A thermal-hydraulic model is developed to simulate and study the dynamic behavior of bulk evaporation and condensation processes in a multiphase nuclear fuel cell. The phase-change process is driven and controlled by internal heat generation and wall heat removal under constant volume condition. The modeling involves variable gravity conditions that allow for performance analysis of the multiphase nuclear fuel for terrestrial and space applications. A complete set of governing equations for both liquid and vapor phases is developed and numerically solved. The model is used to simulate the operation of a multiphase nuclear fuel cell at zero-gravity and microgravity levels. The temperature and phase distribution, the flow field, and the evolution of the liquid-vapor interface are computed and demonstrated.