BOUNDING ESTIMATES FOR CRITICAL EVENTS WHEN DIRECTLY DISPOSING HIGHLYENRICHED SPENT NUCLEAR-FUEL IN UNSATURATED TUFF

This paper examines the possibility of criticality in a nuclear waste repository. The estimated probabilities are rough bounds and do not en tirely dismiss the possibility of a critical condition; however, they do point to the difficulty of creating conditions under which a critic al mass could be assembled (i.e., corrosion of containers, separation of neutron absorbers from the fissile material, and collapse or precip itation of the fissile material). In addition, should a criticality oc cur in or near a container, the bounding consequence calculations show ed that fissions from one critical event are quite small (< similar to 10(20) fissions, if similar to aqueous and metal accidents and experi ments). Furthermore, a reasonable upper bound of total critical events of 10(28) fissions corresponds to only 0.1% of the number of fissions represented by the spent nuclear fuel inventory in a repository conta ining 70,000 metric tons of heavy metal (MTHM) (the expected size for the proposed repository at Yucca Mountain, Nevada).