ML20245A382
| ML20245A382 | |
| Person / Time | |
|---|---|
| Issue date: | 06/14/1989 |
| From: | Remick F Advisory Committee on Reactor Safeguards |
| To: | Zech L NRC COMMISSION (OCM) |
| References | |
| ACRS-R-1360, NUDOCS 8906210294 | |
| Download: ML20245A382 (2) | |
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Ei% Jr E ADVISORY COMMITTEE ON REACTOR SAFEGUARDS !
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l The Honorable Lando W. Zech, Jr.
Chairman i U.S. Nuclear Regulatory Comission I Washington, D.C. 20555 {
Dear Chairman Zech:
SUBJECT:
RELIABILITY AND DIVERSITY During the 349th and 350th meetings of the Advisory Comittee on Reactor Safeguards, May 3-6, 1989 and June 8-10, 1989, respectively, we discussed the implementation status of the anticipated transients without scram (ATWS) .
rule. Our Subcommittee'on Instrumentation and Control Systems also met with '
representatives of the staff and the industry on April 21, 1989 to review the i progress being made regarding this matter.
It appears that reasonable progress is being made, especially in light of some of the difficulties that have arisen in the interpretation and applica-tion of the rule. However, during the course of our discussions of compli- )
ance with the rule, two issues arose that we consider to have enough general I significance to deserve further attention.
The first of these is the significance and application of diversity in '
systems that use redundancy to achieve high levels of reliability. The ATWS i rule requires that diversity be used in an effort to further improve reli-ability. The staff interprets the rule to require diversity even if, in a !
particular application, there is no evidence that its use increases reli- l ability. It appears, indeed, that this interpretation would be used even in ;
situations in which, by virtue of comercial availability of components,
! maintenance considerations, or other relevant factors, diversity might reduce
, the reliability of a particular system. This seems to us to be contrary to l the spirit of the ATWS rule which is aimed at increasing the overall reli-ability of the rapid shutdown system. Furthermore, we believe that in any
, situation in which diversity is considered as a means to increase reli- 1 l
ability, it should be kept in mind that reliability is the objective, and not i diversity per se. Thus, if diversity is to be required, effort should be-made to ensure that it will contribute to increased reliability rather than making the system less reliable.
I 8906210294 890614 PDR ACRS R-1360 PDC I
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The Honorable Lando W. Zech, Jr. June 14,-1989 i The second issue, which also came up during the discussion of the use of i diversity, has to do with the possible influence of aging on the occurrence j of common mode failures. The staff reasoned that even if diversity were not i important during the first forty years of plant life, it might avoid develop- J ment of common mode failures from " wear out," that might occur if operation i beyond the original forty-year license is approved. We believe such concern i may arise from a misunderstanding. While it is true that " wear out" of components does cluster around some "mean-time-to-wear-out," this time should-be well known from test or experience, and components should be replaced or
- overhauled early enough to avoid it. Time-in-service for components that j have not been replaced should be far enough - removed from " wear cut" that 1 failure due to wear out (i.e., " aging") should not be a contributor to common mode failures.
We believe some further consideration of these two issues by the staff is merited, not only as they may bear on the application of the ATWS rule, but because of their significance generally.
Sincerely,
)
Forrest J. Remick Chairman i
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