Study Of An Estimation Method Of Initiating Event Frequency In On-line Risk Monitoring Systems

Probabilistic Safety Assessment (PSA) method has been widely applied in terms of design,safe operation,and supervision of Nuclear Power Plants (NPPs). As one of the special applications of PSA, Living PSA supports operational decision-making of a NPP. Risk Monitoring is one of the key directions of Living PSA. It is mainly used for risk-informed decision making process, providing time-varied risk information for the operating personnel and regulators of NPP. On-line Risk Monitoring System (OLRM) calculates the risk information on-time by updating the risk model and related reliability data of NPP on time, in order to assist the operation, maintenance of NPP and give support to regulators for decision-making.Initiating event frequency is a significant influencing factor of quantitative safety analysis of NPP. During the power operation stage, initiating event frequency may change because of the configuration, operating environment, and other factors. The paper firstly split all the initiating event groups in PSA model of NPP, in accordance with the features of OLRM,then merged the initiating events to identify a new OLRM initiating events list. Secondly,according to the characteristics of each initiating event, the estimation methods of initiating event frequency were classified——fixed-value type and model-estimated type. Based on that,the appropriate method was selected to assess the frequency of different initiating events that may occur under different conditions.Initiating event frequency of supporting systems may vary with the system configuration significantly. This paper assessed the failure frequency of Component Cooling Water System of NPP by fault tree method, Monte Carlo method and Markov method, according to components and operating procedures of the system. The applicability of three modeling methods in OLRM model was analyzed.Finally, according to the features of OLRM and needs from personnel of NPP, an initiating event frequency monitoring software of OLRM was developed. The structure and functions of the software were introduced...