Minimum Critical Masses of Arrays of Fissile Oxide and Metal Elements in Water

Minimum critical masses are calculated for arrays of fissile metal and oxide rods or metal plates spaced in water. The composition of the fissile materials investigated were 96% ²³⁹Pu and 4% ²⁴⁰Pu or 93.4% ²³⁵U and 6.6% ²³⁸U. In addition, minimum critical masses were computed for arrays of plutonium and uranium metal cubes spaced in water. These studies were made to aid the criticality engineer in evaluating fabrication and storage problems involving the handling of various fissile shapes in hydrogenous media. Results were calculated in terms of array minimum critical masses as a function of the volume-to-surface ratio (V/S) of an array element. The minimum critical mass for cube arrays was found to remain constant over a wide range of V/S values, while the minimum critical mass of plate arrays always decreased with decreasing V/S rod arrays exhibited an intermediate behavior. Oxide arrays generally had smaller critical masses than corresponding metal arrays because of their smaller self-shielding factors.