inlet temperature. Data points for different systems are shown on this graph. The abscissa is marked with potential candidate working fluid boiling temperatures. A UF4- fueled vapor core, operating in a closed Rankine cycle (essentially constant radiator temperature), clearly offers the capability of order-of-magnitude improvement in radiator performance. 2.4 Fuel and Working Fluid The selection of fissile fuel and working fluid determines the properties of the partially-ionized plasma exiting the vapor core, and subsequently, establishes the operating conditions of the energy conversion system and the closing of the cycle. It is apparent that only closed-cycle fuel loops would be considered for space or terrestrial application of fissile reactor fluids. For the 1960s' high temperature space propulsion rocket study, metallic uranium vapor at about 10000 K was the fuel of choice. Due to serious problems of containing and/or separating U vapor at these temperatures, the use of gaseous UF6 was studied. During INSPI's 1987 gas core research work, it became apparent that a condensable fluid with suitable thermodynamic and chemical properties would bring the gas core power project into a feasible temperature range and acceptable cycle-closing conditions. The Rankine vapor core-magneto conversion systems have established their superiority over other GCR cycles for space power generation. Using both temperature
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