The Safety Depressurization System (SDS) is a new design feature incorporated into Ulchin Nuclear Power Plant Units 3 & 4, Korea Atomic Energy Research Institute (KAERI) designed two loop 2825 MWt Pressurized Water Reactor (PWR) plants. The SDS is designed to mitigate the beyond design basis event of Total Loss of Feed-water (TL0FW). There have been analyses to determine the minimum required bleed valve capacity for SDS to prevent core uncovery. However, the long term thermal hydraulic behavior of primary system and resultant containment response during a Feed and Bleed (F & B) transient were not detailedly analyzed yet. It is desirable that the long term F & B by the operation of SDS valves and High Pressure Safety Injection (HPSI) pumps could lead to the completion of a controlled primary system depressurization and cooldown to Shutdown Cooling System (SCS) entry conditions without violating the existing Equipment Environmental Qualification (EEQ) pressure and temperature profile.
The thermal hydraulic behavior of the primary system is analyzed during long term F & B operation with the various combination of the available number of HPSI pumps, bleed valves and the initiation time of F & B operation. The analysis is performed by CEFLASH-4AS Realistic Evaluation Model (REM) computer code which employs two mass, two energy, and one mixture momentum equation. The mass and energy release defined by the REM is used as input to the CONTRANS computer code to calculate containment pressure and temperature response. The resultant pressure and temperature profiles following the F & B transient depending on various modes of active Containment Heat Removal System (CHRS) operation are compared with existing EEQ curve. It is found that the SCS entry condition can be reached within reasonable time period without violating the EEQ curve by proper operation of SDS valves and HPSI pumps and active components of CHRS.