A risk informed defense-in-depth framework for existing and advanced reactors

The philosophy known as ‘defense-in-depth’ has been used extensively in nuclear reactor safety evaluations and in justifying regulatory decisions. An evolving number of definitions of this philosophy have been proposed, primarily from a regulatory perspective. The purpose of this paper is to review the current definitions of defense-in-depth and to offer solutions to the technical issues identified from this review. A more general definition of defense-in-depth is proposed that can be used for any reactor concept. The proposed definition includes an explicit consideration of how the inherent characteristics of the reactor set the foundation for the safety case, an examination of how design features of a reactor employ the strategies of accident prevention and mitigation, and how to quantify the importance of design features responsible for prevention and mitigation. Application of this approach is demonstrated with use of examples for pressurized water reactors and modular high-temperature gas-cooled reactors.