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from an appropriate weighting of a hydrogen spectrum based upon a free gas scattering kernel and a reflector spectrum based upon the proper scattering kernel (such as the Parks kernel for graphite) at the reflector temperature. The spectral weighting would be determined by the spatial distribution of the hydrogen density and temperature. The decision to use either the mixed spectrum method or the effective hydrogen cross section method should be based on the particular reactor being studied. The effective cross section method may be adequate for most parameter studies, while the mixed spectrum method might be required for detailed core design studies. If a very elaborate mixed spectrum calculation is necessary to obtain multigroup cross sections, then the accuracy of the multigroup diffusion approximation may be less than that involved in the spectrum calculation. In this case, the accuracy of the cross sections would provide no additional accuracy in the multigroup results. If additional accuracy is required, one and two dimensional transport theory codes should be used.