The Risk Assessment Of Small LOCA Based On Dynamic Event Tree Method

Accident risk analysis is a part of On-Line Risk Monitor in nuclear power plant.The purpose of On-Line Risk Monitor is to obtain the current state risk information in nuclear power plant.The traditional Event Tree/Fault Tree is a static analysis method.Although it is widely used in the safety field as a classical probabilistic safety assessment method,it has great limitations in describing the dynamic behavior of the system.The Dynamic Event Tree(DET)is a comprehensive simulation method based on accident scenarios.It is an effective PSA tool which can well describe the dynamic behavior of the system,especially has an advantage in dealing with the interaction between process variables and component states.In addition,DET can describe and characterize the accident evolution scenarios well,and seek out the potential information in the process of accident evolution,which is helpful for the system safety risk prediction and analysis.In this paper,a Small Loss of Coolant Accident(SLOCA)with a diameter of 15 mm is taken as the research object.The simulation of the SLOCA under various accident scenarios is completed by Relap 5.0,and the process variables and component states relative to time are obtained,then the DET model based on the simulation results is set up.In the modeling process,the branch principle of the DET model is determined,the attributes of each node in the DET model are defined,and every branches of the DET model are described in detail.Finally,the analysis results of branch nodes of each level and the total risk results of each path are obtained,and the risk contribution rate of each critical state is obtained.The results show that the risk contribution rate of the critical state that low pressure injection system fails to be input has reached more than 90% no matter in the accident sequences of the high-pressure injection system inputted automatically or manually,or in the accident sequences of the low-pressure injection system inputted automatically which indicates that low-pressure injection system plays a very important role in the process of this accident mitigation.This analysis is more targeted and more reasonable than the traditional event tree analysis,because it takes full account of the dynamic characteristics of the accident scenarios.