ML19329A995
| ML19329A995 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 01/31/1977 |
| From: | Rusche B Office of Nuclear Reactor Regulation |
| To: | Bender M Advisory Committee on Reactor Safeguards |
| References | |
| NUDOCS 8001290600 | |
| Download: ML19329A995 (4) | |
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UNIT 13 STATES NUCLEAR REGULATORY COMMiss!ON k*
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Mr. Myer Bender, Chairman Advisory Comittee on Reactor Safeguards U. S. Nuclear Regulatory Comission Washington, D. C.
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Dear Mr. Bender:
REQUEST FOR INTERPRETATION OF THE COMMITTEE COMMENTS ON DAVIS BESSE UNIT 1 It is the staff's practice to address in a Supplement to its Safety Evaluation Report the formal comments on a specific application made by the. Advisory Comittee on Reactor Safeguards, and as presented in the Committee's report to the Chairman of the Nuclear Regulatory Comission.
In accord with this practice, we have reviewed the Comittee's coments on its review of the Davis Besse Nuclear Power Station, Unit 1; these comments are presented in your report dated January 14, 1977.
In order that the staff's response to the Comittee's comments are consistent with the Comittee's intent, it is essential that the staff's interpretation of that intent be correct. The degree of uncertainty in-the staff's mind as to the correctness of the interpretation it has made of two of the Comittee's coments in its January 14, 1977 report is sufficient to warrant a request to the Comittee to verify the correctness of those interpretations.
The first mattc. involves the following comment relating to seismic design margins:
The structures and components of Davis-Besse, Unit 1, were designed for a Safe Shutdown Earthquake (SSE) acceleration of 0.15g at the foundation level. Because of changes in the regulatory approach to selection of seismic design bases, the Comittee believee that an acceleration of 0.20g would be more appropriate for the SSE acceleration at a site such as this in the Central Stable Region. The Applicant presented the results of preliminary calculations concerning the safety margins of the plant for an SSE acceleration of 0.20g. The Comittee recomends that the l
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r Mr. Myer Bender m.;; c-NRC Staff review this aspect of the design in detail and assure itself that significant margins exist in all systems required to accomplish safe shutdown of the reactor and 4
continued shutdown heat removal, in the event of an SSE at this higher level. The Comittee believes that such an evaluation need not delay the start of operation of Davis-Besse. Unit 1.
The Comittee wishes to be kept infonned.
t We are uncertain of the Comittee's. intent with respect to its recom-mendation that the staff review the design in detail and assure itself that significant margins exist in all systems _ in the event of an SSE of 0.209. One interpretation that could be made of this recom-mendation would involve an extensive analysis program by the licensee to assess the response of all structural components, mechanical com-ponents, and electrical and instrumentation components to a 0.20g seismic event. This program, and the staff involvement in it, would involve a costly undertaking that would not likely be completed until a year or two after comercial operation. We hesitate to accept this interpretation, because we view the value of such an undertaking not to warrant the impact or cost to the staff and licensee.
However, it is difficult to develop an alternative interpretation that would be consistent with the Committee's tenninology; i.e.,
review the design in detail, significant margins, all systems, SSE of 0.20g. We believe an examination of the stresses in selected components in essential systems for the 0.15g design basis earth-quake with a determination of the importance of the SSE-induced stress (or OBE-induced stress if it is controlling) to the overall stress, and a qualitative assessment of the adequacy of those result-ant margins for a slightly increased seismic event, taking into account important effects such as those associated with deconvolution, spectra, damping, etc., should be sufficient to provide adequate assurance of safety. We attempted to orally provide this type of assessment to the Comittee at its enuary 6,1977 meeting, but, on the basis of the Comittee's letw, failed to provide an entirely convincing argument. However, on the basis of all our discussions with the Comittae on the Davis Besse application, we believe an interpretation of the Comittee's report that expands somewhat on this latter approach is more consistent with the Comittee's intent than one that is based on a literal reading of the Committee's state-ment. Accordingly, we have asked the applicant to assume an SSE of 0.20g at the surface, currently accepted procedures for deconvolution, and response spectra and damping values in accordance with current staff practice, and then to calculate the response of selected com-ponents in systems needed for safe shutdown. The components would 0
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be selected on the basis of previous calculations as those likely to envelope the response of all system components.
If the results of the calculation are within current acceptance limits for the loading condition analyzed it will be concluded that an adequate safety margin exists. We expect this task will require several weeks work.
The second matter involves the following coment relating to means for evaluating accidents:
The Comittee recomends that, prior to comercial power 1
operation of Davis-Besse, Unit 1, additional means for evaluating the cause and likely course of various acci-dents, including those of very low probability, should be in hand in order to provide improved bases for timely decisions concerning possible off-site emergency measures.
The Comittee wishes to be kept. informed.
We are uncertain of the intent of the Comittee's coment. We are interpreting it to be consistent with the discussions initiated by J. Ebersole on recent operating license applications reviewed by the Comittee. The matter initially raised by Mr. Ebersole relates to system or plant conditions that are beyond the design basis conditions that the staff currently requires to be addressed.
For example, the staff does not require the condition of loss of all alternating current power to be addressed in an application. The Comittee's coment is being interpreted to require that events of this type, which the staff has heretofore not required be considered because of the low likelihood of occurrence in situations which could lead to serious safety problems, be analyzed by the applicant and additional means be in hand, prior to comercial operation, to l
evaluate the cause and likely) course of such events (or accidents, in the words of the Comittee. There are an unlimited number of degraded situations of low probability that can occur in a power plant. Our current staff policy is not to require consideration of any of these low probability events. We believe that the practice 1
followed by the staff, currently and in the past, and approved by the Comittee in the past, provides an adequate degree of safety.
In view of this we are interpreting the Comittee's comment to relate to those specific degraded plant conditions, beyond the staff's requirements, that were identified by Comittee members during its January meeting.
Further, we will assume that the development of written procedures by the applicant to be available to the operating I
staff will constitute acceptable " additional means." The applicant l
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- r.' Fy.;r Sandar AN.i ' air unects to have.croceduras availabic crior to comercial cooration for (1) total loss of alternating current nowr, (2) momentary loss of direct current nower, (3) short-tarm loss of all service water, and (4) degraded steam generator level control.
This second matter, unlike the first matter, is anpitcable to all plants.
Surely tha matter is not of ca.for safety significance sinca plants other than Davis Besse. Unit 1, are currently being licensed without the mattar being addressed at all, or in a less rigorous manner than required of the Davis Besse, tinit 1, apolicant.
In view of the lack of justified immediato safety concern and the general acplicability of the matter, it would aopear that this matter could be more efficiently considered as a generic item than one that might be addressed on some plants late in their review schedules. tie would anoreciata the Cennittee's view on this consideration.
If either of the interpretations we have made of the two Comittee cow.nts discussed above are counter to the intent of the Committee, we would a::nreciate prompt notificetion to this effect along with a more definitive description of tae Co.:nittae's true intant.
Sincerely, o g.ut 3::. a y senc.h:4e i
Ben C. Rusche, Director Office of Huclear "cactor Regulation Distributien Docket File EDO Reading NRR Reading RCDeYoung Reading BCRusche EGCase RHeineman HDenton R8oyd
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