Criticality Studies with Plutonium Solutions

Abstract

A series of criticality experiments were performed with plutonium (4.6% 240Pu) nitrate solution in stainless steel spheres of 11.5-, 14-, and 15.2-in. diam. Reflectors of water, concrete, paraffin, and stainless steel were used; experiments were also performed on the 15.2-in. sphere unreflected. The spheres were made critical with plutonium concentrations varying from 24 to 435 g Pu/liter and molarity varying from 0.2 to 7.7. The minimum critical volumes for Pu(NO3)4 in water containing 4.6% 240Pu were determined to be about 22 and 11 liters, respectively, for bare and reflected spheres at a concentration of 175 g Pu/liter. The effect of a 0.030-in. cadmium shell or a 4-in. air gap between the reflector and the vessel reduced the reflector worth to that of a nominal reflector (1-in. of water or less) for the concentrations of plutonium measured. Comparisons were made between experimental and theoretical results using multigroup diffusion theory.