Reliability Assessment for a Decay Heat Removal System in a SFR using the Reliability Physics Model

A sodium-cooled fast reactor (SFR), one of the Gen-IV reactors, has adopted a passive safety system for decay heat removal. The passive safety system composed solely of passive components relies on natural forces such as gravity, natural convection, and conduction, rather than external driving forces or human actions. The passive safety system is considered to have a higher reliability than an active one since it is almost completely independent of component failures or human errors. Probabilistic risk assessment (PRA) is expected to play a major role in the design and licensing framework for future reactors. Because the safety of a SFR largely depends on the reliability of the decay heat removal system adopting a passive safety feature, it is indispensable to assess the reliability of the system. As the passive safety system does not consist of active components, the existing reliability assessment methodology which uses component failures and human errors would not work for the process. Therefore, the reliability physics model has been employed as a reliability assessment methodology for a decay heat removal system in a SFR. The system fails to perform its function when the load of the system exceeds the capacity of the system. This failure is called as functional failure. There are two types of uncertainty affecting the load and the capacity, namely model uncertainty and parametric uncertainty. The load function can be obtained from propagation of these uncertainties. The methodology embraces selection of the accident scenario and failure criteria for making the capacity distribution, identification of important uncertain parameters which affect the performance of the system, uncertainty analysis for making the load distribution, assessment of the reliability of the decay heat removal system, and sensitivity analysis. This methodology has been applied to the reliability assessment of the decay heat removal system in Prototype Gen-IV Sodium Cooled Fast Reactor (PGSFR), which is currently under development in Korea. The reliability of the decay heat removal system composed of the active decay heat removal system (ADHRS) and the passive decay heat removal system (PDHRS) was estimated under an unprotected loss of heat sink (ULOHS) accident by using MARS-LMR code