Criticality Comparison of Highly Enriched Uranium Dispersed Over Any Volume as Solid Particles, and as a Solution

Abstract

An optimised rectilinear configuration containing a homogeneous solution of uranium metal with 1.3kg 235U and water was compared to the same water-moderated and reflected configuration with uranium metal being modelled in progressively smaller lumps. The results of KENO-V.a calculations with a benchmarked ENDF/B-V library showed that, for this highly-enriched uranium (HEU) to be in a critical state, the fissile material must either be arranged in a water moderated configuration of at least 250,000 evenly spaced HEU chunks with optimum reflection, or dissolved completely in acid. A scenario involving metallic 1.3kg 235U in a flooded environment would not be sufficient to cause a critical configuration. Some dissolving agent such as acid must be present to dissolve the uranium into a solution, to achieve the spacing between uranium particles that is required for a critical configuration. The macroscopic crosssection of the uranium atoms is compared to the cross section of the uranium spheres to demonstrate that a self-shielding effect remains even when the particles are smaller than 0.1 mm diameter.