A quantitative assessment method for safety-critical signal generation failures in nuclear power plants considering dynamic dependencies

In nuclear power plants, safety-feature actuations are executed by automated signals from plant protection systems or by manual signals from human operators. To date, there have been many studies in relation to reliability analyses of automated protection systems and the situation assessment process of human operators in the event of different types of accident circumstances. These studies focus on reliability analyses of the protection systems independent from the occurrence of accidents and on the situation assessment process of operators on the assumption that a certain accident has occurred. In contrast, the present paper proposes a quantitative analysis method to assess the entire process, including the occurrence of an accident, recognition of plant state changes, and the generation of safety-critical signals by automated systems and/or human operators. A dynamic system modeling method is used in order to address the sequential dependent relations between occurrences of accidents and component failures of instrumentation and control systems, and a quantitative model for the situation assessment process is adopted for the analysis of signal generation from human operators. The proposed method is applied to the example situation of a loss-of-coolant accident.