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nuclear characteristics of the system. Typical thermal flux profiles of critical reactor configurations are shown in Figure 7. The thermal flux attenuation is seen to be very insensitive to changes in the uranium density profile, in the case of low fuel density. An average uranium density of N235 = 1019/cc was used in these two calculations.

        Based on the four group diffusion theory calculations, the uranium plasma reactor reference model is defined as follows:

        Core radius                          4 ft

        Core height                          8 ft

        Graphite reflector thickness         2 ft

        Graphite temperature at core
          interface                          40000K

        Uranium composition                  100% U-235

The critical mass of U-235 will depend on the uranium density distribution, the plasma temperature distribution, and pressure, and the hydrogen coolant density and temperature distribution.

        These parameters will have to be specified in

order to compute the critical mass. The diffusion theory calculations indicate that average U-235 densities on the order of 0.5 x i019/cc to 1.6 x I019/cc will be required to achieve a critical plasma reactor at the desired