Effects of Secondary Depressurization on Core Cooling in PWR Vessel Bottom Small Break LOCA Experiments with HPI Failure and Gas Inflow

Abstract

The effects of steam generator(SG) secondary depressurization, as one of accident management measures during a beyond-design-basis loss-of-coolant accident(LOCA) at pressurized water reactor(PWR), are experimentally investigated at the Large Scale Test Facility(LSTF) which simulates a 4-loop PWR by full-height and 1/48 volumetric scaling. Multiple instrument-tube break at the reactor vessel bottom, equivalent to 0.2% cold leg break, was simulated in the experiments with high pressure injection(HPI) system failure and non-condensable gas inflow from the accumulator injection system(AIS). The experiments showed that secondary depressurization to achieve primary system cooling at a rate of −55 K/h was sufficient to achieve core cooling by the low pressure injection(LPI) system when the gas inflow was prevented, while it was insufficient for core cooling in case of the gas inflow. The rapid secondary depres- surization, however, was successful to achieve core cooling by the LPI actuation irrespective of the AIS gas inflow because it contributed to lower the primary pressure and conserve primary coolant mass as shown in a Pressure-Mass map. The RELAP5/MOD3 code analyses well predicted these thermohydraulic phenomena including SG heat transfer coefficient which was significantly degraded by the gas accumulation.