Risk-Informed Defense-in-Depth Strategy for Nuclear Power Plant

Abstract As one of the basic safety design theory, Defense-In-Depth (DID) concept has been proven effective for ensuring the safety of nuclear reactors by safety operation of hundreds of units for decades. The classic defense-in-depth concept containing five defense layers is built on mainstream light-water reactor type. And after Fukushima nuclear accident, improvements of the DID concept was proposed by IAEA and WENRA, including adding sublevel for additional Design Extension Condition (DEC), and emphasizing the independence between defense levels. However, there are several problems during DID practical application; and current DID concept based on deterministic method has limitations for innovative design or advanced technology application, such as non-light water reactor designs, because current DID strategy evolves from LWR type and earlier deterministic hypothesis. How to develop a more comprehensive, holistic, and technology-inclusive DID strategy is explored in this study. A risk-informed (RI) method is adopted to improve the DID strategy by combining the traditional DID concept with a risk-informed approach. Considering the gradual development of risk assessment methods and tools and acceptance process of risk insight for nuclear industry, a progressive risk-informed DID strategy is proposed in this article, including three RI solutions with different optimized elements, which are implemented through an integrated risk-informed DID design process balancing between deterministic and risk perspectives, and allocating resources to weakness that might be overlooked.