The external active thermal control system (ATCS) of International Space Station (ISS) in the assembly-complete configuration has 24 radiator panels on each side, designed to reject the Station's waste heat to deep space. The radiators use liquid ammonia as the working fluid.The effective sink temperature of the radiators may vary from -112° C to 116° C (-170° F to 240° F) depending on several factors including the orbit parameters and attitude and orientation of the Station. At sink temperatures below the ammonia freezing point of -77.7° C (-107.9° F) and low heat load conditions, the radiator flow tubes may freeze. Therefore, the radiators are designed to withstand freezing and thawing.The purpose of this study is to develop a detailed SINDA/FLUINT model of the ATCS radiators and use it to predict the freeze/thaw cycles of the radiator flow tubes at extremely cold environments, and examine its impact on the performance of the ATCS loop. To account for the energy of fusion of ammonia during freezing and thawing and to simulate the gradual formation of ice on tube walls that leads to blockage of flow in fully frozen tubes, an alternate ammonia property subroutine has been developed for this study that estimates the ammonia properties in the vicinity of the freezing point and at subfreezing temperatures. The results of the study include predictions of the panel level freezing/thawing patterns at different cold environments. The results also include predictions of the radiator pressure drop variations, the rate of formation and melting of ice on the flow tube walls, and the flow distribution in open tubes during freezing and thawing.