An Assessment of Heterogeneous Effects on System Reactivity for Criticality Safety Analyses with LEU+ and HALEU Materials

To develop the criticality safety basis for any system, process, or package, the worst-case configuration of materials resulting in the maximum system reactivity must be determined. It is commonly accepted that in terms of the maximum system reactivity, at the lower enrichments used in current commercial practice (i.e., 5 wt% ²³⁵U), a heterogeneous configuration is bounding of a homogeneous mixture of fissile and moderating materials. However, a common assumption made is that with increasing enrichment, a homogeneous system can be bounding. With increased industry interest in utilizing higher enrichments for commercial applications with low-enriched uranium (LEU+) (≤10 wt% ²³⁵U), and high assay low-enriched uranium (HALEU) (≤20 wt% ²³⁵U) materials, it has become increasingly important to verify any assumptions and to have a better understanding of the expected system behavior at these higher enrichments.

The SCALE code system was used to assess the effects of heterogeneity on system reactivity with varying enrichments and system configurations for a UO₂ and water system, typical of a transportation package criticality analysis. The purpose of this assessment was to provide insight on the effect of material heterogeneity on system reactivity with increasing enrichment. The results of this study confirm that for systems with a higher hydrogen-to–fissile material (H/X) ratio, the homogeneous mixture of material may be bounding for HALEU materials. However, for systems with a lower hydrogen-to–fissile material ratio (H/X ≤ 200), a heterogeneous configuration of contents is expected to be bounding for most LEU materials. Overall, for any LEU system, including HALEU material, heterogeneous reactivity effects should always be considered.