Impact of fissile minor actinides on the criticality safety of transport of fissile material

In the current IAEA regulation for the safe transport of radioactive material and all previous editions of this regulation, the fissile materials are defined as the materials containing any of the 4 following nuclides ²³³U, ²³⁵U, ²³⁹Pu and ²⁴¹Pu. It is well-known that other nuclides are able to sustain a fission chain reaction and that some of them have a lower subcritical mass limit than the four usual fissile nuclides, for example ^242mAm, ²⁴³Cm and ²⁵¹Cf. In cases where sufficient quantities of these nuclides would be present in a material, the guidance of the regulation advices to perform a criticality safety assessment as per the requirements of paragraphs 673-686. When these minor actinides are not separated from fission products, uranium or plutonium (for example, in irradiated UOX or MOX fuel), they have no impact on nuclear criticality safety and the formulation of SSR-6 is acceptable. However, if these fissile minor actinides come from an enhanced separation process, they could have a significant impact on reactivity even for limited quantities (several grams), in particular for some nuclides. Moreover, even if many minor actinides have a significant subcritical mass limit (much higher than ²³⁵U), when these nuclides are irradiated (for example, as experimental fuel), it becomes more complicated to justify subcriticality if their initial mass is important. Indeed, during irradiation, neutron capture by these nuclides, for example ²⁴¹Am, will produce fissile minor actinides with very low subcritical mass limits. (author)