Modeling crust fracture and water ingression through crust during top-flooding strategy for corium cooling

Abstract

During an LWR severe accident, top flooding strategy is usually used in order to achieve the coolability of corium. Water ingression phenomenon, where water percolates through cracks in the crust, is identified as one of the phenomena that enhance the heat removal during cooling of the corium via top flooding. In this study, a model for the water ingression phenomenon is suggested. The model assumes that the morphology of fractured crust is determined during the period before onset of water ingression. Especially, the present model predicts the morphology of the crust using two thermal-stress based criteria: a tensile-strength criterion and a toughness criterion for the fracture. In addition, unlike previous models that assume heat removal is limited hydraulically to the dryout heat flux, heat flux during water ingression is estimated considering the phenomenon where quenching is limited thermally by large thermal energy of crust. When the current model was used to estimate the heat removal rate during the quenching of the fractured crust, it showed 26.5% of root-mean-square error (RMSE) for data (Lomperski and Farmer, 2007), which was improved from 39.3% RMSE of the existing hydraulic-limit based models (Lomperski and Farmer, 2007). In addition, comparing removed corium energies calculated from the present model with one calculated without considering water ingression, we found that water ingression can remove three-times larger energy when the concrete content in the corium is lower than 5 wt%.