Impact of new prompt fission neutron spectrum evaluations and the associated cross-correlations on critical assemblies using MCNP6 - 14385

Two recent re-evaluations of the neutron-induced prompt fission neutron spectrum (PFNS) of uranium and plutonium actinides have included some cross-correlated uncertainties that have not been released in a new nuclear data library. The first re-evaluation included cross-isotope correlations of the PFNS within each of the uranium and plutonium suites of actinides below the second-chance fission threshold. The second, more recent re-evaluation includes incident energy-dependent correlations of the ²³⁹Pu(n,f) PFNS. Both of these new evaluations and their cross-correlations, given the importance of these actinide quantities and because the impact of the incident energy-dependent correlations has never been studied previously, could have an impact on a variety of criticality safety applications. For example, the ke uncertainty due to the uranium PFNS uncertainties in the Big Ten critical assembly is underpredicted by ∼10% when the cross-isotope correlations in the uranium PFNS are ignored. (authors)