To evaluate the released radioactive materials to an environment under a nuclear severe accident, it is necessary to analyze the ex-vessel phenomena. The releasing diameter is one of the most essential parameters to estimate the ex-vessel phenomena. The diameter is enlarged because the wall of a reactor vessel melts due to the high temperature of corium flow. In this study, a hole-ablation model was developed based on the one-dimensional heat balance equations for molten corium flow, crust layer, and molten layer of the reactor vessel, respectively. Moreover, the heat transfer coefficient for the situation of discharging flow was obtained using computational-fluid-dynamics. As a result, we proposed the enhancement factor, 1.57, to be multiplied by the heat transfer coefficient for an external turbulent flow. We found out that the strong effect of the critical thickness of the molten wall layer on the thicknesses of the crust layer and the molten wall layer. We suggested the relationship between the critical thickness of the molten wall layer and the ratio of the superficial velocity of the molten wall layer to that of the corium discharging flow. The proposed model was compared with the Pilch model: the mean averaged error (MAE) of the present model was 9.51%, whereas that of the Pilch model was 36.13%. In the case of the low superheat of the corium where the crust can be produced, the Pilch model without the crust formation model predicts the much larger final-hole diameter than the present model does.