ML25168A076

07-15-83 ACRS Report on the Proposed Final Rule on Anticipated Transients Without Scram

ML25168A076
Person / Time
Issue date:	07/15/1983
From:	Mark J Advisory Committee on Reactor Safeguards
To:	Palladino N NRC/Chairman
References
Download: ML25168A076 (1)

Text

UNITED STATES NUCLEAR REGULATORY COMMISSION ADVISORY COMMITTEE ON REACTOR SAFEGUARDS WASHINGTON, D. C. 20555 July 15, 1983 Honorable Nunzio J. Palladino Chairman U.S. Nuclear Regulatory CorT111ission Washington, D. C. 20555

SUBJECT:

ACRS REPORT ON THE PROPOSED FINAL RULE ON ANTICIPATElJ TRANSIENTS WITHOUT SCRAM (ATWS)

Dear Dr. Palladino:

During its 279th meeting, July 7-9, 1983, the Advisory Committee on Reactor Safeguards reviewed the proposed Rule on ATWS.

A Subcommittee meeting was held with members of the NRC Staff on May 27, 1983.

Tne Committee also had the benefit of the documents listed.

For BWR plants, the Rule requires installation of an alternate rod injection (ARI) system and a standby liquid control system (SLCS) of a specified control capacity.

The Rule also requires that the SLCS must be capable of automatic initiation for new plants and for other BWR plants that already have the capability to automate this system.

Automatic trip of the recircu-lation pumps is also specified, although we understand that this feature is already operational on all plants through Commission Order.

PWR plants are required to have equipment diverse from the reactor protec-tion system installed to assure automatic initiation of auxiliary feedwater and of turbine trip. In addition, reactors manufactured by tne ~abcock &

Wilcox Company (B&l) and Combustion Engineering, Incorporated (CE) are required to install a diverse scram actuation system.

While not in the current version of the Rule, an amendment will be proposed to require Westinghouse reactors to install a diverse scram actuation system as well.

The NRC Staff is recommending that licensees establish a reliability assur-ance program for their plants' reactor trip systems.

We support efforts to enhance reliability assurance for such critical plant systems; however, we believe a comprehensive program that encompasses all plant safety-related systems would be more effective.

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Honorable Joseph May 13, 1981 Although the actions taken by the NRC were timely as a direct response to the Browns Ferry Unit 3 incident, we believe that this incident should have inspired a very considerable acceleration of the Commission's efforts to resolve the ATWS issue. This has not been the case.

We recommend strongly that the Commission act promptly and with high priority to resolve ATWS.

In this regard, we reiterate the recommendations made in our letter of April 16, 1980 on ATWS.

Sincerely,

~~

Chairman 1913

Honorable Nunzio July 15, 1983 We recommend that instrumentation needed to accomplish safe shutdown of the plant following an ATWS event be qualified to perform acceptably following the high pressure or other environmental effects to which the instrumenta-tion might be exp,osed during the event.

We also recommend that the possible effects of high p,ressure on the integrity of primary system pump seals and the pressure boundary of instruments or other components not covered by the ASME Boiler and Pressure Vessel Code be explored, and that the NRC Staff assure itself that pump seal or other primary system coolant leakage caused by an ATWS does not significantly increase the likelihood of severe damage to the core.

To further decrease the probability of an ATWS, we recommend that efforts be made by plant operators to decrease the number of transients that challenge the reactor protection system.

For example, operating experience has shown that a large portion of plant transients results from malfunctions of the feedwater system.

Re.cent information. al so i ndi cat es that the number of reactor scrams per plant is significantly higher in the U.S. than in certain foreign countries (Japan for example).

We recomirerrd an investigation to determine if practical means exist to, reduce challenges to the reactor trip system.

We are concerned about the potential for confusion when part of the reactor trip system is safety grade and part is not.

Neither the diverse shutdown logic systems required by the Rule nor the heat tracing for boron solution storage systems are, safety grade.

The same is true for the alarm systems that indicate whether the heat tracers are operative.

These matters may warrant further consideration.

The NR~ Staff has suggested that an ATWS contribution to core melt frequency of 10- /reactor year or less would be acceptable.

Since the risk to the public from such an accident may vary markedly between PWRs and BWRs, it is not clear that a single core melt probability criterion should be applied uniformly to all LWRs.

If a major release of5 radioactive material from the containment is likely, the criterion of 10- /reactor year is, at best, marginal for any single cause, such as ATWS, and further reductions in likelihood should be considered.

The NRC Staff's estimates of ATWS likeli-hood are best estimates, with considerable uncertainty, and thus may not reflect a reasonable degree of assurance that the actual risk is as low as it should be.

We recommend that considerations of such risk be included, particularly for reactors having high surrounding population densities.

We recommend that designers of new plants give consideration to designs that will reduce reactor trip system challenges as well as enhance reli-ability.

We note that while the Rule requires new 8WRs to enhance the capability of the SLCS, it is silent concerning the need for additional mitigating features for PWRs.

For example, there is no requirement for 1915

Honorable Nunzio July 15, 1983 additional pressure relieving capability for B&I~ and CE plants.

We disagree with this position and recommend that the NRC require licensees to explore means for enhancing the ATWS mitigative capabilities for new PWR plants.

We support the provision that the schedule for implementation be one negoti-ated between the licensee and the NRC, but caution that care be exercised to assure the timeliness of the changes required.

We believe that the proposed Rule is an appropriate approach to dealing with the ATWS issue, but recommend that consideration be given to the above comments before a Rule is issued in final form.

The Committee plans a later review of the proposed amendment dealing with the Westinghouse trip system.

References:

Sincerely,

~CB J. J. Ray'9--

Chairman I. Memorandum from ~1. L. Ernst, NRC, to R. F. Fraley, ACRS, "Amendments to 10 CFR Part 50 Related to Anticipated Transients Without Scram {ATWS), 11 dated June 27, 1983.

2.

U.S. Nuclear Regulatory Commission, "Generic Implications of ATWS Events at the Salem Nuclear Power Plant, 11 USNRC Report, NUREl:.i-1000, Volume 1, dated April 1983.

3.

U.S.

Nuclear Regulatory Corrmission, "Generic Actions for Licensees and Staff in Response to the ATWS Events at Salem Unit l, 11 USNRC Report, SECY-83-248, dated June 22, 1983.

4.

Letter from E. P.

Rahe, Jr., Westinghouse Electric Corporation, to H. R. Denton, NRC,

Subject:

Westinghouse Comments on Proposed Amend-ment to ATWS Rule, dated June 22, 1983.

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