Year
2014
Abstract
It is a universal requirement for characterization of radioactive items and special nuclear materials a Total Measurement Uncertainty (TMU) value is determined for each of the measured quantities such as nuclide activity. For Non-destructive Assay systems, the TMU analysis is typically performed on an individual item basis. However, in many cases, materials are transported, stored and disposed of in large groups for example by overpacking smaller packages into a larger container or emplacing packages into groups for final disposal. The current standard practice for package-group data analysis is usually to treat each package as independent and uncorrelated and use a simple summation / averaging method (or in some cases summation of TMU in quadrature) to define the overall characteristics and associated uncertainty in the package group. In reality, many groups of packages are assayed on the same system, so there will be a large degree of co-dependence in the individual uncertainty elements. Many uncertainty terms may be significantly reduced when addressing issues such as the source position and variability in matrix contents over large populations. The systematic terms encompass both inherently “two-directional” random effects (e.g. variation of source position) and other terms that are “one-directional” i.e. designed to account for potential sources of bias. An analysis has been performed with population groups from a variety of non-destructive assay platforms. From this analysis, a new model has been developed that yields a general case algorithm for determination of overall TMU for batches of containers.