Abstract: By integrating advanced static and dynamic energy conversion systems, space lithium-cooled reactors provide reliable electric power outputs at the hundred-kilowatt level, enabling sustained operation of scientific instruments, life-support systems, and propulsion units in extraterrestrial environments. The unique operational requirements of these reactors necessitate precise control over startup and shutdown processes, both of which involve complex solid-liquid phase transitions within the lithium coolant. Due to lithium’s relatively high melting point (454 K), the working fluid remains in a solid state prior to launch, ensuring safe transport and storage. During reactor core startup, the coolant gradually undergoes melting and thawing, facilitating heat transfer and enabling reactor activation. In planned shutdown scenarios, the coolant eventually solidifies completely, requiring a controlled remelting process to restore operational readiness. The solidification and melting phase transitions are accompanied by significant volumetric changes, including contraction and expansion, as well as dynamic movement of the coolant. These phenomena alter heat transfer boundaries and introduce additional complexity into the simulation of phase transition processes. Traditional modeling approaches, such as the enthalpy method and the equivalent heat capacity method, are applied to simulate phase change behavior. However, these methods prove inadequate for accurately capturing the movement of coolant boundaries, particularly under conditions involving large temperature gradients and non-uniform heat flux distributions. The moving boundary enthalpy method is commonly employed to circumvent direct tracking of the solid-liquid interface, but this approach requires the division of the computational domain into extremely fine control volumes, resulting in excessive computational demands and limited applicability for system-level analysis. To overcome these challenges, this work developed an improved freezing front tracking model capable of accurately tracing the evolution of the solid-liquid interface during phase change events. The model partitioned the phase-changing coolant into distinct solid, liquid, and mixed-phase control volumes. Temperature diffusion equations were formulated and solved for the solid and liquid regions, while coupled equations governing temperature evolution and interface movement were established for the mixed-phase region. Implicit discretization schemes were implemented to enable efficient and robust numerical computation, allowing for high-precision temperature distribution and detailed interface tracking even when employing relatively coarse computational grids. Comparative analysis involving a water freezing case demonstrate that maximum relative error of the freezing front tracking model is 0.80%, significantly outperforming the moving boundary enthalpy method, the maximum relative error of the moving boundary enthalpy method is 2.93% under similar grid conditions. Further modeling of the solidification process in SP-100 space lithium-cooled reactor coolant channels of varying sizes revealed that the complete solidification time increases with channel dimension, and that inward contraction of the coolant leads to an increase in radiative thermal resistance, thereby prolonging the time required for full solidification. Simulation results of the melting process within a 0.05 m diameter channel under an average heating heat flux density of 5 kW/m² indicate a complete melting time of 8 279 s, with pronounced local temperature differences and a peak coolant temperature of 587 K. These findings demonstrate that the improved freezing front tracking model offers superior accuracy and computational efficiency compared to traditional methods, especially in scenarios involving coarse grids and complex boundary movement. The results provide valuable insights into the thermal management and phase transition dynamics of space lithium-cooled reactors, supporting the development of reliable thawing strategies and enhancing reactor safety, performance, and operational flexibility in future space missions.