Assessment of passive residual heat removal system cooling capacity

Abstract

Advanced passive PWR relies on passive safety systems to provide core cooling capacity and deal with design basis accidents and beyond design basis accidents. However, the passive safety system is lack of practical operating experience and their performance is heavily influenced by other systems. The cooling capacity of passive residual heat removal system (PRHR), which is designed to remove decay heat when normal heat removal approach is not available, requires specific assessment during different accidents. In this study, a detail model of advanced passive PWR, including Reactor Coolant System (RCS), simplified secondary side and Engineered Safety Features (ESF), has been built using mechanism accident analysis code. The plant transient has been simulated, and cooling capacity of PRHR been analyzed during loss of normal feedwater and main feedwater line rupture. Conservative assumptions were made specially based on different accident scenarios and one of the two fail-open valves arranged in parallel at the PRHR heat exchanger (HX) outlet line was assumed not open, as the worst single failure. The progress of the two accident sequence is calculated and the thermalhydraulic behavior of RCS is investigated and the main transient parameters are obtained, including primary side pressure, steam generator pressure, pressurizer water level. The cooling power and system response are calculated. The results show that PRHR, with CMT injection, can remove the decay heat from RCS to IRWST, keeping the pressures of RCS and steam generators remaining below 110 percent of the design values and the pressurizer overfilling is prevented. Sensitivity study has been performed to study the system resistance effects on the capacity of PRHR, which shows that increase in system resistance coefficient reduces the cooling capacity of PRHR.