Nuclear criticality safety calculations for a K-25 site vacuum cleaner

A modified Nilfisk model GSJ dry vacuum cleaner is used throughout the K-25 Site to collect dry forms of highly enriched uranium (HEU). When vacuuming, solids are collected in a cyclone-type separator vacuum cleaner body. Calculations were done with the SCALE (KENO V.a) computer code to establish conditions at which a nuclear criticality event might occur if the vacuum cleaner was filled with fissile solution. Conditions evaluated included full (12-in. water) reflection and nominal (1-in. water) reflection, and full (100%) and 20% {sup 235}U enrichment. Validation analyses of SCALE/KENO and the SCALE 27-group cross sections for nuclear criticality safety applications indicate that a calculated k{sub eff} + 2{sigma} < 0.9605 may be considered safely subcritical. Thus, a system with a calculated k{sub eff} + 2{sigma} {ge} 0.9605 is considered unsafe and may be critical. Critical conditions were calculated to be 70 g U/L for 100% {sup 235}U and full 12-in. water reflection. This corresponds to a minimum critical mass of approximately 1,400 g {sup 235}U for the approximate 20.0-L volume of the vacuum cleaner. The actual volume of the vacuum cleaner is smaller than the modeled volume because some internal materials of construction were assumed to be fissile solution. The model was an overestimate, for conservatism, of fissile solution occupancy. At nominal reflection conditions, the critical concentration in a vacuum cleaner full of UO{sub 2}F{sub 2} solution was calculated to be 100 g{sup 235}U/L, or 2,000 g mass of 100% {sup 235}U. At 20% {sup 235}U for the 20.0-L volume of the vacuum cleaner. At 15% {sup 235}U enrichment and full reflection, critical conditions were not reached at any possible concentration of uranium as a uranyl fluoride solution. At 17.5% {sup 235}U enrichment, criticality was reached at approximately 1,300 g U/L which is beyond saturation at 25 C