The International Space Station thermal control system is a one-phase ammonia loop. However, under some operating conditions two-phase flow is introduced into the system. Safe equipment operating envelopes during the two-phase flow periods, which usually are transient in nature, must be established either experimentally or computationally. The computational methodology has been developed to a point to make the computational approach feasible and can be used to establish such operational envelopes. Two-phase flow analysis of the active thermal control system (ATCS) ammonia pump was conducted to determine the safe operating envelopes. Analysis was conducted to determine the pump dynamic environment under two operating conditions: (1) fill of the entire ammonia loop during the initial on-orbit system fill; (2) fill of a radiator Orbital Replacement Unit (ORU). Critical design parameters, such as pump pressure, void fraction, and mass flow rate, were calculated for candidate system-fill scenarios. The computations were performed with the FLOW-NET program, which was modified to accommodate a stalled pump condition caused by a pump upstream pressure decrease.