ML19290D040
| ML19290D040 | |
| Person / Time | |
|---|---|
| Issue date: | 07/14/1976 |
| From: | Moeller D Advisory Committee on Reactor Safeguards |
| To: | Gossick L NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| Shared Package | |
| ML19290C953 | List: |
| References | |
| FOIA-80-587 ACRS-R-0725A, ACRS-R-725A, NUDOCS 8002150533 | |
| Download: ML19290D040 (3) | |
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UNITED STATES f7h NUCLEAR REGULATORY COMMISSION
-q /- f ADVISORY CCf.".11TTEE ON RE ACTOR S A PEGUARDS
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riasmuc ion. o. c. 2csss July 14, 1976 Mr. L. V. Cossick
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Executive Director for Operations b-U. S. Nuclear Regulatory Coc:sission g
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Dear Mr. Gossick:
In its report of April 3,1975, to Chairman Anders on the Reactor Safety Study (RSS), 'n' ASH-1400, the ACRS expressed its belief that many of the techniques used in the RSS can and should be used by reactor designers to i= prove safety and by the h2C Staff as a
.tupplement to safety assesscent.
During recent reviews, th'e Cormittee has learned of ongoing efferts in the NRC Regulatory Research programs to test fault-tree methedology for possible application to an evaluation of the sin'gle-failure criterion, redundancy require =ents, and testing and maintenance requirements.
l The Com.ittee reco = ends that this methodology be applied in inves-tigating the areas listed below:
(1)
In recent months varying approaches to syste= design for residual heat removal (RHR) and ECC systems have been proposed by the same PWR reactor 9cndor for differen: models (e.g., tuo 1007. capacity systems versus three 50*' (or possibly larger) capacity syste s).
It appears that RSS techniques afford an opportunity of providing an assess =cnt of the relative and absolute reliability of such l
alternative designs, albeit with uncertainties.
4 (2). The ACRS has received a reor,rt that current regulatory requirc=ents for ECC systc=s in German PWR's require an additional redundant system (which cuy be representative of a system under maint.cnance).
The ACRS recom=cnds that an assessment be made of the merits and costs as well as potential disadvantages of such a requiren sat compared to current U.S. requirements.
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. July 14, 1976
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Hr. L. V. Cossick There are many systems which are normally in operation, and which (3) are required in varying degree and for varying time periods to cooling water mair,tain saf e shutdown (for exampic, the component system). Many of these systems currently employ two trains in order to meet the single failure criterion.
The ACRS recommends that the methodology of RSS be applied to such systems, and coupl:d with other considerations, an assessment be made of whether the single-failure criterion as currently applied This assessment should allow for the frequency and is adequate.
time periods of possible downtime for one of two vital systems for repair, and should include an assessment of the adequacy of measures for repair.
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(4) The ACRS recoer. ends that the reliability of isolation of the high pressure primary system from connecting low pressure systems be lants and for proposed plants, in the light assessed for existing p'uences of a loss of such isolation..
of the potential conseq The ACRS believes that by examination of these (and similar) systems by the methodology of R33, further insight into potential probic=s or into improvements in safety can be obtained, and that an improved basis for the setting of priorities and the allocation of resources may be provided.
j The Committee suggests'that such efforts be pursued both by NRC and by i,
the appropriate groups in the nuclear industry.
3 Sincerely yours,
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Dade W. Moeller Chairman O
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Use of Probabilistic Assessment of Reliability Honorable. William A. Anders January 14, 1976 LTplicit in the use of a probabilistic gJa1 is the application of proba-bilistic mthods in the analysis of the reactor systems. Although L'SH-1400 provides assistance in this arca, data for som systers are still suffi-ciently coarse that engineering judg:rnt must be used, both in synthesizing the analytical models and in choosing appropriate input data. Lbder the
' circumstances there is a need for some conservatism in the choice of mcdel$
and the selection of data. Even so, there are a nurcher of. approaches to the redelling that ma'y prove to have equal validity and the ACPS suggests that both the hTC Staff and the vendors give further consideration to vr.clous alternatives. For exa ple it may be feasible to treat the time-varying nederator tc perature coefficient probabilistically.
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