A description is presented on how to perform fundamental analyses for accumulators used to maintain pressure control in closed-loop fluid systems. This includes how to charge the loop to meet pump net positive suction pressure (NPSP), accommodate leakage and ground fill uncertainties, and maximize operating and nonoperating pressures. Additionally, the no-fault propagation requirement must be satisfied such that a failure of one accumulator will not fail another. A SINDA/FLUINT model of the early external active thermal control system (EEATCS) was developed.The EEATCS is used to perform the cooling of the United States (U.S.) Laboratory (USL) during early assembly stages of the International Space Station (ISS) to support assured early research. The EEATCS provides the ability to transport the thermal load generated by the USL to space by thermal radiation via two deployable radiators. The EEATCS can be described as an indirect heat rejection system composed of two ammonia flow loops. Each loop contains accumulators, radiators, a heat exchanger, piping, and an independent pump and flow control subassembly (PFCS). The PFCS contains the flow control valve (FCV), which controls the radiator bypass flow in each loop.Each ammonia loop contains five accumulators to accommodate fluid expansion and leakage as well as maintaining pressure control. Analysis concentrated on transient accumulator response to hot and cold start-ups into a beta zero orbital environment. Analysis also reviewed the response to an accumulator bellows rupture.The model results helped give a good understanding of the accumulators and their response to transient conditions for the EEATCS for its support to the ISS.