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{{#Wiki_filter:UNITED STATES NUCLEAR REGULATORY COMMISSION  
{{#Wiki_filter:UNITED STATES NUCLEAR REGULATORY COMMISSION  
-7 (2J~&po WASHINGTON.
-7 (2J~&
: 0. C. 10555 June 21, 1990 CHAIRMAN C - , .-. ri r : 7: Mr. William R. Griffin , Executive Secretary Town of Plymouth Office of the Selectmen
WASHINGTON. 0. C. 10555 June 21, 1990 CHAIRMAN C -
'- . 11 Lincoln Street ::: 1 ., : Plymouth, Massachusetts 02360  
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. Griffin:==
==Dear Mr. G r i f f i n :==
I am responding to your letter of April 24, 1990, coacerning the direct torus vent at the Pilgrim Nuclear Power Station. I referred the 12 specific questions you raised in your letter to the Nuciear Regulatory Commission (NRC) staff, and their detailed responses are enclosed.
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.
Some additior,al backgrsund information that may be helpful to you is also enclosed.
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.
I hope the information wt are pruvidin~
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.
will lrad to a berrei. understanding of the generic issues associated with venting, and, in particular, how they relate to the Piigrim Nuclrir Power 5tbiion. If you have any further questicns, pleisr contacr mc n? Mr. T. T. l$lsr;in, Administrator of HFtC's Region I office. Xr. Mar~in can be rezched by teiephcne et (215) 337-5'5s.
I hope t h e i n f o r m a t i o n w t a r e p r u v i d i n ~ w i l l l r a d t o a b e r r e i.
Sincerely,  
u n d e r s t a n d i n g of t h e g e n e r i c i s s u e s a s s o c i a t e d w i t h v e n t i n g, a n d,
in p a r t i c u l a r, how t h e y r e l a t e t o t h e P i i g r i m N u c l r i r Power 5 t b i i o n.
I f y o u h a v e a n y f u r t h e r q u e s t i c n s, p l e i s r c o n t a c r mc n?
Mr.
T. T. l$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


==Enclosures:==
===Response===
: 1. Background Information
\\
: 2. Responses to Concerns 3. SECY-89-017
I Q 2:
: 4. Inspection Report No. 50-293/86-07
R ~ s D o ~ s ~ :
: 5. lcspecticn Report No. 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.
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?
Question 1 (0 1): What are the decontamination factors for the 0001 for 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 (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.
\ I Q 2: R~sDo~s~:
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.
various isotopes?
Please provide a graph of offsite radiation doses based on the possibility of a vacuum breaker valve remaining open at ID%, 25%.
In other words, how well dbes the wet well pool scrub out the fission by-products, keeping the radioactive particles from releasing to atmosphere?
50% and 100%.
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 to 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 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 basis for the staff's position in 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.
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 penetration consists of a vacuum breaker and an air operated butterfly valve in series. During normal operation, both valves are closed; the vacuum breaker is maintained closed by the weight of the disk, and the butterfly valve is maintained closed by positive actuator air pressure.
 
In the event of a loss-of-coolant accident (LOCA), the increasing wetwell pressure will add to the closing pressure of the vacuum breaker. As a result, it is anticipated that during the entire positive pressure history within the containment, neither valve in the penetration will move from its closed position.
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.
However, at the end of the pressurization phase, there is a potential for creating a negative pressure in containment.
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.
This would occur only after the steam release from the reactor coolant system has ceased. As the aetwell pressure approaches atmospheric, the butterfly valve is opened, thereby allowing the vacuum breaker to properly function.
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.
The vacuum breaker would begin to open when the wetwell pressure becomes slightly suh-atmospheric.
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.
Air froc the reactor buildir.9 would restore the wetwell pressure back ro atmospheric.
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.
The above sequence descripticn has focused on the Design Basis Accident (DBA). However, the sequence is equally valid for a large number of potential severe accident scenarios.
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.
The dif- ferer,ces would he limited to the pressure rise rate and the maximum pressure and temperature values reache6 Curing the event. These Gifferences, however, would not alter the events ss describts above. Therefore for purposes of consideration of vacuum breaker failure, the staff's conclusions can be considered applicable for both DBA and severe accident events. Therefore, during the entire positive pressure profile of the event, the penetration has two closed barriers in series. It is only during the end of the pressurization phase that the penetration is aligned into its vacuum breaker role. Because of this double barrier protection and the fact that both valves are not expected to change position during the pressurization phase of the event, the staff has concluded that failure of the penetration as a leak tight barrier is not credfble and need not be considered in the design basis. Q 3: The NRC has recommended venting at the containment design pressure  
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.
?.s a minimum, or in the czse of Pilprim, at 60 psi. G!hy is the Pilgrim DXS rupture disk set at half that, at 3G psi? Response:
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.
The fact thbt the Pilgrim DTVS rupture disk is designed tc rupture at 30 psi is not related to the MRC's recommendation that specified the venting pressure at the containment design pressure.
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.
Thc set pressure for the rupture disk does not control the venting pressure because there are two closed isolation valves in the flow path.
The above sequence d e s c r i p t i c n has focused on t h e Design Basis Accident (DBA).
These two valves are normally closed and will be opened manually by the operator if venting is needed. Pilgrim's venting pressure in this case is consistent with the recomnendations contained in Emergency Procedure Guide1 ines (EPG) , Revision 4. These guide1 i nes have been approved by the staff. The maximum containment pressure at which the operators are expected to open the vent valve is 56 psig (not 60 psi), which is consistent with the NRC recomnendation on venting pressure.
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 rupture disk is designed to serve as an additional leakage barrier at pressures below 30 psi. It is designed to open below the containment design pressure, but will be intact up to a pressure equal to or greater than those pressures that cause an automatic containment isolation during any accident conditions.
The d i f -
Therefore, its presence in the line can effectively eliminate thc negative consequences of inadvertent actuation of the vent valves at pressures below 30 psi. The set pressure of 30 psi for the rupture disk satisfies these design objectives.
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.
Q 4: What is the minimum containment pressure allowed by procedures at which the operators could open the DTVS outbcard containment valve, AG-5025? Response:
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.
Use of the direct torus vent will be in accordance wirh a~provri EPG requirements ar~d will be ccntroll~d by Emergency Operatin?
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.
Procedures (EOPs) . There is rlc spec if ied ainimum ccntainment pressure ellowed by the BWR Owners Group EPGs, Revision 4, at which the operarors could open the DTVS outboard ccntainment valve.
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.
There is a primary containment pressure limit (PCPL) of 56 psig. Plant-specific supporting analyses are used to indicate when the operators should begin the venting procedure.
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.
These analyses considered a number of plant parameters, including the pressure rise rate. These actions ensure that venting is used only if needed, that the conditions are beyond the design-basis-accident assumptions, and that the pressures in the containment do not exceed the PCPL limit. Q 5: Please provide information on the reliability of the hydrogen and oxygen concentration monitors at Pilgrim. What percentage of the time have hoth systems been accurste!:'
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.
functioning?
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:
Respor~se:
The post-accident hydrogenloxygen analyzers were icstalled in &ri~.sry 1985 as par; of the post-TMI desicn modificctions.
The post-accident hydrogenloxygen analyzers were i c s t a l l e d i n
Since th~ installation, one train (of two) wzs inoperable for tcc days in November 1985, asd one trzin was inoperible for four days in January 1986, for a total of six days. At no time were both trains inoperable simultaneously. Technical Specification 3.7.A.7.c allows the reactor to cperate for up to 7 days if one train is inoperable.
& 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.
Q 6: Response:
Since t h
Q 7: Response:
~
Q 8: Response:
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.
q 9: Response:
A t no time were both t r a i n s inoperable simultaneously.
In addition, the containment atmospheric oxygen analyzer, which mnitors the oxygen concentration during normal operation, has been extremely reliable.
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.  
The plant staff conservatively estimated this analyzer to have a reliability that exceeds 98 percent. Does the NRC concur that the use of the DTVS does not involve an unreviewed safety question?
 
Yes. As documented in NRC Inspection Report No. 50-233/88-07, dated Way 6, 1988, the NRC inspected the installed DTVS design configuration and the licensee's evaluation and determined that they were acceptable. Venting has been approved under previous versions of the EPGs. The dlrect torus vent is initiated by procedures under conditions specified by the EPGs. Because the outbozrd valve, A0-5025, is sealed closed and subject to leak testing, this valve satisfies the provisions of 10 CFR Part 50, Appendices A and J, which are the regulations for containment isolation and leak testing, respectively.
Q 6:  
Therefore, the NRC concurred that the use of the DTVS does not involve an enreviewed safety question.
 
Goes the NRC concur that tl~c use of the DTVS does not require channes to Pilgrim's Technical Specifications?
===Response===
Yes: the NRC agrees that rhe use of the DTVS does not recuire changes to Pilgrim's Technical Specifications.
Q 7:  
Cur inspection reports, which were noted in the previous responses, incluaed consideration of possible TS changes, and WE determined th none were needed.
 
Does the NRC judge the DTVS to improve the safety at Pilgrim? Yes. The DTVS provides an improved containment venting capability for decay heat removal. The DTVS will prevent the majority of postulated loss of decay heat removal sequences from resulting in core melt and will mitigate the consequences of the residual sequences involving core melt where venting through the suppression pool is found necessary.
===Response===
Additional safety benefits of DTVS are discussed in the previous background paragraphs.
Q 8:  
Does the NRC conclude that the installation and use cf the DTVS are acceptable under the provisions of iO CFR 50.59? Yes. As WE hoted in =he response to Question 6, the staff inspected the design of DTVS at Pilgrim and fcund the installed system and the associated analysis scceptable.
 
Venting had been approved under previous versions of the EPGs. The direct torus vent is initiated by procedures under conditions specified by the EPGs. In addition, the installation or use of the direct torus vent will not increase the probability of a new accident. Therefore, the installation and use of the DTVS are acceptable under the provisions of 10 CFR 50.59. Furthermore, in a supplemental assessment of October 12, 1988, the NRC staff concluded that the Safety Enhancement Program (SEP) modifications being implemented in accordance with 10 CFR 50.59, including the DTV modification, would enhance the overall plant safety and performance of Pilgrim. Q 10: Does the NRC conclude that Boston Edison has adequately considered the technical issues germane to the DTVS? Response:
===Response===
Yes. Based on the noted inspections and reviews of the Pilgrim SEP, the NRC staff concludes that the safety issues associzted with the DTVS have been adequately considered.
q 9:  
F 11: Why was the automatic reclosure on high radiation of valve AO-5025 deleted during the design revisfcn of the system? Response:
 
The reclosure of v~lve AO-5025 was deleted because this reclosure, if performed at high radiation levels, would isolate the vefit flok path when venting is needel tc mitigate the overpressure chal:er,ge.
===Response===
Thus, automatic reclosure could defeat tht purpose of the vEKt design. Q 12: Generic Letter 89-16 indicates some benefits of a hardened wet well vent to reduce core damage frequencies during SBO [station blackout]
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.
and ATWS [anticipated transient without scram] accident scenarios.
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.
Is this true for Pilgrim? Response:
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. The isolation valves, A0-5025 and AO-50425, are designed with ac independent power supplies.
Yes.
These two valves are powered from essential dc power and are backed up with diverse nitrogen actuation capability.
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.
Therefore, in case of an SBO event, the valves would be available for venting. The venting concept is mainly designed to slow overpressure transients of the contain- ment. During some ATWS events, the pressure in the containment wj!l rapidly increase.
Venting has been approved under previous versions of t h e EPGs.
Ventin! pressure could be reached in a matter of minutes rather than hours. Therefore, venting may not prevent containaent failure because of tCe high conteinment pressurization rate but would provide adcitlonal time to scram the reactor and delay the core me1 t.
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.
Enclosure 1 Backqround Information Related to Pilgrim Station's Direct Torus Vent System (DTVS1 On January 23, 1989. the NRC staff presented its recomendations on Mark I containment performance improvements and other safety enhancements to the Comnission in SECY 89-017. It represented the completion of the staff efforts on the Containment Performance Improvement (CPI) Program for Mark I containments.
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.
The program was established to determine what actions, if any, should be taken to reduce the vulnerability of containments to severe-accident challenges.
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.
From this point of view, the staff proposed that hardened vent capahility would enhance plant capabilities with regard to both severe accident prevention and mitigation.
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 ?
Sum* low probability scenarios in which multiple failures occur can lead to containment failure. Containment failure from these scenarios can result in a loss of cooling water which is used to remove decay heat. The installation of a hardened vent greatly reduces the likelihood of early containment failure and, therefore, reduces the risks to the public because cooling capability is maintained. For other sequences for which core melt is predicted, ventinp could be effective in delaying containmect failure and in mitigating the reierse of fission products.
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.
Although venting of the containmefit is currently included in BWR emergency operating procedures to improve the survivability cf the conteinment, which acts as the last barrier for an ~ncCrttr0lled release of radiation, it generally uses a vent path that includes ductwork with a low design pressure.
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
Venting under high-pressure savere-accident conditions coule fail this ductwork, release the containment atmosphere into the reactor building, and damage equipment or contaminate equipment needed for accident recovery.
none were needed.
Venting through this ductwork may hamper or complicate post-accident recovery activities.
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 ?
The installation of a reliable hardened wetwell vent allows for controlled venting through the wetwell while providing a path with significant scrubbing capability of fission products to the plant stack and prevents damage to equipment needed for accident recovery.
Yes.
Based on the staff's recommendation, the Comnission directed the staff to allow the licensees that elected to incorporate this plant improvement to install a hardened wetwell vent in accordance with the Commission's regulations (10 CFR 50.59). Plant specific backfit analyses were directed for the remaining plants with Mark I containments.
The DTVS provides an improved containment venting c a p a b i l i t y for decay heat removal.
Where these analyses supported imposition of a hardened vent, the staff wes directed to issue orders requiring this modification.
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.
Prior to the Com;;lission decision in this mttter, numerous discussicns with b~tp' industry groups and individual licensees were conducted.
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.
These discussions included meetings with Boston Edison (the licensee for Pilgrim).
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?
The purposs of these discussions was to gather ell available informaticn relative to the hardened vent to enable the staff to mke an informed decision.
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.
During this process, Boston Edison proposed to install the Direct Torus Vent System (DTVS). The licensee had concluded that it had sufficient infonation to commit to a specific design for hardened wetwell vents. The proposed modification was consistent with the staff's generic finding for Mark I plants. However, the staff did not use the Pilgrim design as a test case, as is indicated in your letter.}}
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.}}

Latest revision as of 02:56, 12 January 2025

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)
v • d • e


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.

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