Abstract: To study the mechanism of UO₂ freezing in fast reactor,the mechanism of models for UO₂ freezing in steel tubes are introduced in this paper,including conduction-limited freezing,bulk freezing and Fuel Caps freezing model.As an example,the GERSER tests are simulated and compared under these models.The conclusion is that Fuel Caps freezing model is a better mechanism model for interpretation of liquid UO₂ freezing in steel tubes,however,further experiments and model improvement are needed to study the freezing mechanism,including the effects of the pressure difference and flow regimes of melt leading edge.