Greatly improved transuranic waste assay accuracy using neutron signal imaging

Differential dieaway-based, passive-action neutron (PAN) assay systems have been used extensively for high-sensitivity bulk transuranic (TRU) waste assays for more than 10 years - at US Department of Energy sites, US commercial nuclear facilities, and internationally. In common with all nondestructive assay techniques applied to bulk TRU wastes (i.e., 208-{ell} drums or larger packages), matrix effects are the dominant source of assay error in PAN measurements. Even prudent use of properly carried out calibrations in close mockups of the actual waste drums does not solve this assay accuracy problem. It is easily demonstrated with seemingly innocuous waste forms such as combustibles or burnables (i.e., rags, paper, etc.) that measured passive or active assay responses may vary a factor of 2 or more from the drum center to its outer radius. A proper matrix correction can be made to TRU waste measurements only if one knows exactly where within the waste drum the neutron signals originate. The authors have developed what believed to be a satisfactory solution to this TRU waste measurement problem: an imaging PAN (IPAN) assay system that not only provides sensitive waste matrix identification but also provides passive and active neutron signal imaging.