Analysis of loss of residual heat removal system for CPR1000 under cold shutdown operation

After Fukushima accident, the safety assessments within the full range of operation conditions for the current fleet of nuclear power plants in China are requested to be performed by the national nuclear safety administration. Accordingly, twelve typical cases were simulated to study the thermal-hydraulic behavior of loss of residual heat removal system for CPR1000 under cold shutdown operation with RELAP5/MOD3.3 code. The transient behaviors of system pressure, coolant temperature, water inventory, coolant boiling and core uncovering time, and other important parameters under 12 different plant configurations are predicted. The results indicate that the system pressure of cases with intact system increases continuously to the set value of 3.8 MPa for opening the relief valve in the RHR system without any mitigation action. The starting time of coolant boiling highly depends on the initial water inventory in the RPV as well as its behavior during the initial stage after losing the RHR system, while the starting time of core uncovering highly depends on the plant configurations. The results of the study can provide references for the plant safety improvement and guideline for an operator to strategize a mitigation action.