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September 17, 1999 Dr. William D. Travers Dear Dr. Travers: SUBJECT: PROPOSED RESOLUTION OF GENERIC SAFETY ISSUE-145, "ACTIONS TO REDUCE COMMON CAUSE FAILURES" During the 465th meeting of the Advisory Committee on Reactor Safeguards, September 1-3, 1999, we reviewed the proposed resolution of Generic Safety Issue (GSI)-145, "Actions to Reduce Common Cause Failures." During our review, we had the benefit of discussions with representatives of the NRC staff. We also had the benefit of the documents referenced. Recommendations
Discussion Common-cause failures of redundant safety systems have been of concern ever since quantitative estimates of the availability and reliability of these systems were developed starting in the early 1970s. CCFs are intended to represent causes of dependent failures that are not modeled explicitly in the probabilistic risk assessment (PRA). The fact that a class of diverse failure causes must be modeled has created unusual challenges for the analyst. The difficulty is compounded by the realization that the operating experience contains a wealth of information on potential CCFs, i.e., partial failures that could have evolved into the complete failure of redundant components within a "small" period of time. The efforts over the last 25 years to understand CCFs have been successful. The rate of occurrence of complete CCFs has been steadily decreasing (see attached Figure). Both the industry and the NRC staff have been sensitized to the significance of CCFs. A major contributor to this success has been the work sponsored by the former Office for Analysis and Evaluation of Operational Data and continued by the Office of Nuclear Regulatory Research, to collect and analyze relevant operational experience, as well as disseminating this information. On July 30, 1998, the staff issued NRC Administrative Letter 98-04 to inform the licensees about the availability of CCF database, CCF analysis software, and associated technical reports. Subsequently, the staff transmitted the multi-volume report NUREG/CR-6268 on CCF through a letter dated July 30, 1998. We are concerned that, although this report will eventually be used by PRA analysts, utility managers who could take specific actions to further reduce the potential for CCFs in the near term are unlikely to read this massive report. We, therefore, believe that before GSI-145 is declared as resolved, an additional Administrative Letter should be issued summarizing the insights from the CCF project in a way that will be useful to plant managers. We are somewhat concerned that the staff does not plan to determine whether the licensees are implementing any actions based on the insights of NUREG/CR-6268 to reduce the potential for CCFs. However, given the general awareness of the CCF issue that we mentioned earlier, we do not believe that this is a sufficient reason to justify delaying the resolution of GSI-145. The staff should, of course, be vigilant to identify any signs that the downward trend in the CCF rate has reversed. Sincerely, /s/ Dana A. Powers References : |