Analysis of Critical Experiments with Organic-Moderated Assemblies

The results of critical experiments, performed with organic-moderated platetype assemblies containing uranium enriched to 90% in ²³⁵U, in the zero-power reactor ROSPO, are reported. Several cores, differing in critical radius (19.5 to 47.5 cm) and in the ratio of uranium-to-stainless-steel plate number, have been investigated. The comparison with the reactivities calculated by a standard two-group calculation procedure shows an overestimate of the _eff's (up to 1.95% for the smallest critical core) with a systematic dependence on the core radius. A satisfactory agreement is found for large-size cores (R ≥ 40 cm). It is shown that simple calculational improvements, such as a four-group evaluation of the nuclear constants, and a more detailed treatment of core-radial reflector interface zone, lead to a homogeneously good agreement (within 0.25% ) over the whole range of core dimensions.