An Improved Modeling Method for ISLOCA for RI-ISI and Other Risk Informed Applications

摘要

In this study, an improved modeling method for Interfacing Systems LOCA (ISLOCA) was developed in order to accurately and easily evaluate ISLOCA risk for risk informed applications. In the developed method, an event tree for identifying ISLOCA core damage sequences is constructed for each selected ISLOCA path. In developing an ISLOCA event tree, failure of multiple pressure barriers, failure of isolation, and failure of mitigation are considered. Then, for each identified ISLOCA core damage sequence, a fault tree is constructed. In an ISLOCA fault tree, not only the failure of components themselves but also failure of the support systems and operator errors are modeled. Once fault trees for all ISLOCA core damage sequences are developed, they are integrated into a linked core damage frequency (CDF) fault tree with fault trees of other initiating events' core damage sequences. Also, the ISLOCA fault trees are integrated into level 2 Probabilistic Risk Assessment (PRA) logic as a part of Large Early Release Frequency (LERF) fault tree. Then using the linked CDF (or LERF) fault tree, minimal cutsets for total CDF (or LERF) or for ISLOCA or other initiating events can be generated by evaluating associated gates in the linked fault tree. Either point estimate or probability distribution of CDF (or LERF) can be evaluated using the generated minimal cutsets and failure data. Furthermore, the impacts of failure of any components in ISLOCA paths on CDF (or LERF) can be easily evaluated just by setting the related components to fail and evaluate the linked fault tree. Also, since the each ISLOCA fault tree is logically combined with fault trees of other ISLOCA paths and those of other initiating events in the linked CDF and LERF fault trees, the impacts of a component which is shared by more than one ISLOCA paths or failure of support systems related with ISLOCA components can be directly evaluated from the linked CDF (LERF) fault tree model. Fault trees for ISLOCA through hot leg injection lines of the intermediate head safety injection system of a reference plant were developed as an example implementation of the developed method. Also, through two example applications, it was demonstrated that the linked CDF (LERF) fault tree model with the improved ISLOCA model could accurately evaluate the conditional CDF and LERF for a pipe segment break in an ISLOCA path for risk informed in-service inspection (RI-ISI) and the risk increase due to two leaking check valves in an ISLOCA path for risk informed decision making. The developed ISLOCA method was implemented in the PRA model for a reference plant and the PRA model was successfully used for the update of risk evaluations for RI-ISI. The developed ISLOCA modeling method will enable more accurate and easier evaluations of risks associated with ISLOCA in the future risk informed applications.