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Effect of Thermal Motion of Hydrogen Nuclei

        In the previously discussed zero power plasma reactor with a graphite reflector, the thermal neutron spectrum was described by a Maxwellian distribution corresponding to the graphite temperature. When the reactor operates at significantly high power levels, the hydrogen coolant required for heat removal will exist at temperatures which result in a large increase in the thermal motion of these scattering nuclei. When neutrons in thermal equilibrium with graphite have a scattering collision with hydrogen nuclei at a higher temperature, the neutrons will gain kinetic energy. If the collision rate with hydrogen is comparable to that with graphite, the characteristic temperature of the neutron spectrum will be shifted to a higher value. This shift, or "hardening," of the spectrum will influence reactor criticality by reducing the neutron density at energies where the uranium absorption cross section is large.

        Based on two dimensional criticality calculations of gaseous core reactors containing hydrogen, Hyland et al. (30) have concluded that neutron scattering collisions with hydrogen have a negligible effect on the critical mass. This conclusion does not appear to be justified since the "upscattering" effects of hydrogen are not included. The calculations are based on the assumption that the thermal