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thermal group cross sections, a more rigorous and possibly more accurate method should be considered. The fundamental approach to the treatment of thermal neutron scattering is through application of appropriate scattering kernels within the framework of thermalization theory. In the hydrogen cooled plasma core reactor, the thermal group averaged cross section of hydrogen, Es , is determined by a mixed neutron spectrum. The spectrum results from scattering collisions with both the high temperature hydrogen and the relatively low temperature reflector material. The scattering cross section Es in (2.34) was obtained using the TEMPEST code (34). This code averages microscopic cross sections over a thermal neutron spectrum based upon one of the following approximations:


        1. The Wigner-Wilkins light moderator equation

        2. The Wilkins heavy moderator equation

        3. The Maxwellian distribution

Since each of these approximations is characterized by a single moderator temperature, it appears that E in (2.34) is determined from a spectrum based upon a temperature and scattering kernel corresponding to either hydrogen or the reflector, rather than from a mixed spectrum. A more rigorous calculation of Es would require the determination of the mixed spectrum. Such a spectrum could be computed