Criticality Data for Spherical ²³⁵U, ²³⁹Pu, and ²³⁷Np Systems Reflector-Moderated by Low Capturing-Moderator Materials

The critical dimensions of spherical systems moderated and reflected by low-capturing materials such as D₂O, BeO, Be, and C were investigated. A parametric study of the critical mass of enriched uranium, plutonium, and neptunium is examined and tabulated. The results obtained expand on the understanding of reflector-moderated critical systems, and they show regions of unstable criticality for ²³⁵U and ²³⁹Pu reflected cores at intermediate densities. This instability is illustrated by calculations of the positive reactivity coefficient of volume expansion. The coefficient is positive, not negative, in the intermediate density region for ²³⁵U and ²³⁹Pu systems. For ²³⁷Np cores reflected by the same moderator, the effect is negligible. The critical dimensions were calculated with the DANTSYS codes using the Hansen-Roach cross-section libraries. This study is both a summary of mostly unpublished calculations and new calculations. Experimental data for these configurations are extremely limited. These are examined in the text when applicable.