The melting point of some generation IV reactors, such as Lead-Cooled, Molten Salt, is considerably higher than ambient temperatures. Under reactor shutdown, maintenance, refueling, or the activation of emergency cooling systems, the coolant may overcool and solidify. This poses a significant risk, as coolant solidification is more likely to occur under these conditions. As a result, coolant solidification accidents have been designated as one of the design-basis accidents for generation IV reactors that use high melting point coolants.

This study aims to establish a one-dimensional solidification model with mushy zone effect based on energy conservation and enthalpy porous medium model.

Firstly, a solidification layer thickness model was established based on the energy conservation equation, and a source term model with mushy zone was established based on the enthalpy porous medium model. The velocity and temperature distribution models were obtained based on the boundary layer theory. Secondly, the established one-dimensional model was compiled and coupled into the ASYST system program, forming a new system safety analysis program ASYST-SF (Adaptive SYStem Thermal-hydraulics with Solidification Function), and the designed molten salt solidification experiment was used for verification. Finally, the filling behavior of FLiBe in the pipe was simulated using ASYST-SF.

The experimental verification results show that the overall model error is less than ±10%, meeting the requirements of reactor system safety analysis. The evolution behavior of fluid temperature, solidification layer thickness, and pressure drop of the pipe filling behavior under typical working conditions is somehow illustrated.

The model and calculation results in this study are of great significance for improving the operational safety of molten salt reactors.