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        f = functional relation between Pc' MU, and T The functional relations which define pressure

trajectories are determined numerically using the UPLAZ-2 program. For the uranium density corresponding to a given critical mass and a given temperature, the UPLAZ-2 equations are solved for the pressure P. By solving the equations for temperatures from 50000K to 120,000K, the resulting pressures define trajectories along which the uranium mass is constant. The nuclear calculations discussed in Chapter III indicate that a critical reactor would require uranium densities in the neighborhood of i019/cc. Calculated pressure trajectories for several uranium densities are shown in Figure 12. The most important trend shown by these curves is that as the critical uranium density increases, extremely high pressures are required in order to operate the reactor at high temperatures. Therefore, every effort should be made to reduce the critical mass by optimizing the reactor geometry, the reflector geometry and materials, and the fluid flow characteristics. From Figure 11, it is seen that if the critical uranium density can be reduced from 1.0 x i019/cc to 0.5 x 1019/cc, the pressure required to operate the reactor at 100,0000K is reduced from 950 atm to 480 atm.