Year
2007
Abstract
Radioactive wastes that are bound for the waste Isolation Pilot Plant (WIPP0 or other disposal sites are subject to characterization before they can be shipped. Characterization includes site Acceptable Knowledge (AK), Visual Examination (VE), X-ray Nondestructive Examination (NDE), and radiometric Nondestructive Assay (NDA). These characterization methods are developed to meet applicable government regulations, and are usually used together to describe a waste’s contents. NDA techniques include neutron or gamma assays (or both) depending on the nature of the waste. NDA measurement results are generally reported on a radioassay data sheet, formatted to meet the requirements of each site’s TRU waste program manager. Results include a list of nuclides, mass, activity, and associated uncertainties, and the minimum detectable activity for each nuclide. The individual nuclide results are used to compute the TRU alpha activity, 239Pu equivalent activity, 239Pu fissile gram equivalent, decay heat, and total Pu mass for each drum. The individual uncertainties are propagated to derive uncertainties for each of these quantities. Each individual value is reported with a Total Measurement Uncertainty (TMU), into which all the measurement uncertainties are propagated in standard fashion. Typical measurement uncertainties include matrix-source inhomogeneity, source distribution, and lumpiness in addition to counting statistics. The individual uncertainties are evaluated to determine the relative importance of each, so that major contributors to the overall measurement uncertainty can be minimized if possible. This paper will discuss the contributors to TMU for typical gamma and neutron assays, and discuss the relative importance of each one.