Evaluation of corium cooling in external core catcher system

An evaluation of ex-vessel core catcher system of a sample advanced light water reactor (ALWR) was presented. It was assumed that the thermal power of the reactor was 4000MWt and that the spreading area of a molten corium was approximately 80 m2. The core catcher was designed to cool down the molten corium through a combined injection of water and gas from the bottom of the molten corium, which could be effective in the reduction of rapid steam generation and in the suppression of steam explosion. By using the MELCOR 1.8.6 code, the calculations of molten core concrete interaction (MCCl) phenomena were performed for a representative severe accident scenario of the ALWR, that is, the 6-inch large break loss of coolant accident without safe injection. The composition of the molten corium, the decay power level, and the concrete ablation depth with time were obtained by the MCCI calculations. The corium cooling history in the core catcher after the coolant injection was evaluated to calculate the temporal steam generation rate by considering an energy conservation equation. They were used as the major inputs for the temporal calculations of containment pressure which was performed by using the GASFLOW code. Several cases with a change of water and gas injection rates were calculated to evaluate the effect of combined injection of water and gas. It was confirmed that the water and gas mixture injection system was an effective corium cooling method in the ex-vessel core catcher to preclude a possible steam explosion and to suppress quick release of the steam