A nuclear criticality safety analysis of the PACT-8 melter for the INEL Pit-9 comprehensive demonstration project

Between 1967 and 1969, ∼3100 m3 of transuranic (TRU) waste from the Rocky Flats Plant was buried with other hazardous and low-level radioactive wastes in a 40- x 116-m disposal pit located at the Idaho National Engineering Laboratory's Radioactive Waste Management Complex. This pit, designated as pit 9, is estimated to contain ∼40 Ci of 238Pu, 1.7 kCi of 239Pu, 0.4 kCi of 240Pu, 3.0 kCi of 240Pu, and 3.1 kCi of 241Am. During the nearly 30 yr that have elapsed since burial, normal surface-water movement coupled with the disintegration of the original containers have contributed to limited migration of radionuclides past the boundaries of the original disposal containers. Sensitivity to issues germane to buried TRU waste has led to the decision to retrieve this material. Because the remediation operations involve fissile materials, a credible nuclear criticality safety (NCS) analysis must be performed as part of the safety evaluation process to demonstrate the improbability of any contingencies contributing to an inadvertent criticality. Environmental NCS analyses are problematic because no experimental benchmarks with comparable ranges of scale and fissile material concentration exit that can be applied to environmental situations