Calculation of the Minimum Critical Mass of Fissile Nuclides

The OB-1 method for the calculation of the minimum critical mass of fissile actinides in metal/water systems was described in a previous paper. A fit to the calculated minimum critical mass data using the extended criticality parameter is the basis of the revised method. The solution density (grams/liter) for the minimum critical mass is also obtained by a fit to calculated values. Input to the calculation consists of the Maxwellian averaged fission and absorption cross sections and the thermal values of nubar. The revised method gives more accurate values than the original method does for both the minimum critical mass and the solution densities. The OB-1 method has been extended to calculate the uncertainties in the minimum critical mass for 12 different fissile nuclides. The uncertainties for the fission and capture cross sections and the estimated nubar uncertainties are used to determine the uncertainties in the minimum critical mass, either in percent or grams. Results have been obtained for ²³³U, ²³⁵U, ²³⁶Pu, ²³⁹Pu, ²⁴¹Pu, ^242mAm, ²⁴³Cm, ²⁴⁵Cm, ²⁴⁹Cf, ²⁵¹Cf, ²⁵³Cf, and ²⁵⁴Es. Eight of these 12 nuclides are included in the ANS-8.15 standard.