Covariances of Prompt Fission Neutron Spectra

Following the outcomes and the recommendations of the Working Party on Evaluation Cooperation (WPEC) SG-26 the impact of the uncertainties and energy correlations in the prompt neutron fission spectra (PFNS) was studied in several critical benchmarks, both thermal and fast. Two different approaches to calculate the sensitivity of keff with respect to the fission neutron spectra, including the new so-called normalized sensitivity method, were used as a means to check and assure the mathematical correctness of the derived fission spectrum covariance matrices. A new Monte Carlo (MC) method has been proposed and applied to produce the covariance matrices of the PFNS for the neutron induced fission on 235U, 238U and 239Pu nuclei. The MC method was validated by comparison with the matrix derived analytically using the so-called File-30 formalism in ENDF terminology. Several sets of covariance matrices of the PFNS of 235U, 238U and 239Pu nuclei were derived from the uncertainties assigned to the model parameters of the Watt and the recently proposed Kornilov PFNS parameterizations.