Year
2007
Abstract
The quantification of gamma emitting radionuclides in large waste packages has traditionally been based on segment response functions for homogeneous matrices with uniform activity distributions. This is usually justified on the grounds that in the absence of additional information, the assumption of uniformity would, on the average when taken over a large number of containers, approximate the situation for randomly packed waste. The stream average (or cumulative) assay result would therefore presumably not be biased. In practice, however, the matrix in a particular container may be quite heterogeneous, consisting of chunks of material with void space between. Additionally, the activity distribution may not match the density distribution of the matrix. An example is surface contamination chunks of material in a heterogeneous container. The impact of these deviations from the simplistic assumptions of matrix and source uniformity have not been extensively studied and reported previously. In this work we look at the problem using a numerical simulation tool especially constructed to model irregular density and activity distributions. We illustrate for two familiar assay conditions - the gamma-ray assay of 200 liter drums and SLB-2 waste boxes - how the presence of substantial void space results in a higher detection efficiency than the usually applied uniform model would predict. We show how with reasonable knowledge of the waste form, an allowance may be made and in particular how a more appropriate total measurement uncertainty (TMU) contribution may be estimated.