Independent Verification of the Mitigating Systems Performance Index (MSPI) Results for the Pilot Plants (NUREG-1816)
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Manuscript Completed: October 2004
Date Published: February 2005
D.A. Dube (NRC Project Manager)1, C.L. Atwood2,
S.A. Eide3, B.B. Mrowca4, R.W. Youngblood4, D.P. Zeek3
2905 Covington Road
Silver Spring, MD 20910
3Idaho National Laboratory
P.O. Box 1625
Idaho Falls, ID 83415-3850
11140 Rockville Pike
Rockville, MD 20852
1Division of Risk Analysis and Applications
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
NRC Job Codes J8263 and Y6370/Y6636
In its Reactor Oversight Process (ROP), the U.S. Nuclear Regulatory Commission (NRC) currently uses performance indicators to quantify safety system unavailability (SSU) for four important nuclear power plant systems. Over time, the NRC staff has identified a number of concerns related to the use of these indicators, including the use of short-term unavailability to approximate unreliability, the use of generic performance thresholds irrespective of variations in risk significance, and potential double-counting as a result of support system failures cascading onto front line systems. Moreover, the way the SSU indicators currently measure unavailability is inconsistent with the definition in the NRC’s Maintenance Rule, as well as the indicators promulgated by the World Association of Nuclear Operators and the Institute of Nuclear Power Operations.
This report describes the background, technical issues, and pilot project leading to the development of a more risk-informed performance indicator, known as the Mitigating Systems Performance Index (MSPI). The MSPI addresses most of the concerns related to the use of the current indicators. The NRC staff extensively tested and improved the MSPI methodology during a 12-month pilot plant application phase that involved 20 nuclear power plant units of varying design. The staff also evaluated technical issues related to the new indicator’s sensitivity to probabilistic risk assessment (PRA) modeling detail. In addition, the staff compared the MSPI results to the existing indicators, as well as findings from the significance determination process (SDP). The analysis indicates that the MSPI appears to consistently provide a better measure of integrated system performance than the current SSU performance indicators.