Development of an improved solidification model for molten salt reactor applications

The freezing of molten salt in flow systems is one of the major safety concerns for molten salt reactors (MSRs), since solidification in drain lines or passive heat removal systems can obstruct emergency cooling lines and trigger uncontrolled temperature excursions in the reactor core. To predict this behavior, an improved solidification model is developed, which can be adopted in thermal-hydraulic system codes. Building on an existing fuel salt freezing formulation, the present model adds an energy equation for the pipe wall to restore the wall thermal inertia that the earlier model neglects. The governing equations are cast in control-volume form with semi-implicit numerical solution and solved in a MATLAB program for assessment. The model is first applied to the conceptual FLiBe and water cases, where it reproduces the existing model without the wall and predicts slower freezing once the wall thermal inertia is included. The model is then validated against dynamic and static water-freezing experiments and showing better agreement with measured data than the reference models. The results indicate that the proposed model captures the physics of molten salt freezing and provides a useful basis for MSR safety analysis within the MARS code.