Cross-section adjustment in the fast energy range on the basis of an Asymptotic Progressing nuclear data Incremental Adjustment (APIA) methodology

The data assimilation benchmark launched by the “Subgroup 33” on ''Methods and issues for the combined use of integral experiments and covariance data'' of the Working Party on Evaluation Cooperation (WPEC) of the OECD Nuclear Energy Agency Nuclear Science Committee is investigated by means of a recently proposed Asymptotic Progressing nuclear data Incremental Adjustment (APIA) technique. More specifically, two distinct APIA simulations having a different sequence are compared on the basis of the deterministic code system ERANOS. The analysis mainly uses a priori cross-sections along with covariance data in 33 groups both stemming from the TENDL evaluation. This basic nuclear data, which is preprocessed before the APIA methodology is used, is consistent in terms of (1) data source and (2) combined processing on the basis of the Total Monte Carlo method. The similar a posteriori cross-sections being provided, largely avoiding conflicts between partial adjustments as a result of the individual incremental assimilation steps are indicative of consistent adjustments according to a previous study. Correspondingly, the TENDL inelastic scattering cross-sections of 238U and 23Na would need some reductions essentially as a consequence of the assimilation of ZPPR9 experimental data. Another important result of the study is that there is a markedly trend of improvement for the a posteriori as against the a priori ratios of computed (C) to experimental values especially as regards the central 238U fission reaction rate per atom relative to the central 235U fission reaction rate per atom, and also as regards sodium coolant density effects; independently of the APIA sequence, the a posteriori C-values in most cases lie within the one-sigma experimental uncertainty or just a little outside, and the resulting reduced uncertainties of the a posteriori as against the a priori C-values are found correspondingly smaller than the experimental uncertainties. It appears that a suitable adjustment would likely require as a basis the use of consistent cross-sections and covariance data.