United States Nuclear Regulatory Commission - Protecting People and the Environment

Resolution of Generic Safety Issue B-61, "Allowable ECCS Equipment Outage Periods"

February 19, 1999

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

Dear Dr. Travers:


During the 459th meeting of the Advisory Committee on Reactor Safeguards, February 3-6, 1999, we reviewed the proposed resolution of Generic Safety Issue (GSI) B-61, "Allowable ECCS Equipment Outage Periods." During our review, we had the benefit of discussions with representatives of the NRC staff and the documents referenced.


The issues identified under GSI B-61 will be addressed through the implementation of the Maintenance Rule. Therefore, consideration of these issues under the aegis of GSI B-61 is not required, and GSI B-61 should be considered resolved.


GSI B-61, identified in June 1978 and prioritized in November 1983, was described in NUREG-0471, "Generic Task Problem Descriptions." It addresses the risk impact of surveillance test intervals and allowable equipment outage periods. These allowable outage periods, which are largely based on engineering judgment, are defined in Technical Specifications for safety-related systems. The allowable outages represent 20 to 80 percent of the total unavailability of emergency core cooling systems (ECCS).

The staff considered the need to implement a limit on cumulative outage time (COT) and conducted a limited regulatory analysis of the issues in GSI B-61 to evaluate the impacts of COT on systems during unscheduled or corrective maintenance. Results of this analysis revealed that implementation of COT did not meet the substantial added protection criterion specified in the regulatory analysis guidelines. The staff's analysis was limited to consideration of four representative plants. The staff did not compare the results of the analysis with those included in the Individual Plant Examination Insights report. Also, the staff did not perform an evaluation of uncertainties associated with its analysis. Although this analysis was inadequate for the resolution of GSI B-61, this issue should be considered resolved because the concerns identified under this GSI will be addressed through implementation of the Maintenance Rule.



Dana A. Powers


  1. Memorandum dated January 12, 1999, from Thomas L. King, Division of Systems Technology, RES, to John T. Larkins, ACRS, Subject: Resolution of B-61, "Analytically Derived Allowable Equipment Outage Periods."
  2. U. S. Nuclear Regulatory Commission, NUREG-0471, "Generic Task ProblemDescriptions," June 1978.
  3. U. S. Nuclear Regulatory Commission, NUREG-0933, "A Prioritization of Generic Safety Issues," Item B-61: Allowable ECCS Equipment Outage Periods, November 1983.
  4. Memorandum dated June 17, 1983, from F. H. Rowsome, Division of Safety Technology, NRR, to G. C. Lainas, Division of Licensing, NRR, Subject: Safety Evaluation of the Licensees' Response to TMI Action Item II.K.3.17.
  5. U. S. Nuclear Regulatory Commission, NUREG/BR-0058, Revision 2, "Regulatory Analysis Guidelines of the U.S. Nuclear Regulatory Commission," Final Report, November 1995.
Page Last Reviewed/Updated Friday, January 31, 2014