ML20010D610
| ML20010D610 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 06/24/1981 |
| From: | Advisory Committee on Reactor Safeguards |
| To: | Advisory Committee on Reactor Safeguards |
| References | |
| ACRS-1855, NUDOCS 8108280355 | |
| Download: ML20010D610 (7) | |
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MINUTES OF THE MAY 5, 1981 Q1 b(l.
h AD H0C SUBCOMMITTEE ON DECAY HEAT REMOVAL SYSTEMS 2
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u.q'$W The ACRS Ad hoc Subcommittee on Decay Heat Removal Systems met on Ma k1981 at 1717 H Street, N.W., Washington, D.C. The purpose of this mee~ ting w t[oM formulate a response to Commissioner Gilinsky's questions on possible improve-ments to the North Anna #2 RHR and LPI systems and to discuss the NRC-sponsored work at Sandia alternate decay heat removal systems. A copy of the notice for this meeting is included as Attachment A.
A list of attendees is included as Attachment B.
A schedule for this meeting is included as Attachment C.
The handouts for this meeting are included in the ACRS files.
Selected handouts are included as Attachment D.
The meeting was begun at 1:00 pm with a short executive session in which Mr. Ward, the Subcommittee chairman summarized the objectives for the day's meeting.
The meeting was adjourned at 5:00 pm. The meeting was conducted entirely in open session. The subcommittee heard presen-tations from NRC Staff, BNL, Sandia, and VEPC0.
The meeting was attended by D.
Ward, Subcommittee Chairman; J.
- Ebersole, H.
Etherington, and J.
- Ray, Subcommittee members; R. Savio, ACRS Staff; A. Acosta, E. Epler, and P. Davis, Subcommittee consultants.
INTRODUCTION l
Mr. Ward held a short executive in which he discussed the purpose of the day's meeting.
He noted that it was the Subcommittee's intention to develop a response to Commissioner Gilinsky's concerns about the decay heat removal systems in the North Anna plant and to provide comments or any critique of the l
Staff's long range program for generic improvements to decay heat removal systems.
He noted that the matter was to be brought to the ACRS at the May l
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l 8108200355 810624 PDR ACRS 1855 PDR
Ad Hoc Subcmte on DHRS May 5, 1981 meeting for consideration.
Mr. Ward stated that Commissioner Gilinsky had requested that the ACRS reexamine the desirability of adding a heat exchanger to the LPI system on North Anna 2 and environmentally qualify the,RHR system.
Mr. Ward proposed that the following questions be addressed:
1.
Is it desirable to require these modifications for the North Anre plant and for all similar plants?
2.
Is the existing RHR system in its present state capable of functioning in harsh containment environments such as would result from a LOCA or core damage event?
3.
Are these cost-effective improvements and do they represent major vulner-abilities in the North Anna decay heat removal systems?
l 4.
Does this matter immediate resolution or is resolution within the scope of l
the generic programs proposed by the Staff sufficient?
I S.
Are the goals proposed in Task A-45, " Shutdown Heat Removal Requirements",
I appropriate and is the work being given the appropriate priority and level of resources?
SUMMARY
OF STAFF POSITION ON COMMISSIONER GILINSKY'S COMMENTS AND THE STATUS OF TAP-45, F. ORR, NRC STAFF Mr. Orr of the NRC Staff described the North Anna LPI recharge systems and summarized the NRC Staff's position on Commissioner Gilinsky's questions. The RHR is the two-pump, two heat exchanger system located inside the containment
Ad Hoc Subcm2e on DHRS May 5,1981 with the single hot leg suction.
The LPI injects directly into the reactor cold leg does not utilize a separace heat exchanger.
Heat removt.1 must be accomplished through the break or the containment sprays.
The system is similar to the Surry (WASH-1400 system), differing in that the initial suction is provided to the containment spray pumps through a storage tank) as opposed to from the containment sump in Surry.
This protects against damage to the pumps cavitation in small break scenarios.
Commisisoner Gilinsky requested that the NRC Staff evaluate the advantages of adding a heat exchanger to the LPI system and environmentally qualify the RHR system.
Mr.Orr indicated that the North Anna. system as designed meets the NRC requirements and that, within the NRC review guidelines, the su'ggested niodifications do not significantly impro'Je the system.
Mr. Orr indicated that the NRC review is limited to a application of the single failure ~ criteria.
A NRC staff sponsored study was performed by the BNL to examine issues extending beyond a single failure criteria.
The study used risk oriented and utilized the WASH-1400 study in that it examined the North Anna plant in context of the fault trees developed for Surry plant.
The study concluded that the addition of heat exchangers to the PPI system probably did not significantly reduce the probability of core mel t.
The issue of the environmental qualification of the RHR was not ad-dressed in this study.
Mr. Orr noted that Commissioner Gilinsky's concerns would also be applicable to the Surry 1 & 2, Beaver Valley 182, and North Anna 1 plants. Mr. Ebersole asked a number of questions relating to the details of the piping design and to the abili;y of the RHR system to withstand unusual events. The VEPC0 representatives participated in this discussion.
It was noted that~ all electrical switch gear for the RHR system was located external to the containment and would not be affected by LOCA environments.
The pump
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0 Ad Hoc Subemte on DHRS May 5,1981 motors are the only electrical apparatus that are installed within the con-tainment.
The mo+', are not subjected to direct containment spray but would be exposed to LOCA environments.
The degree to which these motors would resist LOCA environment would be established in this discussion.
Procedures have been established for removing decay heat in a refueling / head-off config-uration failure of the RHR system.
BROOKHAVEN NATIONAL LABORATORY RISK ASSESSMENT EVALUATION - A. BUSLIK Mr. Busiik described a NRC sponsored study performed at BNL which was directed towards evaluating reliability improvements that might be gained by adding heat exhangers'to the LPI system. The study utilized results of the WASH-1400 study (the Surry plant).
The study concluded that the addition of the heat exchanger to the LPI system probably would significantly reduce the probabil-ity of a core melt. The study did not address less severe events.
TASK A-45, SHUTDOWN HEAT REMOVAL REQUIREMENTS - A. MARCHESE, NRC STAFF Mr. Marchese of the NRC Staff summarized the work that had been performed on l
the Task A-45 (Shutdown Heat Removal Requirements) since the April 10, 1981 l
ACRS briefing.
It is expected that a first draft of the Task Action Plan will be completed by May 8,1981 and an approved draft would be available to the l
Subcommittee by May 22, 1981. The current plans call for the use of WASH-1400 l
and RSSMAP to assess the decay heat removal system reliability of the six plants which were treated n the studies and the extrapolation of this informa-tion to the extent,'ossible of other operating LWRS as a first step.
The current plans call for the resolution of this unresolved safety issue by the end of 1984, with the implementation of the study ecommendation after this l
Ad Hoc Subcmte on DHRS May 5, 1981 t
period. ~~In response to a question from Mr. Ebersole, Mr. Marchese estimated that the frac. tion of the core melt accidents which are attributable to the failure of the shutdown heat removal systems is about fifteen percent. There was some discussion as to when plant modifications, if required, would be implemented on existing plants.
'Mr. Marchese estimated that modest backfits could be accomplished by 1986.
He stated that he envisioned that there may well be three classes of plants; those that had sufficiently reliable decay heat removal systems, those that would have sufficiently heat removal systems with modest backfits, and those which would require some. sort of add-on
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alternate decay heat removal system.
The Subcommittee indicated that they believed that the work should not' place an undue reliance on the results of risk assessment studh. and noted that the six plants considered may not cover the full range of decay heat removal design configurations.
DISCUSSIONS ON NUREG/CR-1556, " STUDY OF ALTERNATE DECAY HEAT REMOVAL CONCEPTS FOR LIGHT WATER REACTORS / CURRENT SYSTEMS AND PROPOSED OPTIONS"- M. TAYLOR, NRC/RES AND D. BERRY,SANDIA Mr. Taylor and Mr. Berry summarized the NRC sponsored at Sandia on alternate' decay heat removal concepts.
The work performed to date has identified the important transients, evaluated the U.S. and some foreign decay heat removal systems as selected alternate decay heat removal concepts for more detailed study, and has developed basic design ' criteria for these concepts.
The AFWS and HPIS for PWRs and the suppression pool cooling and high pressure service water systems for BWRs have been identified as the important systems in the DHR process.
The design critria which have been identified for the alternate concepts which are believed to be sufficient to reduce reliable systems
Ad Hoc Subcmte on DHRS May 5,1981 t
be self contained, be testable and maintainable, and be automatically initi-ated if less than 30 minutes is available for operator action.
The systems are required to have the capability to operate for 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> without operator intervention.
Some examples of the devices which are being considered are add-on HPI and AFWS trains and high pressure RHR systems.
The benefits associated with System modification will be measured by the reduction in the core melt probability. Cost estimate in conceptual design will be obtained by subcontractors.
Practice for decay heat removal systems for foreign plants was compared to U.S.
practice 'for six PWRs and six BWRs.
Additional infomation was also obtained from the Swiss AE who. acted as consultants for Sandia.
Mr. Berry noted that in the opinion of tne Sandia investigators the Europaons have
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overemphasized redundancy in the decay heat removal systems. Bunkered systems have been used some European designs.
These systems provide advantages both from the standpoiit of sabotage protection.
The evaluation of decay heat removal improvements will be perfomed for a PWR and a BWR.
The Oconee and Grand Gulf plants have been selected (RSSMAP studies exist for both of these plants).
When this part of the work has been completed the study will be ended to the evaluatin of several PWR and BWR designs.
The WASH-1400 and remaining RSSMAP plants will be among those sel ec ted.
Mr. Epler and Mr. Ebersole stressed the importance of the capability of on-line testing, establishing a high system reliability.
They noted that a
Ad Hoc Subcmte on DHRS May 5,1981 system which could be tested without disabling parts of the plant elinimated much of the chance for maintenance / testing error. They additionally noted that bunkered systems provided a decreased vulnerability to sabotage.and a high degree of independence. Mr. Berry was asked to comment on the safety improve-ment that might be provided by the addition of the heat exchanger to the LPI system in the North Anna plant.
Mr. Berry noted that such a heat exchanger could be provided from the engineering standpoint but it was not clear to him that this would improve the overall system reliability.
DISCUSSION AND SUBCOMMITTEE CONCLUSIONS Discussions were held in which Mr. Ward solicited comments from members of the Subcommittee. Mr. Ebersole stated that in evaluating improvements to specifi-cally the North Anna decay heat removal systems and more generally to all decay heat removal systems one should not overemphasize the large accident and should also consider small break accidents and their impact on both plant operation and the public health ar.d safety.
It was generally agreed that Commissioner Gilinsky's concerns were best addressed within the context of the Task Action Plan A-45 and that the deficiencies in the North Anna system, if any, did not require any more immediate action.
Mr. Ebersole commented that the probabilistic risk assessment on which Task Action Plan A-45 appeared to be placing a great deal of reliance does not consider details of the design and field installation and that in ms4y cases the reliability of a' system is highly dependent on these features. Mr. Ward requested that VEPC0 and the NRC be prepared to address the Full Committee on May 7,1981 and gave VEPC0 in-structions. as to what areas need to be addressed in their presentatioas.
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