condensate flow of 043.5 kg/s from coolers of air recirculation system. SIEMENS-KWU states the following response times calculated for an assumed leakage of 2 kg/s of saturated steam as a function of compartment size: Volume capacity Of compartment Pressurizer valve compartments Air supply ducts Reactor well Upper steam generator compartments

Temp. approx.

Humidity air COOh

ROW

approx.

approx.

177m’

10s

0.8 s

250 m’

20

1.3 s

1080 m’

100s

750 m’

Condensate

280 5

ss

200 s

3,s s

5 APPLICATION”’ 5.1 Experience

with Siemens-reactors

(PWR,

BWR)

Prior to the introduction of the BS-concept in Germany (1979)‘,2,” -whose main aspect is to verify quality through production, etc., see Fig. 2-there was in place an extensive research and development experimental study testing worst case conditions and theoretical study of fracture mechanics application with the aim to improve the safety calculation, to preclude critical large breaks and to improve the access of the piping systems for IS1 by avoiding pipe whip restraints. It has to be shown that any through-wall crack will lead to a leak that must be detected long before becoming unstable ( = critical crack length) by an

German LBB procedures ~ No.

Plants

Commercial operation

Type

-

ABC-

145

T

L

1

Obrigheim

311969

PWR

+

2 3 4

Stade Atucha 1 Eiblis A

5 I1972 6 I1974 2 I d975

PWR PHWR PWR

+

5 6

Bor55ele UnteMutr

1011973 9 11979

PWR PWR

7 a

Biblis B Neckar 1

114977 12/1976

PWR PWR

9 10

GGrgan Grafenrheinfeld

1011979 611982

PWR PWR

+ + +

II 12

Gmhnde Philippsburg

2 I1905 4!1985

PWR PWR

+ +

13 14

Bmkdorf Neckar 2

1211986

PWR

+

15

Trillo

411989 11 I1988

PWR PWR

+ +

16 17

Angn2+3 Atucha 2

18 19

lsar 2 Emsland

2

1

PWR PHWR 4 I 1988 611988

PWR PWR

PWR: MCL; BWR: Feedwaterand Steamline Other piping without backfltting Other piping with backfitting (e. g. leak detection material evaluation, exchange of piping, etc.)

IS

A

C

+ + + +

+ + 1+ + + + +

system,

Fig. 9. Status of Siemens/KWU plants applying the LBB concept.

appropriate leak detection system. The LBB-concept has been applied to all Pre-Convoi and Convoi-plants for the main coolant piping, partially to the surgeline. It was also partially realized by replacement work in nearly all former built Siemens plants. The German LBB-concept was applied also in The Netherlands, Brazil and Argentina. An overview on the applications of LBB to Siemens PWR and BWR is given in Fig. 9.

(Greifswald

l-4)

1991:

(VVER-440/V230).

Slovakia

(Bohunice)

44O/V230).

3 and 4 (together

with

and EdF) was performed. l-4)

to primary

(VVER-440/V230).

piping

(main

coolant

line, surgeline, emergency core cooling line) was

(VVER

a LBB-analysis

VVER

to primary piping system of Kola 1

and 2, Novovoronesh

LBB-application

with Viniaes a study was performed by Siemens on the possibilities of LBB-analysis in Russian plants. GDR

1996: Russia (TACIS91/1.2

LBB-application

1996: Bulgaria (Kozloduy

1989: Russia (Viniaes). Together

440/23(J). Siemens performed the primary piping system.

2) (VVER-440/V230).

Within the WANO-Six-Months Program, under the leadership of Consortium EdF-Siemens together with Russian and Bulgarian partners a LBB-analysis of primary piping systems (main coolant line, surge line) was performed.

Framatome

5.2 Experience with WER-reactors in respect to LBB-concept (LBB confirmed under defined restrictions)

1990:

1992: Bulgaria (Kozloduy

on

Siemens performed a LBB-analysis of main coolant line and surge line in cooperation with a Czechoslovakian plant operator and research institutes (NRI, Prag; Welding Institute Bratislava). 1991: Vienna, Austria. IAEA Participation of

Siemens and Framatome in an IAEA-Experts-meeting on integrity of LBB-components (February, 1991).

performed

by Siemens in cooperation

with Bulga-

rian partners (PHARE-program). 1996: Russia (RBMK). A TACKS-program

performed AEA

to analyse LBB by a consortium

was of VTT,

and Siemens.

REFERENCES 1. KuBmaul, K.. Developments in nuclear pressurevessel and circuit technology in the Federal Republic of Germany. In SMiRT-6, Post Conference Seminar No. 8, Session1, Paper no. 1, Paris, 1981. 2. KuRmaul, K., German basic safety concept rules out possibility of catastrophic failure. Nuclear En,qinrering International, Dec. 1984,41-46.

! Ih

G. BartholonlP

:. Reactor Safety Commission (RSC), 173. Session of l7.‘.19X7-, 19x9 1. Bartholom6, %., Steinbuch, R. and Wellein, R.. Preclusion of double-ended circumferential rupture of the main coolant line. Nuclear Engirzeerirzg und Design, 1982,72(l), 97-105. 5. Roos. E., Herter, K.-H., Julisch, P., BartholomC, G. and Senski. G.. Assessmentof large scale pipe tests by fracture mechanics approximation procedures with regard to leak-before-break. Nuclear Engineering and

6. Bartholom6. G.. Break preclusion concept 111( rrbrm;lnv In SISSI 94. Saclay IrWrnutionul Swzirw 011 .St~wTliriii Integrity. INSTN, Saclay, France, 109-t. In I’riiri~iplc.5 (I\ fracture mrdiunics applicatiori irl ,Vlldwr f’o~~w Pliitlri. pp. 309-331, 7. Bartholomd, G., Keim, E., Kastner. W.. Knoblach. U’. and Wellein, R., Application of LBB in German NPP. In Leak-Before-Break

in Reactor Pipirlg

cd

Vr.w~lr

9.5,Lyon, France. 1YYS.paper P3. pp. P3.1-P3.15.

LBB