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 NUCLEAR REGULATORY COMMISSION

-7 (2J~&

WASHINGTON. 0. C. 10555 June 21, 1990 CHAIRMAN C -

r i r

7:

Mr. William R. G r i f f i n E x e c u t i v e S e c r e t a r y Town o f Plymouth O f f i c e of t h e S e l e c t m e n 11 L i n c o l n S t r e e t

1., :

Plymouth, M a s s a c h u s e t t s 02360

Dear 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 Nuclear 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, and 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 backgrsund 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 you 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,

in p a r t i c u l a r, how 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 5 t b i i o n.

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 Region I o f f i c e.

Xr. M a r ~ i n can be 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.

S i n c e r e l y,

E n c l o s u r e s :

1.

Background I n f o r m a t i o n 2.

Responses t o Concerns 3.

SECY-89-017 4.

I n s p e c t i o n R e p o r t No.

5 0 - 2 9 3 / 8 6 - 0 7 5.

l c s p e c t i c n R e p o r t N o. 50-293/PP-12 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

Response

\\

I Q 2:

R ~ s D o ~ s ~ :

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?

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 (OF) for particulate and volatile fission products of about 100, depending upon the accident sequence and the temperature of the water. This means that cbout 1 percent of the particulate ar~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.

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%.

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 ~ o penetrations 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 breaker and an a i r operated b u t t e r f l y valve i n s e r i e s.

During normal operation, b o t h valves a r e closed; t h e vacuum breaker i s maintained closed by t h e weight of the d i s k, and t h e b u t t e r f l y valve i s maintained closed by p o s i t i v e a c t u a t o r a i r pressure.

I n the event of a loss-of-coolant accident ( L O C A ), the i n c r e a s i n g wetwell pressure w i l l add t o the c l o s i n g pressure 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 valve i n the p e n e t r a t i o n w i l l move from i t s closed p o s i t i o n.

However, a t the end of t h e p r e s s u r i z a t i o n phase, there i s a p o t e n t i a l f o r c r e a t i n g a negative pressure i n containment.

T h i s would occur o n l y a f t e r the steam release from the 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 pressure approaches atmospheric, t h e b u t t e r f l y valve i s opened, thereby allowing t h e vacuum breaker t o p r o p e r l y function.

The vacuum breaker would begin t o open when t h e wetwell pressure becomes s l i g h t l y suh-atmospheric.

A i r f r o c t h e r e a c t o r buildir.9 would r e s t o r e t h e wetwell pressure back r o atmospheric.

The above sequence d e s c r i p t i c n has focused on t h e Design Basis Accident (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 accident scenarios.

The d i f -

ferer,ces would he l i m i t e d t o the pressure r i s e r a t e and t h e maximum pressure and temperature values reache6 Curing t h e event.

These Gifferences, however, would not a l t e r t h e events ss d e s c r i b t s above.

Therefore f o r purposes of consideration of vacuum breaker f a i l u r e, t h e s t a f f ' s conclusions can be considered a p p l i c a b l e f o r both DBA and severe accident 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 event, t h e p e n e t r a t i o n has two closed b a r r i e r s i n series.

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 breaker r o l e.

Because o f t h i s double 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 both valves a r e not expected t o change p o s i t i o n during t h e p r e s s u r i z a t i o n phase of t h e event, t h e s t a f f has concluded t h a t f a i l u r e o f the p e n e t r a t i o n as a leak 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 not be considered i n t h e design b a s i s.

Q 3:

The N R C has recommended v e n t i n g a t the containment design pressure

?.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 rupture 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 the MRC's recommendation t h a t specified t h e v e n t i n g pressure a t t h e containment design pressure.

Thc s e t pressure 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 venting pressure because t h e r e a r e two closed i s o l a t i o n valves i n t h e f l o w path.

These two valves a r e normally closed and w i l l be opened manually by t h e operator if venting i s needed.

P i l g r i m ' s venting pressure i n t h i s case i s consistent w i t h the recomnendations contained i n Emergency Procedure Guide1 ines (EPG), Revision 4.

These guide1 i nes have been approved by the staff.

The maximum containment pressure a t which t h e operators are expected t o open t h e vent valve i s 56 p s i g

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

The rupture d i s k i s designed t o serve as an a d d i t i o n a l leakage b a r r i e r a t pressures below 30 p s i. It i s designed t o open below the containment design pressure, b u t w i l l be i n t a c t up t o a pressure equal t o o r greater than those pressures t h a t cause an automatic containment i s o l a t i o n d u r i n g any accident conditions.

Therefore, i t s presence i n the l i n e can e f f e c t i v e l y e l i m i n a t e thc negative consequences of inadvertent a c t u a t i o n o f t h e vent valves a t pressures below 30 psi.

The s e t pressure 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 design o b j e c t i v e s.

Q 4 :

What i s the minimum containment pressure allowed by procedures a t which t h e operators could open the DTVS outbcard containment valve, AG-5025?

Response

Use o f the d i r e c t torus vent w i l l be i n accordance w i r h a ~ p r o v r i EPG requirements a r ~ d w i l l be c c n t r o l l ~ d by Emergency Operatin?

Procedures (EOPs). There i s rlc spec i f i e d ainimum ccntainment pressure ellowed by the BWR Owners Group EPGs, Revision 4, a t which the operarors could open the DTVS outboard ccntainment valve.

There i s a primary containment pressure l i m i t (PCPL) of 56 psig.

P l a n t - s p e c i f i c supporting analyses a r e used t o i n d i c a t e when t h e operators should begin the venting procedure.

These analyses considered a number o f p l a n t parameters, i n c l u d i n g t h e pressure r i s e rate.

These actions ensure t h a t venting i s used o n l y i f needed, t h a t t h e conditions a r e beyond t h e design-basis-accident assumptions, and t h a t the pressures i n t h e containment do n o t exceed t h e PCPL l i m i t.

Q 5:

Please provide information on the r e l i a b i l i t y o f the hydrogen and oxygen concentration monitors a t P i l g r i m.

What percentage of t h e time have hoth systems been accurste!:'

f u n c t i o n i n g ?

Respor~se:

The post-accident hydrogenloxygen analyzers were i c s t a l l e d i n

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

Since t h

~

i n s t a l l a t i o n, one t r a i n ( o f two) wzs inoperable 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 time were both t r a i n s inoperable simultaneously.

Technical S p e c i f i c a t i o n 3.7.A.7.c allows t h e reactor t o cperate f o r up t o 7 days if one t r a i n i s inoperable.

Q 6:

Response

Q 7:

Response

Q 8:

Response

q 9:

Response

I n a d d i t i o n, the containment atmospheric oxygen analyzer, which m n i t o r s t h e oxygen concentration during normal operation, has been extremely 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 estimated t h i s analyzer t o have a r e l i a b i l i t y t h a t exceeds 98 percent.

Does the NRC concur t h a t the use of t h e DTVS does n o t i n v o l v e an unreviewed safety question?

Yes.

As documented i n NRC Inspection Report No. 50-233/88-07, dated Way 6, 1988, t h e NRC inspected t h e i n s t a l l e d DTVS design 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 evaluation and determined t h a t they were acceptable.

Venting has been approved under previous versions of t h e EPGs.

The d l r e c t torus vent i s i n i t i a t e d by procedures under conditions s p e c i f i e d by the EPGs.

Because t h e outbozrd valve, A0-5025, i s sealed closed 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 valve s a t i s f i e s t h e provisions o f 10 CFR P a r t 50, Appendices A and J, which are t h e r e g u l a t i o n s f o r containment i s o l a t i o n and leak t e s t i n g, respectively.

Therefore, t h e NRC concurred t h a t t h e use o f the DTVS does n o t i n v o l v e an enreviewed s a f e t y question.

Goes the NRC concur t h a t t l ~ c use o f the DTVS does not r e q u i r e channes t o P i l g r i m ' s Technical S p e c i f i c a t i o n s ?

Yes: the NRC agrees t h a t rhe use o f the DTVS does n o t r e c u i r e changes t o P i l g r i m ' s Technical Specifications.

Cur inspection r e p o r t s, which were noted i n t h e previous responses, incluaed consideration o f p o s s i b l e TS changes, and WE determined t h

none were needed.

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

Yes.

The DTVS provides an improved containment venting c a p a b i l i t y for decay heat removal.

The DTVS w i l l prevent t h e m a j o r i t y of postulated l o s s o f decay heat removal sequences from r e s u l t i n g i n core 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 core m e l t where venting through t h e suppression pool i s found necessary.

A d d i t i o n a l s a f e t y benefits of DTVS a r e discussed i n the previous background paragraphs.

Does the NRC conclude 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 acceptable under t h e provisions o f iO CFR 50.59?

Yes. As WE hoted i n =he response t o Question 6, t h e s t a f f inspected the design 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 associated a n a l y s i s scceptable.

Venting had been approved under previous versions o f the EPGs.

The d i r e c t t o r u s vent i s

i n i t i a t e d by procedures under conditions s p e c i f i e d b y t h e EPGs.

I n addition, 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 increase the p r o b a b i l i t y of a new accident.

Therefore, t h e i n s t a l l a t i o n and use of the DTVS a r e acceptable under 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 October 12, 1988, t h e NRC s t a f f concluded t h a t t h e Safety Enhancement Program (SEP) m o d i f i c a t i o n s being implemented i n accordance w i t h 10 CFR 50.59, i n c l u d i n g the 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 Edison has adequately considered t h e technical issues germane t o the DTVS?

Response

Yes.

Based on t h e noted inspections and reviews 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 the safety issues associzted with t h e DTVS have been adequately considered.

F 11:

Why was the automatic r e c l o s u r e on h i g h r a d i a t i o n of valve AO-5025 deleted d u r i n g t h e design r e v i s f c n o f t h e system?

Response

The reclosure o f v ~ l v e AO-5025 w a s deleted because t h i s reclosure, 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 path when venting i s needel t c m i t i g a t e the overpressure chal:er,ge.

Thus, automatic r e c l o s u r e could defeat 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 vent t o reduce core damage frequencies d u r i n g SBO [ s t a t i o n blackout] 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] accident 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 valves, A0-5025 and AO-50425, a r e designed w i t h ac independent power supplies.

These two valves a r e powered from essential dc power and are backed up w i t h d i v e r s e n i t r o g e n actuation c a p a b i l i t y.

Therefore, i n case o f an SBO event, t h e valves would be a v a i l a b l e f o r venting.

The v e n t i n g concept i s mainly designed t o slow overpressure t r a n s i e n t s o f t h e contain-ment.

During some ATWS events, t h e pressure i n t h e containment wj!l r a p i d l y increase.

Ventin!

pressure could be reached i n a matter o f minutes r a t h e r than hours.

Therefore, v e n t i n g may n o t prevent containaent f a i l u r e because o f tCe h i g h conteinment p r e s s u r i z a t i o n r a t e b u t would provide a d c i t l o n a l time t o scram the reactor and delay t h e c o r e me1 t.

Backqround Information Related t o P i l g r i m Station's D i r e c t Torus Vent System (DTVS1 On January 23, 1989. the NRC staff presented i t s recomendations on Mark I containment performance improvements and other safety enhancements t o the Comnission i n SECY 89-017.

It represented the completion of the s t a f f e f f o r t s on the Containment Performance Improvement (CPI) Program f o r Mark I containments.

The program was established t o determine what actions, i f any, should be taken t o reduce the 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, the s t a f f proposed t h a t hardened vent 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 accident prevention and mitigation.

Sum low p r o b a b i l i t y scenarios i n which multiple 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 loss of cooling 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 greatly reduces the likelihood o f early containment f a i l u r e and, therefore, reduces the r i s k s t o the public because cooling c a p a b i l i t y i s maintained.

For other sequences for which core melt i s predicted, ventinp could be e f f e c t i v e i n delaying containmect f a i l u r e and i n m i t i g a t i n g the reierse o f f i s s i o n products.

Although venting o f the containmefit i s currently included i n BWR emergency operating procedures t o improve the s u r v i v a b i l i t y cf the conteinment, which acts as the l a s t barrier f o r an ~ n c C r t t r 0 l l e d release of radiation, i t generally uses a vent path that includes ductwork w i t h a low design pressure.

Venting under high-pressure savere-accident conditions coule f a i l t h i s ductwork, release the containment atmosphere i n t o t h e reactor building, and damage equipment o r contaminate equipment needed for accident recovery.

Venting through t h i s ductwork may hamper or 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 allows f o r c o n t r o l l e d venting through the wetwell while providing a path with s i g n i f i c a n t scrubbing c a p a b i l i t y o f fission products t o the 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, the Comnission directed the s t a f f t o allow the licensees t h a t elected t o incorporate t h i s plant improvement t o i n s t a l l a hardened wetwell vent i n accordance with the Commission's regulations (10 CFR 50.59).

Plant s p e c i f i c b a c k f i t analyses were directed for the remaining plants w i t h Mark I containments.

Where these analyses supported imposition o f a hardened vent, the s t a f f wes directed t o issue orders requiring t h i s modification.

P r i o r t o the Com;;lission decision i n t h i s mttter, numerous discussicns with b ~ t p '

industry groups and individual licensees were conducted.

These discussions included meetings w i t h Boston Edison ( t h e licensee f o r Pilgrim).

The purposs of these discussions was t o gather e l l available informaticn r e l a t i v e t o the hardened vent t o enable the s t a f f t o m k e an informed decision.

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 that i t had sufficient i n f o n a t i o n t o commit t o a specific design f o r hardened wetwell vents.

The proposed modification was consistent w i t h the s t a f f ' s generic finding f o r Mark I plants.

However, the s t a f f did not use the P i l g r i m design as a t e s t case, as i s indicated i n your l e t t e r.