Abstract: The boiling pool in single subassembly is an important period in the severe accidental course of fast reactors,which may result in the propagation of molten UO₂ out of subassembly.To understand the mechanism of boiling pool clearly,the single-component mechanism model was established in this paper: the drift flux model was selected to predict the distribution of void fraction,and the temperature field and thickness of the fuel crust that encloses the boiling pool was obtained by solving the enthalpy model.According to the flow types,some modification was made on the Greene correlation.Results indicates that with the reducing of heat discharging ability of coolant,the boiling pool will be formed,and the imbalance between internal heat and outward heat transferring is the origin of vapor formation.The Greene correlation is fit for the molten pool before the generation of vapor.After the vapor is formed,the correlation should be modified according to the flow types.