R-Matrix analysis of U-235 neutron transmission and cross-section measurements in the 0- to 2.25-keV energy range

A new R-matrix analysis of the U-235 cross-section data in the 0- to 2250-e V energy region is presented. The analysis was performed with the SAMMY com puter code that has recently been updated to permit, for the first time, in clusion of bath differential and integral data within the analysis process. Fourteen differential data sets and six integral quantities were used in t his evaluation: two measurements of fission plus capture, one of fission pl us absorption, six of fission alone, two of transmission, and one of eta, p lus standard values of thermal cross sections for fission and capture, and of KI and the Westcott g factors for both fission and absorption. An excell ent representation was obtained for the high-resolution transmission, fissi on, and capture cross-section data as well for the integral quantities. The result is a single set of resonance parameters spanning the entire range u p to 2250 eV, a decided improvement over the present ENDF/B-VI evaluation, in which II discrete resonance parameter sets are required to cover that sa me energy range. This new evaluation is expected to greatly improve predict ability of the criticality safety margins for nuclear systems in which U-23 5 is present.