The two-stage model of changing the thermal and phase state of the active zone melt in the cooled subreactor crucible trap when interacting with the sacrificial material and when crystallizing the diluted corium after the gravitational inversion of its oxide and metal components is presented. The simulation is based on the generalized formulation of Stefan's task. The results of the end-to-end calculation of the COMSOL Multiphysics package show that the thermokinetic processes examined generally reduce the temperature and density of the corium, reduce heat flows on the trap body, minimize the release of hydrogen and radioactive fission products with their retention in a controlled thermal and phase state until full crystallization.