CHAPTER III

       NUCLEONIC ANALYSIS OF EXTERNALLY MODERATED URANIUM PLASMA


       The multigroup diffusion approximation to the

Boltzmann equation is used to make a parametric investigation of some of the essential nuclear characteristics of an externally moderated uranium plasma. These calculations provide a quantitative check on the assumption that the neutron energy spectrum in the uranium plasma is primarily determined by the moderating characteristics of the surrounding reflector. The results of these calculations are used to define a reference model of the plasma core reactor. This model is used to study the coupling between the plasma and nucleonic characteristics of a fissioning uranium plasma.

        The neutron energy spectrum in graphite is calculated using the GAM-I code (32) for the epithermal portion of the spectrum and the TEMPEST code (34) for the thermal portion. A value of 1.125 eV is used for the thermalepithermal energy boundary (maximum graphite temperature corresponds to 0.35 eV), and the U-235 fission spectrum is used as the neutron source. The calculated spectrum represents the energy distribution of neutrons which originate from fissions in a central U-235 plasma and are


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