As molten corium is released from the reactor pressure vessel (RPV) under severe accidents, the molten corium is going to experience various complicated phenomena. If there is no steam explosion, the corium is going to be ejected through the reactor vessel, fragmented, and cooled through the interaction between the corium and water existing in the cavity during its falling stage.
Our research group has recently developed a COrium COolability Analysis tool named COCOA. COCOA is the transient calculation tool for the evaluation of the phenomena from the molten corium release to debris formation through the corium falling stage of FCI. COCOA was corporately developed to calculate the transient results of the pressure, the released energy, the average water temperature, fragmented particle size distribution, particulate debris fraction etc., during the corium falling stage of the FCI phase.
COCOA is validated against FARO, which is a large-scale corium experiment. Comparing to COMETA, which is a 2D severe accident analysis code, COCOA showed better accuracies on the prediction of the pressure, the released energy, and the particulate debris fraction during the corium falling stage of the FCI phase. COCOA gives 16.9% and 26.5% of NRMSD for the pressure and the released energy, while COMETA does 42.2% and 36.0% of NRMSD. Also, COCOA and COMETA predict the particulate debris fraction with 27.22% and 39.25% of RMSE, respectively. Since COCOA is a semi-empirical model-based simple 1-D tool, only small computational effort is required for the calculation.