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neutron spectrum is determined by the graphite temperature, and do not include the spectral hardening due to hydrogen at a higher temperature. While it may be true that hydrogen scattering collisions are negligible in certain restricted cases, this is not expected to be true in most cases of interest. The extent to which hydrogen influences criticality will be determined by the mass of hydrogen in the core, and by the spatial distribution of the hydrogen density and temperature. Thus, in order to properly evaluate the effects of hydrogen, the heat transfer and fluid flow characteristics of hydrogen must be considered simultaneously with the neutron scattering problem.

        A more rigorous treatment of hydrogen scattering

has been used by Herwig and Latham (31) in their multigroup diffusion theory calculations of spherical gaseous core reactors containing hot hydrogen. By defining an effective macroscopic hydrogen scattering cross section, the effects of high temperature hydrogen on critical mass are investigated. The effective cross section is defined by considering the scattering collision rate of a Maxwellian distribution of neutrons at temperature Tn with a Maxwellian distribution of hydrogen atoms at T   For a neutron density n and a hydrogen density NH, the collision rate is