Abstract: To analyze the safety characteristics of liquid metal cooled space reactor restarting in orbit, the effects of shrinkage void on both lithium coolant freezing and melting processes were investigated using a numerical scheme, which was based on a single (solid/liquid) cell approach. Results show that void formation at the wall appreciably reduces the solid-liquid interface melting velocity, and the center void has little effect on the freezing and melting processes. Meanwhile, the wall void causes a substantial rise in the wall temperature during melting process. However, in the case of Li, the maximum wall temperature is much lower than the melting temperature of PWC-11, which is used as the structure material in the space reactor system. Hence, it is concluded that a formation of hot spots is unlikely during the startup or restarting of the space reactor system.