Evaluation of two phase natural circulation mass flow rate in the reactor cavity under IVR ERVC condition for different thermal power

The IVR (In Vessel corium Retention) through the ERVC (External Reactor Vessel Cooling) is known to be an effective means for maintaining the reactor vessel integrity during a severe accident in a nuclear power plant. This measure is adopted in the AP600, AP1000, and Loviisa nuclear power plants as a design feature for severe accident mitigation, and in the OPR (Optimized Power Reactor)1000, the APR (Advanced Power Reactor)1400 and the APR+ as an accident management strategy. Many studies have been performed to evaluate the IVR ERVC. Simulations of two phase natural circulation in the reactor cavity of OPR1000 and APR1400 under IVR ERVC conditions have been performed to determine the natural circulation mass flow rate in the annulus between the outer reactor vessel and the insulation using the RELAP5/MOD3 computer code. The maximum heat removal rate of the CHF (Critical Heat Flux) on the outer vessel wall depends on the coolant circulation mass flow rate. As shown in Fig.1, an increase in coolant mass flow rate leads to an increase in CHF on the outer reactor vessel. As shown in Table I, the thermal power of the APR1400 is higher than that of the OPR1000. The present study is focused on the determination of the natural circulation mass flow rate for different thermal power