RELIABILITY DATA SOURCES IN THE PAKS NUCLEAR POWER PLANT

E. HOLLO

Institute for Electrical Power Research, Budapest

J. TÖTH

Paks Nuclear Power Plant, Paks

Hungary Abstract

In the Paks Nuclear Power Plant with four unit in operati-on there is a 10 reactor years operatioperati-onal experience. This fact makes possible to use the results of the operational experience for probabilistic safety evaluations.

For safety evaluations up to nov; we used datas given by the supplier - what was a limited source -, and datas obtained

In the near future for safety performances we would like to use plant specific datas, gained from the Paks Nuclear Power Plant.

This paper describes the existing data sources in the Paks Nuclear Power Plant, which contains information and statis-tical datas, necessary for safety evaluations. These sources are

Incident investigation report Safety related event report

Report of events with reduction of plant power

Component failure data system (for a limited nunber of components).

1 . Int ro duct ion

In Hungary a c t i v i t i e s in p r o b a b i l i s t i c s a f e t y assessment of n u c l e a r power plant started approx. 4 years ago. As part of the level I PSA of Paks NPP several case studies have been p e r f o r m e d . Parallely with p e r f o r m i n g level I PSA d i f f e r e n t p a r t i c u l a r system r e l i a b i l i t y p r o b l e m s have been analysed during the last two years, such as optimal test interval of the s a f e t y s y s t e m s , p o s s i b i l i t y of maintenan-ce of the c o n d e n s e r cooling pumps during o p e r a t i o n of the plant. There are some other p r o b l e m s which have to be sol-ved in the near f u t u r e by means of r e l i a b i l i t y analysis.

The s a f e t y of our plants was aproached in a d e t e r m i n i s t i c way. In our o p e r a t i o n a l manuals there are several techni-cal s p e c i f i c a t i o n s and limiting c o n d i t i o n s of o p e r a t i o n which a r e , we c o n s i d e r , very p e s s i m i s t i c . Some of these tech specs and LCOs might be overviewed using PSA tech-nics .

For the above m e n t i o n e d r e l i a b i l i t y and s a f e t y evaluations up to now we have mainly used r e l i a b i l i t y data taken from p u b l i c a t i o n s .

Since the start up of the f i r s t unit of the Paks NPP (28. dec . 1 982) 10 reactor years o p e r a t i o n a l experience have been gained . ( N o w there are f o u r u n i t s in o p e r a t i o n . ) T h i s could have been s u f f i c i e n t to f e e d back the operational experience and as a form of f e e d back, to use plant speci-fic data for P S A . U n f o r t u n a t e l y not a total reliability data c o l l e c t i o n was initiated immediately a f t e r the start up of the plant.

Especially many problems we had c o n c e r n i n g the reliability data of the components.

In this paper we b r i e f l y describe those data sources which

2. Existing Data Sources

At the present time in the Paks N u c l e a r Power Plant there are several i n c i d e n t and m a l f u n c t i o n recording s y s t e m s . These report systems were established to p r o v i d e i n f o r m a t i o n for the o p e r a t i o n a l o r g a n i z a t i o n s . At the time of the f o r m a t i o n of the r e q u i r e m e n t s for the f o r m and c o n t e n t s of these reports the PSA was not in general use in Hungary. T h e r e f o r e such points of view which c o u l d guarantee the u s a b i l i t y for P S A , could not have been taken into c o n s i d e r a t i o n . N e v e r t h e l e s s these reports contain a lot of u s e f u l i n f o r m a t i o n for s a f e t y e v a l u a t i o n s . Just the proper way should be f o u n d , how to extract and evaluate the i n f o r m a t i o n from the i n c i d e n t r e p o r t s .

As far as the component f a i l u r e r e p o r t i n g system is c o n c e r n e d , the s i t u a t i o n is worse. The c o m p o n e n t (or equipment) f a i l u r e recording s y s t e m , established at the Paks N u c l e a r Power Plant a f t e r the start up of the f i r s t u n i t s was, one c o u l d say "too simple" and inhomogenous. All

t h e d i f f e r e n t m a i n t e n a n c e o r g a n i z a t i o n s ( m e c h a n i c a l , ! < & C , e l e c t r i c a l ) r e c o r d e d f a i l u r e i n f o r m a t i o n in d i f f e r e n t way concerning both the f o r m and the c o n t e n t . The basic

i n f o r m a t i o n for the c a l c u l a t i o n of r e l i a b i l i t y parameters of the components were not r e c o r d e d or were r e c o r d e d in an i n s u f f i c i e n t way. For t h i s reason a f a i r l y new system must be e s t a b l i s h e d , to get f u l l f a i l u r e s t a t i s t i c s of the c o m p o n e n t s . The concepts of such a r e l i a b i l i t y data system were d e s c r i b e d by the authors of this paper about two years ago/Y/ U n f o r t u n a t e l y during the last two years the develop-ment of the above m e n t i o n e d system has not gone in a way

and at a rate as we had planned it. This fact has f i r s t of all reasons concerning o r g a n i z a t i o n and i n t e r e s t .

organizations are envolved in the data r e c o r d i n g , the less the accuracy of the r e c o r d e d data is.

For these reason in January of 1988 we started a limited scope data recording system for a limited number of compo-nents ( so called stand by s a f e t y s y s t e m s ) , which may be operated e n t i r e l y by only one organization which is inte-r e s t e d in the accuinte-racy of the c o l l e c t e d data.

Now we would like to describe very b r i e f l y those types of reports which in some way could be used as r e l i a b i l i t y data s o u r c e s , including the new l i m i t e d scope c o m p o n e n t f a i l u r e recording s y s t e m .

2.1. I n c i d e n t investigation report S a f e t y related event report

Incident investigation must take place in case, when an occurence is c l a s s i f i e d by the e l e c t r i c a l network dis-patcher as an "incident" ( loss of power-production event ).

The requirements to the content of the report is descri-bed in Appendix A. The "incident" c l a s s i f i c a t i o n is not d e f i n e d by the f a c t , whether the nuclear s a f e t y of the plant is concerned or n o t .

It is d e f i n e d by the consequences from the operational point of view of the electrical n e t w o r k .

T h e r e f o r e for those cases when the o c c u r e n c e is not c l a s s i f i e d as an incident by the network d i s p a t c h e r , but

As we can see both kinds of reports contain p r a c t i c a l l y the same i n f o r m a t i o n . In both reports there may be f o u n d the d e s c r i p t i o n of the i n i t i a l s t a t e , a d e t a i l e d d e s c r i p t i o n of the event s e q u e n c e s , e v a l u a t i o n s of the event, d e s c r i p t i o n of the operator a c t i o n s , human respon-s i b i l i t y and the n e c e respon-s respon-s a r y mearespon-surerespon-s to be u n d e r t a k e n . 2 . 2 . Load r e d u c t i o n report

The load r e d u c t i o n report system actually is the f i r s t f o r m of the equipment f a i l u r e r e p o r t i n g s y s t e m . But un f o r t u n a t e l l y this system is expanded o n l y for those equipments whose f a i l u r e causes load r e d u c t i o n . T h e r e f o r e good s t a t i s t i c s may be o b t a i n e d o n l y for those c o m p o n e n t , any failure of which causes load reduction. As far as this recording system - like the incident reporting sys-tem - have e x i s t e d since the start up of the f i r s t unit of the plant, they are suitable for evaluation of the p r o b a b i l i t y of some initiating events.

But for c a l c u l a t i o n of component r e l i a b i l i t y parameters the data c o l l e c t e d in this system are still not adequate.

2 . 3 - F a i l u r e report

As it was m e n t i o n e d e a r l i e r , in J a n . 1988. a new f a i l u r e report system was started for a limited number of systems and c o m p o n e n t s , with l i m i t e d p o s s i b i l i t i e s . For basis of this systems serves the periodical test of the s a f e t y systems a n d , some a u t o m a t i o n s and p r o t e c t i o n s .

The s a f e t y systems and the main automations and p r o t e c t i o n s are t e s t e d once during six weeks according

Special test must be p e r f o m e d for two operable s a f e t y systems in the case, when one of the three systems is inoperable during a p e r i o d of time not exceeding 24 ho-urs , t o o .

In case, when during the test any equipment f a i l s , a f a i l u r e report f o r m has to be f i l l e d in ( s e e Appendix E). By means of this data recording system the r e l i a b i l i t y of those components may be evaluated w h i c h are characterised by f a i l u r e per d e m a n d . Systematic d o c u m e n t a t i o n of the p e r f o r m e d tests and detected f a i l u r e s will assure the accurate number of f a i l u r e s and demands for all c o m p o n e n t s .

3. Use of Generic and Plant S p e c i f i c Data

As was indicated in preceeding chapters for s a f e t y evaluations up to now d a t a obtained from d i f f e r e n t existing data banks and generic data f i l e s have been u s e d . Plant s p e c i f i c data given by tbe supplier or extracted from plant operational statistics were used in few s p e c i f i c cases.

For c o m p i l a t i o n and a u t o m a t i s e d r e t r i e v a l of g e n e r i c data a c o m p u t e r i z e d data base is being set up Z . P r e s e n t l y the data base involves I N I T I A T I N G EVENT AND C O M P O N E N T F A I L U R E DATA f i l e s .

I N I T I A T I N G EVENT cathegories a r e : LOCA and T R A N S I E N T o n e s . COMPONENT FAILURES are grouped on

- t y p e s , as T E C H N O L O G I C A L , E L E C T R I C A L , and C/I f a i l u r e s , - m o d e s , as STAND-BY and O P E R A T I O N A L o n e s .

For c o m b i n a t i o n of data f r o m d i f f e r e n t sources simple avarage rule using weighting f a c t o r s is u s e d . Files listed

NUREG-3862 Transient IE Frequences , 1985 EPRI-801 ATWS F r e q u e n c i e s , 1978 ANO-1 ARKANSAS IREP Study , 1982 RKS 85-25 R e l i a b i l i t y Data Book , 1985 NKA SAK-1 PRA Uses and T e c h n i q u e s , 1985

(A Nordic P e r s p e c t i v e )

EUR 10696 CEC Benchmark E x e r c i s e , 1986

For i l l u s t r a t i o n of compiled generic f i l e s t r u c t u r e s a sample case sub-file is given in Appendix F.

Plant s p e c i f i c operational and f a i l u r e data have l i m i t e d l y been used in the f o l l o w i n g areas:

- to d e f i n e mean time b e t w e e n f a i l u r e s and average repair t i m e s , e.g. for c o n d e n s e r cooling pumps within t h e i r maintenance p o l i c y study,

- to f o r m u l a t e auxiliary a s s u m p t i o n in n u m e r i c a l d a t a f o r m f o r given c a l c u l a t i o n , e.g. f o r F R A N T I C c o d e t o d e f i n e q . u n a v a i l a b i l i t y of test o v e r r i d e c a p a b i l i t y within s a f e t y system test period o p t i m i z a t i o n s t u d y ,

- to s c r e e n causes of plant outages and load r e d u c t i o n s , to estimate d i s t r i b u t i o n o f f a i l u r e s w i t h i n d i f f e r e n t f a i l u r e modes and s y s t e m s ,

- to v e r i f y a - p r i o r i assumed event s e q u e n c e s through operational tests or real cases , e.g. to make clear m i s s i o n times w i t h i n r e d u c t i o n of f e e d w a t e r event

sequence study,

to e s t a b l i s h r e a l i s t i c s u c c e s s c r i t e r i a for redundant s a f e t y system t r a i n s , e.g. within small and large LOCA event sequence s t u d i e s .

4. Gone lus ion

Data contained in different plant reports described in