235U Resonance Structure Effect on Computed Characteristics of Critical Assemblies in Large Physical Testing Facility with Softened Neutron Spectrum

Abstract

An unusual change of the contributed reactivity of uranium samples with 90% enrichment depending on the size was observed during experiments on the critical assembly of the large physical testing facility with a softened neutron spectrum: the reactivity became negative upon insertion of thin samples. The agreement between the calculations and experimental data in the description of this unusual change of reactivity was obtained by making changes in the radiative capture cross section of ²³⁵U and the factors of resonance self-screening in the group approximation and by taking account of the possible missed narrow resonances of ²³⁵U in the point representation description of the experiment.