Plutonium Denaturing by ²³⁸Pu

This paper analyzes whether reactor plutonium after denaturing by increasing its isotopic content of ²³⁸Pu to 6 to 8% can be regarded as proliferation resistant. In this case the utilization of such denatured reactor plutonium would become unsuitable for a nuclear explosive device (NED) because the high-explosive lenses surrounding the plutonium would melt or their elevated temperature would lead to self-ignition. Eight different plutonium isotopic mixtures with increasing ²³⁸Pu content are analyzed, and their critical masses if surrounded by a 5-cm-thick reflector of natural uranium are determined. This allows calculation of the alpha-particle heat power generated in the plutonium sphere by ²³⁸Pu and other plutonium isotopes. Then, three levels of technology with regard to the size of such hypothetical NEDs (HNEDs) and the technological level of high explosives are defined. On the basis of material data available in the open scientific literature, the radial temperature profiles in such HNEDs of an assumed configuration are calculated, and it is found that for low-technology HNEDs the limiting temperatures are exceeded for a ²³⁸Pu content of 1.6%. For high-technology HNEDs these limiting temperatures are exceeded for a ²³⁸Pu content above ~6% or somewhat more. Such denatured plutonium can be considered as proliferation resistant, similarly as uranium with <20% ²³⁵U or <12% ²³³U.