Risk-Informed, Performance-Based Regulatory Implications of Improved Emergency Diesel Generator Reliability

Abstract

The Nuclear Regulatory Commission's (NRC) steady progress towards risk-informed performance-based regulation (RIPBR) prompted the practical application of this regulatory tool in order to demonstrate its potential benefits. This practical demonstration makes up one part of an Idaho National Engineering and Environmental Laboratory (INEEL) sponsored project entitled Integrated Models, Data Bases and Practices Needed for Performance-Based Safety Regulation. Project members selected the emergency diesel generator system as a candidate for assessment because of its high risk importance for core damage frequency (CDF) as well as for its failure to exhibit fulfillment of its current maintenance objectives. An analysis of current NRC maintenance and inspection requirements of the emergency diesel generators at the Millstone 3 nuclear power plant was performed by the project members. Maintenance and inspection items identified as unnecessary or harmful to the EDG qualified as candidates for removal from the current surveillance schedule. Expert testimony and comparisons with similar non-nuclear utility industries aided in the identification of candidate items. Calculations of the subsequent risk, reliability, safety, and economic implications revealed several benefits of the inspection alterations. The modified inspection provided improved backup power availability and defense in depth during the refueling outage. A sensitivity analysis performed on the EDG basic events affected by inspection alteration showed that a 50% reduction in these basic event failure rates would decrease the EDG system failure probability by 13.9%. The altered inspection also shortens the plant's refueling outage critical path therefore decreasing the risk of fuel damage and improving the risk profile of the plant outage. Transfer of the revised inspection to performance while the plant is operating at power resulted in identical refueling outage benefits. Performance of the inspection at power requires an increase in the allowed outage time (AOT) of the plant. The subsequent rise in core damage frequency due to the increased AOT is considered negligible.