Abstract
In this paper, a typical passive nuclear power plant small break LOCA is taken as the research object. Based on the theory of Risk Informed Safety Margin Characterization methodology, the development process of the accident is studied. The specific sequence is selected to be studied. The important systems are discretely branched (such as Automatic Depressurization System), and the important uncertainty parameters are sampled (such as ADS valve and IRWST valve resistance coefficient). The event tree is re-established, and the success criterion of the head is modified. The fault tree is reestablished. After that the uncertainty parameters are sampled for specific sequences and the accident simulation of the whole plant is carried out for each condition. Thus, the frequency of each sequence and the conditional failure probability under the specific condition are obtained, and the core damage frequency (CDF) value based on RISMC method is finally calculated.
This analysis is mainly aimed at the configuration and sensitivity of ADS. The selected sequence 45,53,64 are generated from the risk significant core damage sequence S in the original model. Through the analysis and calculation of RISMC method, it is concluded that sequence 45 is ok sequence, and the assumption in the original model is too conservative. OK cases with epidemic proportions existed in sequence 53; sequence 64 is consistent with the original hypothesis, which is a CD sequence. Therefore, the calculated value of CDF is reduced to a certain extent.
This paper verifies the feasibility of RISMC method in application on passive NPPs and also proves that the method can remove unnecessary conservatism. more realistic assessment of the risk helps us quantify the safety margin of nuclear power plants better.
