ML16224A800

Evaluation of the Risks During Shutdown and Low-Power Operations for U.S. Nuclear Power Plants

ML16224A800
Person / Time
Issue date:	04/09/1992
From:	Ward D Advisory Committee on Reactor Safeguards
To:	Taylor J NRC/EDO
References
Download: ML16224A800 (3)

Text

D920409 Mr. James M. Taylor Executive Director for Operations U.S. Nuclear Regulatory Commission Washington, D.C. 20555

SUBJECT:

EVALUATION OF THE RISKS DURING SHUTDOWN AND LOW-POWER OPERATIONS FOR U.S. NUCLEAR POWER PLANTS During the 384th meeting of the Advisory Committee on Reactor Safeguards, April 2-4, 1992, we reviewed the staff's program to evaluate the risks posed by U.S. nuclear power plants during shutdown and low-power operations. Our Subcommittee on Plant Operations considered this matter during its April 1, 1992 meeting.

During these meetings, we had the benefit of discussions with representatives of the NRC staff and NUMARC. We also had the benefit of the referenced documents. In our letter of August 13, 1991, we commented on the staff's program for carrying out this evaluation.

The current status of the staff's review and the proposed plans for future staff actions are described in SECY-92-067, "Evaluation of Shutdown and Low-Power Operation" and in NUREG-1449 (Draft Report for Comment), "Shutdown and Low-Power Operation at Commercial Nuclear Power Plants in the United States." The staff expects to complete its formal regulatory analysis by late summer 1992.

We believe that the staff has done a commendable job in evaluating risks from shutdown and low-power operations and in developing a plan for future actions. We were particularly impressed by the insights gained by the staff participants who visited 17 nuclear power plants to observe low-power and shutdown operations. These insights are described in Section 3 of NUREG-1449 and provide what we regard as the principal bases for the staff's technical findings and conclusions. We were also positively impressed with the industry's efforts and believe that industry positions and recommendations expressed in NUMARC 91-06, "Guidelines for Industry Actions to Assess Shutdown Management," are consistent with and complementary to the staff's. We encourage continued interaction with NUMARC on this issue.

The staff's evaluation provided satisfactory responses to earlier comments in our August 13, 1991 letter to you. We believe that the staff's actions relative to the concerns noted in that letter are appropriate. However, we have the following additional comments.

We support the Office of Nuclear Regulatory Research's efforts on developing two plant-specific low-power and shutdown PRAs.

However, these PRAs will not be completed in time for the results to provide useful input into decision making on possible further regulatory actions resulting from this study. Therefore, we

recommend that these efforts be refocused to emphasize development of better methods to deal with the human factors aspects of these PRAs and toward more definitive separation of the various contributing elements to the core damage frequency (e.g., equipment reliability, maintenance activities, adequacy of instrumentation, and the human factor elements).

Section 6.7.1 of NUREG-1449 discusses the use of freeze seals in the maintenance of otherwise nonisolable nuclear power plant piping systems. We agree with the proposed NRC staff action as described in Section 8.2.3 of NUREG-1449, but recommend an explicit requirement for participation of a knowledgeable structural expert in the licensee's 10 CFR 50.59 review of each freeze seal application. This recommendation is based on our concern that inappropriate use of cryogenic freezing media, which have boiling points well below the nil-ductility transition temperature of carbon and low-alloy steels, could lead to brittle failure of piping systems and thus initiate a loss-of-coolant accident.

The insights gained from this study appear to us to have lessons relevant to full power and other modes of operation. For example, there have been unanticipated plant trips at power caused by poor control over maintenance activities. We recommend that the guidelines under development be evaluated for broadened application to all modes of operation.

The proposed guidelines focus on activities only within the plant boundaries. Activities outside the plant boundaries (within switchyards or on transmission lines) have caused unanticipated plant trips and loss of offsite power events. We recommend that appropriate action, perhaps a utility corporate level directive, be taken to ensure the proper control over such activities.

Sincerely, David A. Ward Chairman

References:

1. U.S. Nuclear Regulatory Commission, NUREG-1449, "Shutdown and Low-Power Operation at Commercial Nuclear Power Plants in the United States" (Draft Report for Comment), February 1992

2. SECY-92-067, dated February 25, 1992, from James M. Taylor, Executive Director for Operations, NRC, for the Commissioners,

Subject:

Evaluation of Shutdown and Low-Power Operation

3. NUMARC 91-06, "Guidelines for Industry Actions to Assess Shutdown Management," December 1991

4. Letter dated August 13, 1991, from David A. Ward, Chairman, ACRS, to James M. Taylor, Executive Director for Operations, NRC,

Subject:

Evaluation of Risks During Low-Power and Shutdown Operations of Nuclear Power Plants

5. Memorandum dated January 10, 1992, from A. Thadani, NRC, for David A. Ward, ACRS,

Subject:

Information on Containment

Hatches

6. NRC Information Notice IN 91-81, "Switchyard Problems That Contribute to Loss of Offsite Power," December 16, 1991