Development of a quantitative safety assessment method for nuclear I&C systems including human operators

Abstract

Conventional PSA (probabilistic safety analysis) is performed in the framework of event tree analysis and fault tree analysis. In conventional PSA, I&C systems and human operators are assumed to be independent for simplicity. But, the dependency of human operators on I&C systems and the dependency of I&C systems on human operators are gradually recognized to be significant. I believe that it is time to consider the interdependency between I&C systems and human operators in the framework of PSA. But, unfortunately it seems that we do not have appropriate methods for incorporating the interdependency between I&C systems and human operators in the framework of PSA. Conventional human reliability analysis (HRA) methods are not developed to consider the interdependecy, and the modeling of the interdependency using conventional event tree analysis and fault tree analysis seem to be, event though is does not seem to be impossible, quite complex. To incorporate the interdependency between I&C systems and human operators, we need a new method for HRA and a new method for modeling the I&C systems, man-machine interface (MMI), and human operators for quantitative safety assessment. As a new method for modeling the I&C systems, MMI and human operators, I develop a new system reliability analysis method, reliability graph with general gates (RGGG), which can substitute conventional fault tree analysis. RGGG is an intuitive and easy-to-use method for system reliability analysis, while as powerful as conventional fault tree analysis. To demonstrate the usefulness of the RGGG method, it is applied to the reliability analysis of Digital Plant Protection System (DPPS), which is the actual plant protection system of Ulchin 5&6 nuclear power plants located in Republic of Korea. The latest version of the fault tree for DPPS, which is developed by the Integrated Safety Assessment team in Korea Atomic Energy Research Institute (KAERI), consists of 64 pages of fault trees which should...