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July 20, 2001

Dr. William D. Travers
Executive Director for Operations
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

Dear Dr. Travers:

During the 484th meeting of the Advisory Committee on Reactor Safeguards, July 11-13, 2001, we reviewed the draft NUREG-1742, Vols. 1 and 2. During this review, we had the benefit of discussions with representatives of the NRC staff and the documents referenced. Our Subcommittee on Reliability and Probabilistic Risk Assessment discussed this matter on June 22, 2001.

We agree with the staff that the IPEEE Program has been generally successful in meeting the intent of Generic Letter 88-20, Supplement 4, "Individual Plant Examination of External Events (IPEEE) for Severe Accident Vulnerabilities - 10 CFR 50.54(f)," and has had a positive impact on safety.

Most IPEEEs are not based on probabilistic risk assessments (PRAs) and cannot be used in risk-informed regulatory decisionmaking. Most licensees chose to employ screening analyses and, consequently, the major external-event contributors to risk cannot be identified. Even with this limitation, the IPEEEs have confirmed the findings of earlier PRAs that earthquake- and fire-initiated accident sequences are important contributors to risk. This demonstrates the need for external-event PRAs to implement risk-informed regulatory decisionmaking.

The IPEEE results demonstrate the importance of human performance. Station blackout with failure to align and initiate the steam-driven auxiliary feedwater pump is one of the dominant earthquake-induced sequences for pressurized water reactors. For boiling water reactors, the dominant sequences following an earthquake include the failure of manual actions to recover power. Several manual recovery actions appear in risk-significant fire-initiated sequences, such as starting diesel generators and opening motor-operated valves when fire-induced cable damage occurs.

The report states correctly that there is no strong technical basis for the human action probabilities that are used in the IPEEEs. There is a clear need to improve the methodology used in the evaluation of the probability of unsatisfactory human performance. This should be identified as an important methodological issue for fire- and earthquake-initiated sequences.

We will comment on the unresolved safety issues and the generic safety issues that were addressed by the IPEEE Program after the staff responds to public comments on the report.