The critical U235-to-thorium ratios in single-region slurry reactors were calculated for various ratios of H2O to D2O in the moderator. Reactors studied had diameters between 3-1/2 and 5-1/2 ft, (bare sphere diameters), temperatures of 280°C, and slurry concen¬trations between 50 and 900 g thorium/liter.nIt was found that the addition of small amounts of H2O to D2O (< about 20%) causes the critical mass ratio to become smaller and the minimum ratio for a given reactor size to occur at higher thorium concentrations. For example, in a 4-l/2-ft reactor the minimum ratio was 0.13 g U235/g Th at the conditions of 150 g thorium/liter and 99.8% D2O in the moderator. If the moderator was 20% H2O, the minimum ratio was 0.04 g U235/g at the concentration of 500 g thorium/liter.
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