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POLICY ISSUE
INFORMATION

SECY-05-0192

October 24, 2005

• PURPOSE
• SUMMARY
• BACKGROUND
• DISCUSSION
• UPCOMING ACTIVITIES
• COORDINATION

PURPOSE:

To inform the Commission of the status of the Accident Sequence Precursor (ASP) Program, provide the annual quantitative ASP results, and communicate the status of the development of the Standardized Plant Analysis Risk (SPAR) models.

SUMMARY:

This report discusses the following activities, which the staff has performed since the last status report (SECY-04-0210), dated November 8, 2004:

• Analysis of the FY 2003 and FY 2004 events to identify precursors (i.e., events with a conditional core damage probability (CCDP) or increase in core damage probability (ΔCDP) that is greater than or equal to 1x10^-6).

• The screening and analyses of events for fiscal year (FY) 2005 to identify significant precursors, defined CCDP or ΔCDP that is greater than or equal to 1x10^-3.

• Evaluation of precursor data to identify statistically significant adverse trends for the Industry Trends Program.

• Revision of SPAR models for all plants for internal initiating events during full-power operation, completion of SPAR models for one lead plant for internal initiating events during low-power and shutdown operations, and completion of SPAR models for two lead plants for the calculation of large early release frequency (LERF), and completion of SPAR models for four plants for external events.

• dentification of organizational conflict of interest concerns with our contractor, Idaho National Laboratory, for several projects including those for SPAR model development.

In addition, this report summarizes related upcoming activities for the next 12 months.

BACKGROUND:

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 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 to provide annual quantitative ASP results.

ASP Program

The NRC established the ASP Program in 1979 in response to the Risk Assessment Review Group report (see NUREG/CR-0400, dated September 1978). The ASP Program systematically evaluates U.S. nuclear power plant operating experience to identify, document, and rank the operating events that were most likely to have led to inadequate core cooling and severe core damage (precursors), accounting for the likelihood of additional failures.

To identify potential precursors, NRC staff reviews plant events from licensee event reports (LERs), inspection reports, and special requests from NRC staff. The staff then analyzes any identified potential precursors by calculating a probability of an event leading to a core damage state. A plant event can be one of two types: (1) an occurrence of an initiating event, such as a reactor trip or a loss of offsite power (LOOP), with any subsequent equipment unavailability or degradation or (2) a degraded plant condition depicted by unavailability or degradation of equipment without an occurrence of an initiating event.

For the first type, a conditional core damage probability (CCDP) is calculated. This metric represents a conditional probability that a core damage state is reached, given an occurrence of an initiating event (and any subsequent equipment failure or degradation).

For the second type, an increase in core damage probability (ΔCDP) is calculated. This metric represents the increase in the probability of reaching a core damage state for the period that an equipment or a combination of equipment is deemed unavailable or degraded from a nominal core damage probability for the same period for which the nominal failure or unavailability probability is assumed for the subject equipment.

An event with a CCDP or a ΔCDP greater than or equal to 1x10^-6 is considered a precursor in the ASP Program. The ASP Program defines a significant precursor as an event with a CCDP or ΔCDP greater than or equal to 1x10^-3.

Program objectives. The ASP Program has the following objectives:

• Provide a measure for trending nuclear power plant core damage risk.

• Provide a partial check on dominant core damage scenarios predicted by probabilistic risk assessments (PRAs).

• Provide feedback to regulatory activities.

• Evaluate the adequacy of NRC programs.

The NRC also uses the ASP Program to monitor performance against the safety goal established in the agency's Strategic Plan. (See NUREG-1100, Vol. 21, dated February 2005.) Specifically, the program provides input to the following performance measures:

• Zero events per year identified as a significant precursor of a nuclear reactor accident (i.e., CCDP or ΔCDP greater than or equal to 1x10^-3).

• No more than one significant adverse trend in industry safety performance with no trend exceeding Abnormal Occurrence Criterion I.D.4.

Program scope. The ASP Program is one of three agency programs that assess the risk significance of issues and events. (The other two programs are the Significance Determination Process (SDP) and the Event Response Evaluation Process, as defined in Management Directive 8.3, “NRC Incident Investigation Program”). Compared to the other two programs, the ASP Program assesses the significance of a broader range of operating experience at U.S. nuclear power plants. For example, when compared to the SDP, the ASP program analyzes initiating events as well as degraded conditions where no deficiency in the licensee's performance was identified. In addition, because of the broader objectives of the ASP Program, ASP analyses will often provide a more detailed evaluation of events, including uncertainty and sensitivity analyses. Attachment 3 to this paper documents the differences and scopes of the three programs.

SPAR Model Development Program

The objective of the SPAR Model Development Program is to develop standardized risk analysis models and tools that staff analysts use 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. For example, the revised SPAR models include a new, improved loss of offsite power/station blackout (LOOP/SBO) module, which the staff used in evaluating station blackout risk as part of the agency's efforts to address issues related to the reliability of the Nations's electric power grid.

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, although these input data may be modified to be more plant- and event-specific, when needed. The system fault trees contained in the SPAR models are not as detailed as those contained in licensees' PRA models. However, benchmarking performed during the onsite quality assurance review of the SPAR models indicated that the core damage frequency from the SPAR models are no more than 15 to 20 percent different when compared to the estimates from the licensee PRA models.

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, the Office of Nuclear Reactor Regulation (NRR), and the NRC's regional offices. In August 2000, the SMUG completed the SPAR Model Development Plan, which addresses the following models:

• Internal initiating events during full-power operation (Revision 3 SPAR models)
• Internal initiating events during low-power and shutdown (LP/SD) operations
• External initiating events (including fires, floods, and seismic events)
• Calculation of large early release frequency (LERF)

DISCUSSION:

This section summarizes the status, accomplishments, and results of each program since the previous status report, SECY-04-0210, dated November 8, 2004.

Status of the ASP Program and SPAR Model Development Program

The following subsections summarize the status of ongoing activities and the accomplishments of the ASP Program and SPAR Model Development Program. Attachment 1 to this paper provides additional detail.

ASP Program

• Completed all precursor analyses from FYs 2001, 2002, and 2003, with the exception of the ongoing analyses of the control rod drive mechanism nozzles at several plants which the staff is currently analyzing.

• Completed the screening for FY 2004 and FY 2005 events for significant precursors (i.e., CCDP or ΔCDP greater than or equal to 1x10^-3). The staff has not identified any significant precursors for these years. The staff has completed the more detailed analyses for FY 2004 events, and has begun similar analyses for FY 2005 events. These analyses will be finalized following peer review.

• Evaluated precursor data to identify statistically significant adverse trends for the Industry Trends Program.

• Issued (April 2005) the Risk Assessment Standardization Project (RASP) guidelines for internal events during power operations as a collaborative effort involving RES, NRR, and the regions, to standardize the risk assessment of operating events and conditions within the agency.

• Developed expert elicitation guidelines to assist ASP and Significance Determination Process (SDP) analysts in developing and documenting an estimate of plant conditions or equipment functionality in cases where data are insufficient or inadequate.

SPAR Model Development Program

• Developed enhanced Revision 3 SPAR models in response to an NRR user need. This effort involved (1) performing a cut set level review against the respective licensee's plant PRA to each of the Revision 3 SPAR models for the 61 plants that were not pilot plants in the Mitigating Systems Performance Index (MSPI) Development Program, and (2) incorporating into the Revision 3 SPAR models the resolution of the PRA modeling issues that were identified (a) during the onsite quality assurance (QA) reviews of the Revision 3 SPAR models, (b) during the MSPI pilot program reviews, and (c) based on feedback from model users.

• Completed SPAR model for one lead plant for internal initiating events during LP/SD operations.

• Completed the SPAR model for calculating LERF for the lead plants in the second and third plant classes.

• Incorporated external initiating events (i.e., internal fires, floods, and seismic event sequences) into the Revision 3 SPAR models for Limerick, Salem, Callaway, Wolf Creek, and Kewaunee.

• Developed a user-friendly interface for use with the Revision 3 SPAR models.

• Continued to interact with the Advisory Committee on Reactor Safeguards (ACRS) in its quality review of the SPAR Model Development Program.

ASP Results, Trends, and Insights

This section summarizes the ASP results, trends, and insights, while Attachment 2 provides additional detail.

• The staff completed the final analysis of the FY 2002 event at Davis-Besse Nuclear Power Station, which involved multiple degraded conditions. This event is a significant precursor (ΔCDP = 6x10^-3). No significant precursors were identified in FYs 2003, 2004, and 2005.¹

• Four precursors identified in FY 2002–2004 had a ΔCDP greater than 1x10^-4. These events included the multiple degraded conditions at Davis-Besse, the potential common mode failure of auxiliary feedwater at Point Beach 1 & 2 (original design deficiency), and another potential common mode failure of auxiliary feedwater at Point Beach 2 (potential clogging of recirculation lines).

• No statistically significant trend was identified in the rates of occurrence of all precursors during the period from FY 1993 through FY 2004.² The staff will report on this result in the NRC's Performance and Accountability Report for FY 2005.

• Trending of precursors by CCDP bins is shown below.

• No trend is detected in the ≥10^-3 CCDP bin and a decreasing trend is observed for the 10^-4 CCDP bin. These trends indicate that the occurrence rate of higher risk precursors is constant or decreasing.

The occurrence rate of lower risk (i.e., 1x10^-5 > CCDP > 1x10^-6) precursors is increasing. The increasing trend is due to the grid-related LOOP events caused by the August 14, 2003 Northeast Blackout (3 precursors) and an increase in the scope of events analyzed due to improvements in analysis methods and the SPAR models (20 precursors). Section 2.3 in Attachment 2 discusses this in more detail.

The electrical grid-related LOOP events caused by the August 2003 Northeast Blackout resulted in several agency actions prior to the summers of 2004 and 2005. These included inspections of licensee conformance with applicable NRC regulations and the raising of licensee awareness of the importance of grid reliability.

• The overall risk from ASP events is relatively constant for the period FY 1993 through FY 2004. (See Attachment 2 , Section 3.8.)

UPCOMING ACTIVITIES:

The staff currently plans to engage in the following activities during the next 12 months:

• Identify and complete the preliminary analysis of significant precursors that occur through June 30, 2006, to support the agency's Strategic Plan goals for monitoring performance.

• Complete the final analysis of events for FY 2004, and continue the screening, review, and analysis (preliminary and final) of events for FY 2005 and FY 2006.

• Complete the preliminary assessment of all FY 2005 ASP events to support Agency Action Review Meeting (AARM), by April 2006. In addition, preliminary assessments will also be completed for events occurring during the first quarter of FY 2006 for those events where the inspection reports are completed during that quarter.

• Issue the final results of the ASP trend study in FY 2006.

• Continue enhancing the Revision 3 SPAR models for internal events during power operations.

• Continue developing SPAR models for internal events during LP/SD operations, LERF, and external events in accordance with the approved SPAR Model Development Plan.

• Continue implementing RASP, including streamlining and coordinating ASP and SDP analyses. In addition, the staff will continue to work with internal and external stakeholders to eliminate reviews of ASP analyses for cases where these reviews are considered of minimal value.

In a September 28, 2005 memorandum, the Office of General Counsel identified organizational conflict of interest concerns with our contractor, Idaho National Laboratory, for several projects including those for SPAR model development. The staff is currently considering potential options for resolutions of these concerns. However, this issue may impact upcoming activities and schedules related to the SPAR Model Development program and with programs that utilize the SPAR models.

In summary, the ASP Program continues to evaluate the safety significance of operating events at nuclear power plants and to provide insights to NRC's regulatory programs. The SPAR Model Development Program is continuing to develop and improve independent risk analysis tools and capabilities to support the use of PRA in the agency's risk-informed regulatory activities. SPAR models are used to support the Reactor Oversight Process, the ASP Program, and the Generic Safety Issue resolution process. SPAR models are also used to perform analyses in support of the staff's risk-informed reviews of license amendments, as well as to independently verify the Mitigating Systems Performance Index (MSPI).

COORDINATION:

The Office of the General Counsel has reviewed this Commission paper and has no legal objections to its content.

¹ Screening and reviews of FY 2005 events have been completed through September 30, 2005.

² The trend analyses include preliminary FY 2004 analysis results. In addition, the staff is currently analyzing conditions involving primary water stress corrosion cracking of control rod drive mechanism (CRDM) housings. For the purposes of CCDP bin trend analyses, the CRDM cracking events were placed in the 10^-5 CCDP bin based on preliminary results.

³ If a trend is considered statistically significant it is very unlikely that the trend is solely a result of chance (explained in Attachment 2 , Section 2.0).

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