Core damage frequency observations and insights of LWRs based on the IPEs

On November 23, 1988, the Nuclear Regulatory Commission issued Generic Letter 88-20 which requested licensees in the United States to conduct an Individual Plant Examination (IPE) in order to identify potential severe accident vulnerabilities at their plant and to implement cost-effective plant improvements to reduce or eliminate these vulnerabilities. The licensees, in their submittals, were requested to report the approach/method used in the IPE and the resultant core damage frequency (CDF) along with the dominant accident sequences, the major contributors and any identified vulnerabilities. To date, the majority of the IPEs have been submitted by the licensees. In a follow-up effort to support regulatory activities, the staff (with assistance from Sandia National Laboratories) has initiated an IPE Insights program that involves a global examination of these IPE results to identify insights of potential generic safety significance relative to plant design, operational or maintenance features. Preliminary insights (from the IPEs) confirm that plant characteristics, analytical assumptions and data, for example, have major impacts on the CDF results of the IPEs. In addition, as a result of the generic letter, licensees have both implemented and proposed numerous plant improvements that impact the overall CDF of the plant. A qualitative assessment of the open-quotes importanceclose quotes of these improvements to the industry relative to the CDF is being performed as part of this program. This paper will address these insights; the basis for the observed similarities and variabilities for various classes of plants will be discussed