CONSIDERATIONS OF AUTOCATALYTIC CRITICALITY OF FISSILE MATERIALS IN GEOLOGIC REPOSITORIES

Potential routes to autocatalytic criticality in geologic repositories are systematically assessed. If highly enriched uranium (HEU) or Pu-2 39 are transported and deposited in concentrations similar to natural uranium ore, in principle, criticality can occur. For some hypothesize d critical configurations, removal of a small fraction of pore water p rovides a positive feedback mechanism that can lead to supercriticalit y. Rock heating and homogenization for these configurations can also s ignificantly increase reactivity. At Yucca Mountain, it is highly unli kely that these configurations can occur; plutonium transport would oc cur primarily as colloids and deposit over short distances. HEU solute can move large distances in the Yucca Mountain setting; its ability t o precipitate into critical configurations is unlikely because of a la ck of active reducing agents. Appropriate engineering of the waste for m and the repository can reduce any remaining probability of criticali ty.