Dynamic power conversion technologies, like the closed Brayton cycle (CBC) power conversion system, are being considered for high power nuclear electric propulsion (NEP) missions to explore the outer planets and their moons under Project Prometheus. JIMO, the Jupiter Icy Moons Orbiter mission may require in the range of 100 kWe to the electric propulsion units to achieve reasonable mission timelines of 10 years or less. A key milestone in any NEP mission, besides launch, is the remote, on-orbit startup of the nuclear reactor unit and its progression to higher operating temperatures and thermal power levels. With dynamic power conversion units like CBC, the individual units can be motored up to operating speed within 60 seconds if the heat source is maintained at a high temperature (>950 K) during the engine startup sequence. However, thermal transients are introduced into the nuclear reactor heat source subsystem during this engine startup sequence through the fast reactor's negative temperature coefficient of reactivity. Therefore, the commanded reactivity insertion rate in a fast reactor, to increase its thermal power output during the CBC startup transient, must be limited to a fairly small step size over a controlled time period to provide for a safe operating envelope. The reactor thermal power level, coolant outlet temperatures and coolant flow rates are critical parameters that need to be addressed in the selection and design of the primary heat transport subsystem's components. The transient response of a liquid metal cooled fast reactor, a thermoelectric electromagnetic (TEM) pump, and a liquid metal heat transport loop were evaluated during the startup of a four-unit CBC power conversion subsystem. The goal of the analysis was to assess the minimum time period required, after initial reactor criticality, to reach full electrical power output from the power generating subsystem. The results indicate that an auto-start sequence controller can provide for a safe ascension to full power operation of a fast reactor-CBC space power system.
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