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high temperatures. These densities correspond to U-235 masses of about 25 kg to 75 kg. The choice of U-235 as the fuel for the reference model is based on the availability of both plasma and nuclear data for uranium. Either Pu-239 or U-233 appears to be a better plasma reactor fuel when only the nucleonic criticality effects are considered (23,31). However, since the plasma characteristics of the fuel also have an important effect on the critical mass, the per cent ionization, as a function of pressure and temperature, must be considered when comparing the relative merits of the three fuels.