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Diversity and Defense in Depth

The objective of this research project is to identify appropriate levels of diversity and defense in depth required for mitigating the effects of common-cause failures (CCFs). The licensee is responsible for determining appropriate diverse systems and defense-in-depth design features for mitigating the effects of CCFs, and the responsibility for independently evaluating the licensee's proposed designs lies with the NRC. Guidance for identifying acceptable nuclear power plant safety system diversity and defense-in-depth design approaches is provided in NUREG/CR-6303, “Method for Performing Diversity and Defense-in-Depth Analyses of Reactor Protection Systems.” The intention of this regulatory guidance was to provide licensees and the NRC staff a means for assessing whether additional diversity is required in a digital safety system on the basis of the safety system and nuclear power plant design features.

The deterministic approach described in NUREG/CR-6303, while comprehensive, did not provide specific guidance to licensees for assuring (and for enabling the NRC to confirm) that acceptable diversity and defense-in-depth design strategies had been implemented in a digital safety system design. This conclusion arose from industry experience as licensees began evaluating diversity and defense-in-depth design features using the guidance in NUREG/CR 6303 when developing digital safety system upgrades to their existing analog-based safety systems. For example, recent diversity and defense in depth evaluations have placed significant burden for mitigating CCFs on plant operators instead of on diverse mitigating systems.

The objective of this research project is to improve the guidance in NUREG/CR-6303 by identifying specific combinations of diversity attributes and associated diversity attribute criteria that are acceptable to the NRC for reducing the risk and resulting consequences of unmitigated CCFs in digital safety systems.

Draft guidance is scheduled for completion in late 2007, and final guidance should be completed in mid-2008.