A new model is developed for predicting the two-phase mixture level in a reactor vessel under accident conditions such as LOCA and for predicting rod temperature when core uncovery occurs. The present model can simulate the accident situation in the real plant in which heat-up and flashing phenomena occur simultaneously at the time of an accident. The developed model for predicting the two-phase mixture level describes void fraction as a time-varying and local-varying variable. The time variation of void fraction in this model is solved from the void propagation equation and its spatial variation is solved by the Least Squares Method. And the derivation of governing equations is based on the mass conservation equation combined with the energy equation. Since the present model does not involve any quasi-steady or uniform assumptions on void fraction, the fast transient can be described well. The comparisons of prediction results with the rod bundle test data and the liquid pool flashing test data show that the present model gives good results in both boil-off cases and flashing cases. Therefore, it can be said that the present model is applicable to accident analysis in a real plant with reasonable accuracy. The rod temperature prediction results over the uncovered region is compared with EPRI``s 336 rod bundle test data and the comparisons show that the prediction results overestimate the rod temperature at any location at the early stage of the transient, especially near the steam-water interface. This is resulted from initial wetting of steam probes due to droplets. But since the present model includes droplet effects on heat transfer coefficients, the simulation results show good agreement with the test data over the transient.