Experimental Study on the Boiling Heat Removal at the Upper Surface of the Metallic Layer During Late-Phase Coolant Injection

Sustained heating experiments, named Experiments on Late-phase coolant Injection to ASsess the mitigation of focusing effect of metallic layer (ELIAS), were performed to quantify the boiling heat removal rate at the upper surface of a metallic layer for precise evaluations on the effect of a late-phase in-vessel coolant injection. Heat fluxes from the melt layer to the water pool varied from 250 to 550 kW/m² depending on the surface temperature of the metallic layer. Comparison of boiling heat fluxes between the ELIAS experiments and the calculation using Berenson's film boiling correlation shows that effective heat removal was accomplished via late-phase coolant injection in the ELIAS experiments. In this study, a simple model was developed to evaluate the mitigation of focusing effect in the metallic layer via late-phase coolant injection. The ELIAS experimental data on the heat transfer rate at the upper surface of the metallic layer were used as input data in the simple model. The calculation results for the large-break loss-of-coolant accident in the APR1400 show that the risk induced by the focusing effect is highly dependent on the metallic layer thickness and an enhanced integrity of the reactor pressure vessel could be achieved via late-phase coolant injection during this time period.