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Deprimes occuring in capillary pumped loops (CPLs) with small-diameter evaporator pumps (≤ 16 mm) were examined analytically and experimentally. The proposed deprime mechanism advanced by this paper is nucleation of a pressurized vapor bubble that can blow through the evaporator wick and lead to immediate CPL deprime or unstable operation with a likelihood of deprime at a later time. This failure mechanism is exacerbated by the presence of a non-condensible gas species like hydrogen in the ammonia working fluid. Use of a robust startup evaporator to empty the normal vapor spaces of the remaining CPL evaporators prior to startup is identified as the most effective method for obviating this failure mechanism and ensuring reliable CPL operation.