Applications of Computerized Simulations in Nondestructive Waste Assays

Radioactive calibration standards, especially those containing special nuclear material, are increasingly difficult to obtain. Safety, security and transport regulations that govern such sources, though obviously necessary, are impediments to their widespread use. Computerized simulations can supplement or replace traditional nondestructive assay (NDA) calibration methodologies that use conventional radioactive standards. The new approach is gaining increased acceptance, especially for transuranic waste assay. For example, a gamma ray detector’s absolute response can be characterized using a single radioactive point source, and then practical calibration curves can be constructed by modeling that detector in appropriate counting geometries. The computer models include variables for container-to-detector distance, container shape, and waste matrix composition. The radioactive material may be modeled as a homogeneous or lumpy distribution. Computerized simulations have also been used to determine calibration parameters for passive neutron counting and to devise measurement uncertainty estimates for NDA instruments. This paper briefly describes how such a calibration is performed, and presents some applications where those calibrations are in practical use.