Reflections on the criticality of special actinide elements

During recent years, the list of nuclides known to be capable of supporting a chain reaction has substantially increased. Since the criticality aspects for some of these nuclides differ in important respects from those of the most common fissile nuclides, 92235U, and 94239Pu, a new term, ''fissible'' was recently proposed in nuclear engineering to help distinguish differences. Activation energies for fission have been calculated for 41 of the actinide isotopes which are grouped according to four types of nuclides, those with even-Z and even-N, odd-Z and odd-N, odd-Z and even-N, or even-Z and odd-N. With the possible exception of 92237U, all fissible isotopes listed have even N. The activation energy for fission is less in the case of the even-Z and even-N isotopes, but almost without exception it is the odd-N isotopes that undergo fission with thermal neutrons and which constitute the principal criticality problem. This paper reviews the criticality and fissionability aspects of the fissile and fissible actinide isotopes. The criticality of aqueous mixtures of fissile and fissible isotopes also is briefly discussed, including limits for criticality control. (author)