ML12093A094

Response to April 24, 2990 Letter Concerning the Direct Torus Vent at the Pilgrim Nuclear Power Station

ML12093A094
Person / Time
Site:	Pilgrim, EA-12050
Issue date:	04/02/2012
From:	Carr K Office of Nuclear Reactor Regulation
To:	Griffin W Town of Plymouth, MA
SECY RAS
References
Commission-1, RAS 22182, EA-12-050 IR-88-007, IR-88-012
Download: ML12093A094 (7)

Text

UNITED STATES N U C L E A R REGULATORY COMMISSION WASHINGTON. 0 . C. 10555 (2J~& -7 June 2 1 , 1990 CHAIRMAN C

ri r

- , .-.7 :

Mr. W i l l i a m R . G r i f f i n ,

Executive Secretary Town o f P l y m o u t h O f f i c e of t h e Selectmen '- .

11 L i n c o l n S t r e e t  ::: 1 ., :

Plymouth, Massachusetts 02360 D e a r Mr. G r i f f i n :

I am r e s p o n d i n g t o y o u r l e t t e r of A p r i l 2 4 , 1 9 9 0 , c o a c e r n i n g t h e d i r e c t t o r u s v e n t a t t h e P i l g r i m N u c l e a r Power S t a t i o n . I r e f e r r e d t h e 1 2 s p e c i f i c q u e s t i o n s you r a i s e d i n y o u r l e t t e r t o t h e N u c i e a r R e g u l a t o r y Commission ( N R C ) s t a f f , a n d t h e i r d e t a i l e d r e s p o n s e s a r e e n c l o s e d . Some a d d i t i o r , a l b a c k g r s u n d i n f o r m a t i o n t h a t may be h e l p f u l t o y o u i s a l s o e n c l o s e d .

I hope t h e i n f o r m a t i o n w t a r e p r u v i d i n ~w i l l l r a d t o a b e r r e i .

u n d e r s t a n d i n g of t h e g e n e r i c i s s u e s a s s o c i a t e d w i t h v e n t i n g , a n d ,

i n p a r t i c u l a r , h o w t h e y r e l a t e t o t h e P i i g r i m N u c l r i r Power 5tbiion. I f y o u h a v e a n y f u r t h e r q u e s t i c n s , p l e i s r c o n t a c r mc n?

Mr. T . T . l$lsr;in, A d m i n i s t r a t o r o f H F t C ' s R e g i o n I o f f i c e .

X r . M a r ~ i nc a n b e r e z c h e d by t e i e p h c n e e t ( 2 1 5 ) 3 3 7 - 5 ' 5 s .

Sincerely,

Enclosures:

1. B a c k g r o u n d Information

2. Responses t o Concerns 3 . SECY-89-017

4. Inspection R e p o r t No. 5 0 - 2 9 3 / 8 6 - 0 7

5. lcspecticn R e p o r t N o . 50-293/PP-12

Enclosure 2 Response to Concerns Raised by W.R. Griffin The following items briefly sumnarize current information concerning the hardened vent. They are organized as specific responses to issues raised in your letter to Chairman Carr. You should note that two descriptive terms routinely used within the industry mean the same thing: both the "direct torus vent" and the "hardened wetwell vent describe the vent path to the stack. For purposes of the following responses, they are equivalent.

Question 1 (0 1 ) : What are the decontamination factors for the 0001 for various isotopes? In other words, how well dbes the wet well pool scrub out the fission by-products, keeping the radioactive particles from releasing to atmosphere?

\

Response: Except for the noble gases (consisting of the isotopes of Xenon and Krypton), which are not retained by the pool to any significant degree, the suppression pool is highly effective in scrubbing out and retaining particulate and volatile fission products. Calculations as well as tests indicate thkt the sup-pression pool would be expected t o have a realistic decontaninatior, factor ( O F ) for particulate and volatile fission products of about I

100, depending upon the accident sequence and the temperature of the water. This means that cbout 1 percent of the particulate a r ~ d volatile radioactivity entering the pool would be relezsed to the atmosphere, and about 99 percent would be retained within the pool.

The wetwell pool is highly effective with a OF of about 100 in scrubbing particulate and volatile fission products, but not effective in scrubbing noble gases with a DF of 1.

Q 2: Please provide a graph of offsite radiation doses based on the possibility of a vacuum breaker valve remaining open at ID%, 25%.

50% and 100%.

R ~ s D o ~ s ~ :The staff does not have the off-site radiation dose evaluation requested in your latter. This type of failure was not considered in the Ges!gn basis for the facility since it was not considered to be a credible event. The basis for the staff's position i n this regard is as follows.

The vacuum relief for both the drywell and wetwell is provided by two 100 percent vacuum relief breakers located in t ~ penetrations o

in the wetwell containment shell. These penetrations terminate in the reactor building, which is generally referred to as the secondary containment.

Each p e n e t r a t i o n c o n s i s t s o f a vacuum b r e a k e r and an a i r o p e r a t e d b u t t e r f l y v a l v e i n s e r i e s . D u r i n g normal o p e r a t i o n , b o t h v a l v e s a r e closed; t h e vacuum b r e a k e r i s m a i n t a i n e d c l o s e d by t h e w e i g h t of t h e d i s k , and t h e b u t t e r f l y v a l v e i s m a i n t a i n e d c l o s e d by p o s i t i v e a c t u a t o r a i r pressure.

I n t h e e v e n t of a l o s s - o f - c o o l a n t a c c i d e n t ( L O C A ) , t h e i n c r e a s i n g w e t w e l l p r e s s u r e w i l l add t o t h e c l o s i n g p r e s s u r e o f t h e vacuum breaker. As a r e s u l t , i t i s a n t i c i p a t e d t h a t d u r i n g t h e e n t i r e p o s i t i v e pressure h i s t o r y w i t h i n t h e containment, n e i t h e r v a l v e i n t h e p e n e t r a t i o n w i l l move f r o m i t s c l o s e d p o s i t i o n . However, a t t h e end o f t h e p r e s s u r i z a t i o n phase, t h e r e i s a p o t e n t i a l f o r c r e a t i n g a n e g a t i v e p r e s s u r e i n containment. T h i s would o c c u r o n l y a f t e r t h e steam r e l e a s e from t h e r e a c t o r c o o l a n t system has ceased.

As t h e a e t w e l l p r e s s u r e approaches a t m o s p h e r i c , t h e b u t t e r f l y v a l v e i s opened, t h e r e b y a l l o w i n g t h e vacuum b r e a k e r t o p r o p e r l y f u n c t i o n . The vacuum b r e a k e r would b e g i n t o open when t h e w e t w e l l p r e s s u r e becomes s l i g h t l y suh-atmospheric. A i r froc the reactor b u i l d i r . 9 would r e s t o r e t h e w e t w e l l p r e s s u r e back r o a t m o s p h e r i c .

The above sequence d e s c r i p t i c n has focused on t h e Design B a s i s A c c i d e n t (DBA). However, t h e sequence i s e q u a l l y v a l i d f o r a l a r g e number o f p o t e n t i a l severe a c c i d e n t s c e n a r i o s . The d i f -

ferer,ces would he l i m i t e d t o t h e p r e s s u r e r i s e r a t e and t h e maximum p r e s s u r e and t e m p e r a t u r e v a l u e s reache6 C u r i n g t h e e v e n t .

These G i f f e r e n c e s , however, would n o t a l t e r t h e e v e n t s ss d e s c r i b t s above. T h e r e f o r e f o r purposes of c o n s i d e r a t i o n of vacuum b r e a k e r f a i l u r e , t h e s t a f f ' s c o n c l u s i o n s can be c o n s i d e r e d a p p l i c a b l e f o r b o t h DBA and s e v e r e a c c i d e n t events.

Therefore, d u r i n g t h e e n t i r e p o s i t i v e pressure p r o f i l e o f t h e e v e n t , t h e p e n e t r a t i o n has two c l o s e d b a r r i e r s i n s e r i e s . It i s o n l y d u r i n g t h e end o f t h e p r e s s u r i z a t i o n phase t h a t t h e p e n e t r a t i o n i s a l i g n e d i n t o i t s vacuum b r e a k e r r o l e . Because o f t h i s d o u b l e b a r r i e r p r o t e c t i o n and t h e f a c t t h a t b o t h v a l v e s a r e n o t expected t o change p o s i t i o n d u r i n g t h e p r e s s u r i z a t i o n phase of t h e e v e n t , t h e s t a f f has concluded t h a t f a i l u r e o f t h e p e n e t r a t i o n as a l e a k t i g h t b a r r i e r i s n o t c r e d f b l e and need n o t be c o n s i d e r e d i n the design basis.

Q 3: The N R C has recommended v e n t i n g a t t h e c o n t a i n m e n t d e s i g n p r e s s u r e

?.s a minimum, o r i n t h e czse o f P i l p r i m , a t 60 p s i . G!hy i s t h e P i l g r i m D X S r u p t u r e d i s k s e t a t h a l f t h a t , a t 3G p s i ?

Response: The f a c t t h b t t h e P i l g r i m DTVS r u p t u r e d i s k i s designed t c r u p t u r e a t 30 p s i i s n o t r e l a t e d t o t h e MRC's recommendation t h a t s p e c i f i e d t h e v e n t i n g p r e s s u r e a t t h e containment d e s i g n p r e s s u r e . Thc s e t p r e s s u r e f o r t h e r u p t u r e d i s k does n o t c o n t r o l t h e v e n t i n g p r e s s u r e because t h e r e a r e two c l o s e d i s o l a t i o n v a l v e s i n t h e f l o w p a t h .

These two v a l v e s a r e n o r m a l l y c l o s e d and w i l l be opened m a n u a l l y by t h e o p e r a t o r if v e n t i n g i s needed. P i l g r i m ' s v e n t i n g p r e s s u r e i n t h i s case i s c o n s i s t e n t w i t h t h e recomnendations c o n t a i n e d i n Emergency Procedure Guide1 i n e s (EPG) , R e v i s i o n 4 . These g u i d e 1 ines have been approved by t h e s t a f f . The maximum c o n t a i n m e n t p r e s s u r e a t which t h e o p e r a t o r s a r e expected t o open t h e v e n t v a l v e i s 56 p s i g

( n o t 60 p s i ) , which i s c o n s i s t e n t w i t h t h e NRC recomnendation on venting pressure.

The r u p t u r e d i s k i s designed t o s e r v e as an a d d i t i o n a l leakage b a r r i e r a t p r e s s u r e s below 30 p s i . I t i s d e s i g n e d t o open below t h e c o n t a i n m e n t design p r e s s u r e , b u t w i l l be i n t a c t up t o a p r e s s u r e equal t o o r g r e a t e r t h a n t h o s e p r e s s u r e s t h a t cause an a u t o m a t i c c o n t a i n m e n t i s o l a t i o n d u r i n g any a c c i d e n t c o n d i t i o n s .

Therefore, i t s presence i n t h e l i n e c a n e f f e c t i v e l y e l i m i n a t e t h c n e g a t i v e consequences of i n a d v e r t e n t a c t u a t i o n o f t h e v e n t v a l v e s a t p r e s s u r e s below 30 p s i . The s e t p r e s s u r e of 30 p s i f o r t h e r u p t u r e d i s k s a t i s f i e s these d e s i g n o b j e c t i v e s .

Q 4: What i s t h e minimum containment p r e s s u r e a l l o w e d by procedures a t which t h e o p e r a t o r s c o u l d open t h e DTVS o u t b c a r d containment v a l v e , AG-5025?

Response: Use o f t h e d i r e c t t o r u s v e n t w i l l be i n accordance w i r h a ~ p r o v r i EPG requirements a r ~ dw i l l be c c n t r o l l ~ dby Emergency O p e r a t i n ?

Procedures (EOPs) . There i s rlc spec i f i e d ainimum c c n t a i n m e n t p r e s s u r e e l l o w e d by t h e BWR Owners Group EPGs, R e v i s i o n 4, a t w h i c h t h e o p e r a r o r s c o u l d open t h e DTVS o u t b o a r d c c n t a i n m e n t v a l v e .

T h e r e i s a p r i m a r y containment p r e s s u r e l i m i t (PCPL) of 56 p s i g .

P l a n t - s p e c i f i c s u p p o r t i n g a n a l y s e s a r e used t o i n d i c a t e when t h e o p e r a t o r s s h o u l d b e g i n t h e v e n t i n g procedure. These a n a l y s e s c o n s i d e r e d a number o f p l a n t parameters, i n c l u d i n g t h e p r e s s u r e r i s e r a t e . These a c t i o n s ensure t h a t v e n t i n g i s used o n l y i f needed, t h a t t h e c o n d i t i o n s a r e beyond t h e d e s i g n - b a s i s - a c c i d e n t assumptions, and t h a t t h e p r e s s u r e s i n t h e c o n t a i n m e n t do n o t exceed t h e PCPL l i m i t .

Q 5: P l e a s e p r o v i d e i n f o r m a t i o n on t h e r e l i a b i l i t y o f t h e hydrogen and oxygen c o n c e n t r a t i o n m o n i t o r s a t P i l g r i m . What p e r c e n t a g e o f t h e t i m e have h o t h systems been accurste!:' functioning?

Respor~se: The p o s t - a c c i d e n t hydrogenloxygen a n a l y z e r s were i c s t a l l e d i n

& r i ~ . s r y 1985 as par; o f t h e post-TMI d e s i c n m o d i f i c c t i o n s .

S i n c e t h i~n s t a l l a t i o n , one t r a i n ( o f t w o ) wzs i n o p e r a b l e f o r t c c days i n November 1985, asd one t r z i n was i n o p e r i b l e f o r f o u r days i n January 1986, f o r a t o t a l of s i x days. A t no t i m e were b o t h t r a i n s i n o p e r a b l e s i m u l t a n e o u s l y . T e c h n i c a l S p e c i f i c a t i o n 3.7.A.7.c a l l o w s t h e r e a c t o r t o c p e r a t e f o r up t o 7 days ifone t r a i n i s inoperable.

I n a d d i t i o n , t h e containment atmospheric oxygen a n a l y z e r , w h i c h m n i t o r s t h e oxygen c o n c e n t r a t i o n d u r i n g normal o p e r a t i o n , has been e x t r e m e l y r e l i a b l e . The p l a n t s t a f f c o n s e r v a t i v e l y e s t i m a t e d t h i s a n a l y z e r t o have a r e l i a b i l i t y t h a t exceeds 98 p e r c e n t .

Q 6: Does t h e NRC c o n c u r t h a t t h e use of t h e DTVS does n o t i n v o l v e an unreviewed s a f e t y q u e s t i o n ?

Response: Yes. As documented i n NRC I n s p e c t i o n R e p o r t No. 50-233/88-07, d a t e d Way 6, 1988, t h e NRC i n s p e c t e d t h e i n s t a l l e d DTVS d e s i g n c o n f i g u r a t i o n and t h e l i c e n s e e ' s e v a l u a t i o n and d e t e r m i n e d t h a t t h e y were a c c e p t a b l e . V e n t i n g has been approved u n d e r p r e v i o u s v e r s i o n s o f t h e EPGs. The d l r e c t t o r u s v e n t i s i n i t i a t e d b y procedures under c o n d i t i o n s s p e c i f i e d b y t h e EPGs. Because t h e o u t b o z r d v a l v e , A0-5025, i s s e a l e d c l o s e d and s u b j e c t t o l e a k t e s t i n g , t h i s v a l v e s a t i s f i e s t h e p r o v i s i o n s o f 10 CFR P a r t 50, Appendices A and J, w h i c h a r e t h e r e g u l a t i o n s f o r c o n t a i n m e n t i s o l a t i o n and l e a k t e s t i n g , r e s p e c t i v e l y . T h e r e f o r e , t h e NRC c o n c u r r e d t h a t t h e use o f t h e D T V S does n o t i n v o l v e an e n r e v i e w e d s a f e t y question.

Q 7: Goes t h e NRC c o n c u r t h a t t l ~ cuse o f t h e DTVS does n o t r e q u i r e channes t o P i l g r i m ' s T e c h n i c a l S p e c i f i c a t i o n s ?

Response: Yes: t h e NRC agrees t h a t rhe use o f t h e DTVS does n o t r e c u i r e changes t o P i l g r i m ' s T e c h n i c a l S p e c i f i c a t i o n s . Cur inspection r e p o r t s , which were noted i n t h e p r e v i o u s responses, i n c l u a e d c o n s i d e r a t i o n o f p o s s i b l e TS changes, and WE d e t e r m i n e d t h none were needed.

Q 8: Does t h e NRC judge t h e DTVS t o improve t h e s a f e t y a t P i l g r i m ?

Response: Yes. The DTVS p r o v i d e s an improved containment v e n t i n g c a p a b i l i t y f o r decay h e a t removal. The DTVS w i l l p r e v e n t t h e m a j o r i t y of p o s t u l a t e d l o s s o f decay h e a t removal sequences f r o m r e s u l t i n g i n c o r e m e l t and w i l l m i t i g a t e t h e consequences o f t h e r e s i d u a l sequences i n v o l v i n g c o r e m e l t where v e n t i n g t h r o u g h t h e s u p p r e s s i o n p o o l i s f o u n d necessary. A d d i t i o n a l s a f e t y b e n e f i t s o f DTVS a r e d i s c u s s e d i n t h e p r e v i o u s background paragraphs.

q 9: Does t h e NRC c o n c l u d e t h a t t h e i n s t a l l a t i o n and use cf t h e DTVS a r e a c c e p t a b l e u n d e r t h e p r o v i s i o n s o f iO CFR 50.59?

Response: Yes. As WE h o t e d i n =he response t o Q u e s t i o n 6 , t h e s t a f f i n s p e c t e d t h e d e s i g n o f DTVS a t P i l g r i m and fcund t h e i n s t a l l e d system and t h e a s s o c i a t e d a n a l y s i s s c c e p t a b l e . V e n t i n g had been approved under p r e v i o u s v e r s i o n s o f t h e EPGs. The d i r e c t t o r u s v e n t i s

i n i t i a t e d by p r o c e d u r e s u n d e r c o n d i t i o n s s p e c i f i e d b y t h e EPGs. I n a d d i t i o n , t h e i n s t a l l a t i o n o r use o f t h e d i r e c t t o r u s v e n t w i l l n o t i n c r e a s e t h e p r o b a b i l i t y of a new a c c i d e n t . T h e r e f o r e , t h e i n s t a l l a t i o n and u s e of t h e DTVS a r e a c c e p t a b l e u n d e r t h e p r o v i s i o n s of 10 CFR 50.59.

Furthermore, i n a supplemental assessment o f O c t o b e r 12, 1988, t h e NRC s t a f f concluded t h a t t h e S a f e t y Enhancement Program (SEP) m o d i f i c a t i o n s b e i n g implemented i n accordance w i t h 10 CFR 50.59, i n c l u d i n g t h e DTV m o d i f i c a t i o n , would enhance t h e o v e r a l l p l a n t s a f e t y and performance of P i l g r i m .

Q 10: Does t h e NRC conclude t h a t Boston E d i s o n has a d e q u a t e l y c o n s i d e r e d t h e t e c h n i c a l i s s u e s germane t o t h e DTVS?

Response: Yes. Based on t h e n o t e d i n s p e c t i o n s and r e v i e w s o f t h e P i l g r i m SEP, t h e NRC s t a f f concludes t h a t t h e s a f e t y i s s u e s a s s o c i z t e d w i t h t h e DTVS have been a d e q u a t e l y c o n s i d e r e d .

F 11: Why was t h e a u t o m a t i c r e c l o s u r e on h i g h r a d i a t i o n of v a l v e AO-5025 d e l e t e d d u r i n g t h e d e s i g n r e v i s f c n o f t h e system?

Response: The r e c l o s u r e o f v ~ l v eAO-5025 w a s d e l e t e d because t h i s r e c l o s u r e ,

i f performed a t h i g h r a d i a t i o n l e v e l s , would i s o l a t e t h e vefit f l o k p a t h when v e n t i n g i s n e e d e l t c m i t i g a t e the o v e r p r e s s u r e chal:er,ge.

Thus, a u t o m a t i c r e c l o s u r e c o u l d d e f e a t t h t purpose of t h e v E K t design.

Q 12: Generic L e t t e r 89-16 i n d i c a t e s some b e n e f i t s o f a hardened w e t w e l l v e n t t o r e d u c e c o r e damage f r e q u e n c i e s d u r i n g SBO [ s t a t i o n b l a c k o u t ] and ATWS [ a n t i c i p a t e d t r a n s i e n t w i t h o u t scram] a c c i d e n t scenarios. I s t h i s t r u e f o r P i l g r i m ?

Response: Yes. The i s o l a t i o n v a l v e s , A0-5025 and AO-50425, a r e d e s i g n e d w i t h ac independent power s u p p l i e s . These two v a l v e s a r e powered f r o m e s s e n t i a l dc power and a r e backed up w i t h d i v e r s e n i t r o g e n actuation capability. T h e r e f o r e , i n case o f an SBO e v e n t , t h e v a l v e s would be a v a i l a b l e f o r v e n t i n g . The v e n t i n g c o n c e p t i s m a i n l y designed t o s l o w o v e r p r e s s u r e t r a n s i e n t s o f t h e c o n t a i n -

ment. D u r i n g some ATWS e v e n t s , t h e p r e s s u r e i n t h e c o n t a i n m e n t w j ! l r a p i d l y i n c r e a s e . Ventin! p r e s s u r e c o u l d be reached i n a m a t t e r o f minutes r a t h e r t h a n hours. T h e r e f o r e , v e n t i n g may n o t p r e v e n t c o n t a i n a e n t f a i l u r e because o f tCe h i g h c o n t e i n m e n t p r e s s u r i z a t i o n r a t e b u t would p r o v i d e a d c i t l o n a l t i m e t o scram t h e r e a c t o r and d e l a y t h e c o r e me1 t.

Enclosure 1 Backqround I n f o r m a t i o n Related t o P i l g r i m S t a t i o n ' s D i r e c t Torus Vent System (DTVS1 On January 23, 1989. t h e NRC s t a f f presented i t s r e c o m e n d a t i o n s on Mark I containment performance improvements and o t h e r s a f e t y enhancements t o t h e Comnission i n SECY 89-017. It represented t h e completion of the s t a f f e f f o r t s on t h e Containment Performance Improvement (CPI) Program f o r Mark I containments.

The program was e s t a b l i s h e d t o determine what a c t i o n s , i f any, should be taken t o reduce t h e v u l n e r a b i l i t y of containments t o severe-accident challenges.

From t h i s p o i n t of view, t h e s t a f f proposed t h a t hardened v e n t c a p a h i l i t y would enhance p l a n t c a p a b i l i t i e s w i t h regard t o both severe a c c i d e n t prevention and mitigation.

Sum low p r o b a b i l i t y scenarios i n which m u l t i p l e f a i l u r e s occur can lead t o containment f a i l u r e . Containment f a i l u r e from these scenarios can r e s u l t i n a l o s s of c o o l i n g water which i s used t o remove decay heat. The i n s t a l l a t i o n of a hardened vent g r e a t l y reduces the l i k e l i h o o d o f e a r l y containment f a i l u r e and, therefore, reduces t h e r i s k s t o t h e p u b l i c because c o o l i n g c a p a b i l i t y i s maintained. For o t h e r sequences f o r which core m e l t i s p r e d i c t e d , ventinp c o u l d be e f f e c t i v e i n d e l a y i n g containmect f a i l u r e and i n m i t i g a t i n g the r e i e r s e o f f i s s i o n products. Although venting o f t h e containmefit i s c u r r e n t l y i n c l u d e d i n BWR emergency o p e r a t i n g procedures t o improve t h e s u r v i v a b i l i t y c f the conteinment, which acts as the l a s t b a r r i e r f o r an ~ n c C r t t r 0 l l e drelease of r a d i a t i o n , i t g e n e r a l l y uses a vent path t h a t i n c l u d e s ductwork w i t h a low design pressure. Venting under high-pressure savere-accident c o n d i t i o n s coule f a i l t h i s ductwork, release t h e containment atmosphere i n t o t h e r e a c t o r b u i l d i n g , and damage equipment o r contaminate equipment needed f o r accident recovery. Venting through t h i s ductwork may hamper o r complicate post-accident recovery a c t i v i t i e s . The i n s t a l l a t i o n o f a r e l i a b l e hardened wetwell vent a l l o w s f o r c o n t r o l l e d v e n t i n g through t h e wetwell w h i l e p r o v i d i n g a path w i t h s i g n i f i c a n t scrubbing c a p a b i l i t y o f f i s s i o n products t o t h e p l a n t stack and prevents damage t o equipment needed f o r accident recovery. Based on the s t a f f ' s recommendation, t h e Comnission d i r e c t e d t h e s t a f f t o a l l o w t h e l i c e n s e e s t h a t e l e c t e d t o i n c o r p o r a t e t h i s p l a n t improvement t o i n s t a l l a hardened wetwell vent i n accordance w i t h t h e Commission's r e g u l a t i o n s (10 CFR 50.59). P l a n t s p e c i f i c b a c k f i t analyses were d i r e c t e d f o r t h e remaining p l a n t s w i t h Mark I containments. Where these analyses supported i m p o s i t i o n o f a hardened vent, t h e s t a f f w e s d i r e c t e d t o i s s u e orders requiring t h i s modification.

P r i o r t o t h e Com;;lission d e c i s i o n i n t h i s m t t t e r , numerous discussicns w i t h b ~ t p '

i n d u s t r y groups and i n d i v i d u a l licensees were conducted. These discussions included meetings w i t h Boston Edison ( t h e l i c e n s e e f o r P i l g r i m ) . The purposs of these d i s c u s s i o n s was t o gather e l l a v a i l a b l e i n f o r m a t i c n r e l a t i v e t o the hardened vent t o enable t h e s t a f f t o m k e an informed d e c i s i o n . During t h i s process, Boston Edison proposed t o i n s t a l l the D i r e c t Torus Vent System (DTVS).

The licensee had concluded t h a t i t had s u f f i c i e n t i n f o n a t i o n t o commit t o a s p e c i f i c design f o r hardened wetwell vents. The proposed m o d i f i c a t i o n was c o n s i s t e n t w i t h t h e s t a f f ' s generic f i n d i n g f o r Mark I p l a n t s . However, t h e s t a f f d i d n o t use t h e P i l g r i m design as a t e s t case, as i s i n d i c a t e d i n your letter.