Sensitivity and accuracy considerations for neutron assay of plutonium-contaminated waste in large containers

Since the 1970 innovations have allowed both active and passive neutron techniques to address various safeguards and waste measurement needs in the DOE complex. Much research was focused on satisfjring the 100-nCi/g detection limit for TRU waste in 208-liter drums. The emphasis on measuring drum-sized containers for disposal at WIPP has resulted in improved waste assay capability that now needs to be extended to larger containers. The desire to expedite the decontamination and decommissioning of certtain DOE facilities, and the large waste encountered in that process, has prompted the need for increasingly large disposal containers. Instruments have recently been built to accommodate crates that are nearly 100 cubic feet in volume, such as a B-25 box or Standard Waste 13ox. The density of hydrogen inside a waste container profoundly affects the accuracy of neutron measurements, and the metal content greatly affects sensitivity. Depending on the matrix, and especially the hydrogen content, the response of an instrument to a single point source can vary tremendously within the container. Because the density and composition of metals inside each container are unknown, the observed cosmic ray background rate varies from one container to the next, resulting in a loss of sensitivity for passive counters. In the paper we will explore the magnitude of these problcms for both metal- and hydrogen-bearing matrices in a crate-sized containers