Performing Risk-InformedDefense-in-Depth and Diversity Assessmentsfor Digital Upgrades in Nuclear Plants

摘要

Most of the replacements and upgrades of aging and obsolete instrumentation and controlrnsystems involve transitions from analog to digital technology. The current regulatory guidancernrequires that a defense-in-depth and diversity evaluation be performed for digital upgrades tornsafety systems. This largely deterministic approach sometimes results in significant utility andrnNuclear Regulatory Commission (NRC) resources spent on events, components, and backuprnsystems that do not 1) contribute significantly to plant risk, 2) address other events that may bernsignificant contributors, or 3) improve plant safety. EPRI has recently published a guideline tornassist utilities in performing such evaluations and has submitted it to the Nuclear RegulatoryrnCommission for review and endorsement. Three methods are proposed. The ExtendedDeterministic method is based on the approach outlined in the current regulatory guidance, but itrnalso considers designed-in “defensive measures” and use of risk insights when assessingrnsusceptibilities to common-cause failure. It addresses potentially risk-significant events that mayrnbe overlooked by the current approach. The Standard Risk-Informed method and the SimplifiedrnRisk-Informed method both take into account risk insights provided by the plant-specificrnprobabilistic risk assessment. The EPRI approach examines three factors that determine thernimpact of a digital I&C upgrade on plant risk: a) Impact of the upgrade on I&C channelrnreliability; b) Potential for digital common-cause failure introduced by the upgrade; and c) Therndesign of the plant and the plant mitigating systems available to respond to specific eventsrnaffected by the upgrade (sometimes called the plant safety model). It is the combined effects ofrnthese factors that determine impact on plant risk. This paper describes the EPRI approach forrnassessing defense-in-depth and diversity in the context of the three important risk factors.