A Nuclear Criticality Safety Assessment of the Loss of Moderation Control in 2 1/2 and 10-ton Cylinders Containing Enriched UF<sub>6</sub>

Moderation control for maintaining nuclear criticality safety in 2-1/2-ton, 10-ton, and 14-ton cylinders containing enriched uranium hexafluoride (UF<sub>6</sub>) has been used safely within the nuclear industry for over thirty years, and is dependent on cylinder integrity and containment. This assessment evaluates the loss of moderation control by the breaching of containment and entry of water into the cylinders. The first objective of this study was to estimate the required amounts of water entering these large UF<sub>6</sub> cylinders to react with, and to moderate the uranium compounds sufficiently to cause criticality. Hypothetical accident situations were modeled as a uranyl fluoride (UO<sub>2</sub> F<sub>2</sub>) slab above a UF<sub>6</sub> hemicylinder, and a UO<sub>2</sub> sphere centered within a UF<sub>6</sub> hemicylinder. These situations were investigated by computational analyses utilizing the KENO V.a Monte Carlo Computer Code. The results were used to estimate both the masses of water required for criticality, and the limiting masses of water that could be considered safe. The second objective of the assessment was to calculate the time available for emergency control actions before a criticality would occur, i.e., a "safetime,'' for various sources of water and different size openings in a breached cylinder. In the situations considered, except the case for a fire hose, the safetime appears adequate for emergency control actions. The assessment shows that current practices for handling moderation-controlled cylinders of low enriched UF<sub>6</sub>, along with the continuation of established personnel training programs, ensure nuclear criticality safety for routine and emergency operations.