The (n, γn’) Reaction in Calculations of Fast Reactor Neutron Spectra

The neutron spectrum and criticality of ZPR-3 Assembly 11 have been calculated using estimated cross sections for the ^mU(n, γn’) reaction. Inclusion of this reaction markedly improves agreement between the measured and calculated spectra, and it also decreases the calculated criticality by 0.62%. This change in criticality can be compensated for by lowering the ²³⁸U capture cross section from the ENDF/B-III values to those of a recent microscopic measurement. These and other ramifications of the (n,γn’) reaction in fast reactor assemblies are discussed.