Year
2015
Abstract
In both nuclear safeguards measurements and radioactive waste assay, measurement errors arise and are often undetected. Perhaps the most significant cause of such errors is a lack of knowledge of the material being measured and the consequent inappropriate choice of measurement method. The two factors, which are the dominant sources of error in safeguards and waste measurement, are matrix heterogeneity and the non-uniform distribution of activity in the measured sample. In order to appreciate the contribution made by these factors, the measurement of a variety of radioactive materials and matrices in 200 litre (55 gallon) drums have been modelled using benchmarked MCNP Monte Carlo models. Simulated measurements have been performed for drums with homogeneous matrices with uniform density and activity distribution for a range of density values. These results have been compared to the results of simulations where both drum matrix and activity distribution are non-uniform. The Far Field measurement method has been employed in the simulations. It is the simplest measurement process and is based on the assumption of drum uniformity. As a result, Far Field measurements exhibit the largest measurement errors due to heterogeneity in matrix and activity distribution. Other measurement technologies have been developed to reduce the measurement errors that arise from matrix heterogeneity and non-uniform activity distribution. These measurement technologies include both segmented gamma scanning and tomographic gamma scanning. The reductions in measurement errors using these alternate measurement methods are also considered.