and materials as a criteria, the properties of UF4 were examined and found to be compatible with proposed vapor core reactor concepts and definitively superior when a Rankine cycle is chosen to close the loop. The UF6 and UF4 saturation vapor curves are shown in Fig. 6. UF4 is thermodynamically stable, more chemically stable than UF6, and boils/condenses at temperatures close to the present maximum achievable temperature in space radiators (UF4 boils at 1700 K at 1 atm). This fact allows for a natural cycle point for efficient heat rejection via a condensing radiator. A comparison of the UF4 and U metal saturation vapor curves is shown in Fig 7. At the 10-20 atmospheres of core pressure required for criticality, U metal boils at around 5000 K, a temperature significantly beyond material limitations at different regions in the power cycle. Considering the vapor or liquid recirculation of U metal, criticality, condensation and vaporization problems, UF4 appears to be a more suitable fluid for vapor core systems. However, the use of UF4 requires significant research on the chemical, materials, thermodynamic and electrical conductivity issues critical to ‘magnetic turbine' energy conversion. Also, uranium metal, in liquid form, as micron size droplets dispersed in a vapor working fluid in microgravity, could offer significant potential for further performance gains, especially if the droplets can be separated from the working fluid as it exits the core. Shown on Fig. 8 is a comparison of the UF4 and liquid uranium metal droplet operating temperature and pressure envelopes. The primary concepts using these operating regimes are different, as are their technical limitations; however, they both have very attractive performance potential.
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