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(Information) SECY-03-0049 March 31, 2003
To inform the Commission of the status of the Accident Sequence Precursor (ASP) Program and the development of the Standardized Plant Analysis Risk (SPAR) models. In a memorandum to the Chairman dated April 24, 1992, the staff of the U.S. Nuclear Regulatory Commission (NRC) committed to report periodically to the Commission on the status of efforts to improve the ASP Program. In SECY-94-268, dated October 31, 1994, the staff made two significant changes to the report. First, the staff committed to provide the report annually, and second, the staff began providing annual quantitative ASP results. This report discusses the following activities, which the staff has performed since the last status report (SECY-02-0041), dated March 8, 2002:
The attachment to this paper presents a brief background summary of the ASP and SPAR Model Development Programs and their uses in risk-informed regulation. The staff has not made any significant changes to the programs and their uses since the last status report. This section summarizes the program status, accomplishments, and results for this reporting period. Identification of Significant Precursors The ASP Program provides the basis for one of five performance measures, "no more than one event per year identified as a significant precursor of a nuclear accident," for the performance goal to maintain safety in the reactor safety arena of the NRC's Strategic Plan. The Strategic Plan defines a significant precursor as an event that has a 1 in 1000 (10-3) or greater probability of leading to a reactor accident. During this reporting period, the staff completed its screening and review of events in FY 2002 for potential significant precursors. As reported in the last status report, no significant precursors were identified in FY 2001. The staff did, however, identify three events in FY 2002 as having the potential to be significant precursors. One condition, discovered at the Davis-Besse Nuclear Power Station, involved reactor vessel head degradation and cracking of the control rod drive mechanism (CRDM) housing. The analysis of the condition at Davis-Besse is still ongoing. The second condition, which affected both units at Point Beach Nuclear Plant (counted as two precursors), involved a potential common-mode failure of all auxiliary feedwater (AFW) pumps as a result of a design deficiency in the pumps' air-operated minimum flow recirculation valve. Preliminary results show that the condition at Point Beach Units 1 and 2 does not meet the significant precursor criteria. Moreover, the preliminary analyses indicates that the performance measure (defined above) was not exceeded for FY 2002. The status of both analyses is as follows:
FY 2001 and 2002 Event Analyses The analyses of FY 2001 and 2002 events are ongoing. The current status is as follows:
Delays in issuing preliminary analyses of FY 2001 events are the result of the program's focus on the more complex and potentially risk-important events described above. In addition, in-house resources were assigned to initiate the following improvements in ASP analysis methods to address internal and external stakeholder comments on characterization of precursor events:
Based on an evaluation of the delays experienced during the past year, the staff has initiated two efforts to streamline the analysis and review process in the ASP Program. During the next year, the staff is developing a plan to prioritize and, where appropriate, scale back efforts on events with lower CCDPs and events that are non-controversial. In the longer term, to improve efficiency, NRR and RES are planning to improve consistency between ASP approaches and those used in the Significance Determination Process (SDP), where possible. This effort is part of NRR's SDP improvement initiative. The NRC's Industry Trends Program (ITP) provides the basis for addressing the performance goal measure of "no statistically significant adverse industry trends in safety performance," which is one of the five measures in the NRC's Strategic Plan for the performance goal of maintain safety. One of the indicators that NRR uses to assess industry performance against this measure is the trend of all precursors identified by the ASP Program. The method used for trending precursors is consistent with the analysis used for trending the other indicators in the Industry Trends Program. Consistent with last year's trending analysis, the trend is based on the number of all precursors since FY 1993.(1) Analyses for FY 2000 are preliminary, with final analyses nearing completion (pending resolution of peer review comments). The data for FY 2001 are based on ongoing analyses that have undergone internal staff review. Results. The staff did not observe any statistically significant trend in the occurrence rate of precursors (CCDP or CDP >10-6) during the 1993-2001 period. The following figure shows the occurrence rate (number of precursors per reactor year) for all precursors by fiscal year. The staff did not use data prior to FY 1993 in the trending analysis; however, the figure shows those data for information and to provide perspective. Although the staff did not detect a trend based on data for 1993 through 2001 (shaded bars in the figure), data for the 5-year period from 1997 through 2001 show an increasing number of precursors.
The staff will investigate the nature of the precursors to determine whether there is an explanation for the relatively low number of precursors between 1997 and 1999, and an increasing number of potential precursors in 2000 and 2001. This evaluation will occur after the staff completes its FY 2001 preliminary analysis. The evaluation of trends is the subject of a joint project between NRR and RES as part of the Industry Trends Program. Availability of ASP Analyses and Analysis Details and Results Issuance of ASP analysis reports. In SECY-02-0041, the staff indicated that the issuance of preliminary and final precursor analyses was suspended following the terrorist attacks on September 11, 2001. The staff resumed its issuance of ASP analyses and SPAR models to the licensees in June 2002, based on guidance provided in a Staff Requirements Memorandum on COMSECY-02-0015, "Withholding Sensitive Homeland Security Information from the Public," dated April 4, 2002. Guidance for issuing results of risk analyses to the public is being coordinated between RES, NSIR, and NRR. Posting ASP results on the World Wide Web. The staff is currently implementing two initiatives to make precursor analyses and results available to NRC staff, our contractors, and the public:
The SPAR Model Users Group (SMUG) is composed of representatives from each of the organizations within the agency's program and regional offices that use risk models in regulatory activities. The SMUG meets on a regular basis to provide technical guidance for the SPAR Model Development Program consistent with the approved Integrated SPAR Model Development Plan. This plan conforms to the modeling needs that the SMUG members and their management identified for performing risk-informed regulatory activities. In accordance with this plan, the staff completed the following activities in model and methods development in the past year:
The staff currently plans to engage in the following activities during the next 12 months:
BACKGROUND AND USES OF THE ACCIDENT SEQUENCE PRECURSOR (ASP) PROGRAM AND THE STANDARDIZE PLANT ANALYSIS RISK (SPAR) MODEL DEVELOPMENT PROGRAM The discussion below provides a brief background of the Accident Sequence Precursor (ASP) Program, the Standardized Plant Analysis Risk (SPAR) Model Development Program, and the uses of ASP results and SPAR models in risk-informed regulatory activities. ASP Program The U.S. Nuclear Regulatory Commission (NRC) established the ASP Program in 1979 in response to the Risk Assessment Review Group report (see NUREG/CR-0400, September 1978). The primary objective of the ASP Program is to systematically evaluate U.S. nuclear plant operating experience to identify, document, and rank the operating events that were most likely to lead to inadequate core cooling and severe core damage (precursors), if additional failures had occurred. To identify potential precursors, the NRC staff reviews events and conditions from licensee event reports, inspection reports, and special requests from NRC staff. The staff then analyzes any identified potential precursors and calculates a conditional core damage probability (CCDP) by mapping failures observed during the event onto accident sequences in risk models. An event with a CCDP or a condition with a change in core damage probability (CDP or importance) greater than or equal to 1.0 x 10-6 is considered a precursor in the ASP Program. The ASP Program has the following secondary objectives:
The NRC also uses the ASP program to monitor performance against the Strategic Plan performance goal to maintain safety in the reactor safety arena (see Volume 2, Part 1, of NUREG-1614, September 2000). The program provides input to the following performance measures:
SPAR Model Development Program The Standardized Plant Analysis Risk (SPAR) models are the analysis tools used by staff analysts in many regulatory activities, including the ASP Program and Phase 3 of the Significance Determination Process (SDP). The SPAR models have evolved from two sets of simplified event trees that were initially used to perform precursor analyses in the early 1980s. Today's Level 1, Revision 3 SPAR models for internal events are far more comprehensive than their predecessors. The Level 1, Revision 3 SPAR models comprise a standardized, plant-specific set of PRA-based risk models that use the event tree/fault tree linking methodology. They also use an NRC-developed standard set of event trees and standardized input data for initiating event frequencies, equipment performance, and human performance. These input data can be modified to be more plant- and event-specific, where needed. The system fault trees contained in the SPAR models are not as detailed as those contained in licensees' PRA models. However, benchmarking performed with the SPAR models during the onsite quality assurance review of these models indicates that the models capture 80-85% of the total overall core damage frequency. Revision 3 of the SPAR models includes uncertainty analysis capability through the propagation of uncertainties at the equipment and human performance levels. The SPAR models use results from studies sponsored by the NRC's Office of Nuclear Regulatory Research (RES) to provide an independent validation of input parameters used in a licensee's PRA. These studies include system and component reliability studies, initiating event studies, and a human reliability analysis method. In 1999, the SPAR Model Users Group (SMUG) assumed coordination of model development efforts that support the ASP Program and other risk-informed regulatory processes. This group is composed of representatives from RES and the Office of Nuclear Reactor Regulation (NRR), and regional offices that use reactor plant risk models in regulatory activities. In August 2000, the SMUG completed the SPAR Model Development Plan, which conforms to the modeling needs that SMUG members and their management identified for performing risk-informed regulatory activities. This plan addresses the following models:
Uses The NRC staff uses the ASP methodology, SPAR models, and ASP analysis results to perform the following risk-informed regulatory activities:
1. Consistent with the previous years' trending analyses, the trend is based on the number of precursors since FY 1993. Beginning in 1992, the staff transmitted each preliminary analysis to the responsible licensee for review and comment. As a result of comments received from the licensees, the staff's analyses incorporated credit for additional equipment and recovery procedures. 2. The ASP Program provides one industry trend indicator used in this performance measure. |
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