The Effect of Position on the Criticality of Uranium Spheres in Uranium Solution Cylinders

Thirteen previously unpublished critical measurements on enriched (93.16 wt% ²³⁵U) uranium metal spheres, which are radially but not axially centered in cylinders of enriched (93.16 wt% ²³⁵U) uranium solution, are presented and compared with the results of Monte Carlo calculations. The average reproduction factor calculated for experimentally critical systems was k = 0.989, and this bias showed no systematic variation with the amount the sphere was displaced from center. The sphere size was increased, keeping other parameters constant, until criticality was calculated, at which point the average sphere radius and mass were 2.8 and 8.8%, respectively, greater than the experimental cases. For a given solution cylinder, the minimum critical metal sphere mass occurs when the sphere is centered in the solution. The increase in this critical mass with position along the cylinder axis is also presented parametric in two tank diameters and five solution concentrations. For like asymmetries and at the same concentration, a greater change in the critical sphere mass with position is seen for tanks of large diameter than for smaller tanks. If the tank diameter is fixed, the greater change occurs for solution of high uranium concentration.