Improvements in the prediction of reactivity and reaction rate ratios for uranium lattices

Evidence from a broad range of clean, benchmark experiments has been used to assess the performance of ‘best’ available theoretical methods and differential data for predicting integral thermal reactor properties such as reactivity and reaction rate ratios. It is shown that for improving the consistency of calculated integral parameters for D₂O, H₂O and graphite moderated, uranium fuelled systems, significant modifications to certain currently used nuclear data are desirable. Along with changes in moderator scattering cross-sections, ²³⁸U resonance data and the ²³⁵U fission energy spectrum, some modification of ²³⁵U thermal data has been made which lies within the experimental uncertainties of differential measurements. The ‘new’ data options have been used in various analyses with the WIMS lattice code, and results are presented for the benchmark studies and also for some lattices with more complicated geometries. It is shown that the preferred data generally gives more consistent results for both reactivities and reaction rate ratios over a wide range of conditions, than any previously considered data combinations.