Subcritical Limits for Special Fissile Actinides

Critical masses and subcritical mass limits in oxide-water mixtures were calculated for actinide nuclides other than ²³³U, ²³⁵U, and ²³⁹Pu that have an odd number of neutrons in the nucleus; S_n transport theory was used together with cross sections, drawn from the GLASS multigroup library, developed to provide accurate forecasts of actinide production at Savannah River. The subcritical limits are: 201 g for ²⁴¹Pu, 13 g for ^242mAm, 90 g for ²⁴³Cm, 30 g for ²⁴⁵Cm, 900 g for ²⁴⁷Cm, 10 g for ²⁴⁹Cf and 5 g for ²⁵¹ Cf Association of ²⁴¹Pu with an equal mass of ²⁴⁰Pu increases the ²⁴¹Pu limit to a value greater than that for pure ²³⁹Pu. Association of ^242mAm with ²⁴¹ Am increases the limit for the mixture to that for dry, theoretical density AmO₂ at isotopic concentrations of ^242mAm less than ∼6%. Association of ²⁴⁵Cm with ²⁴⁴Cm increases the limit according to the formula 30 + 0.3 ²⁴⁴Cm/²⁴⁵Cm up to the limit for dry CmO₂. A limiting mass of 8.15 kg for plutonium containing at least 67% ²³⁸Pu as oxide was calculated that applies (provided ²⁴⁰Pu exceeds ²⁴¹Pu) with no limit on moderation.