Total Measurement Uncertainty for Box Segmented Gamma Scanner with Known Source and Geometry

The Canberra Box Segmented Gamma Scanner (BSGS) is one of the many systems available for doing nondestructive assay assessment of radioactive waste for many different types of container. High quality assessment of the radioactive contents of a waste container along with total measurement uncertainty (TMU) is an essential part of government requirements, before shipping of the radioactive waste. While there are many factors affecting the total measurement uncertainty, better knowledge of the waste matrix composition and source positioning can help reduce the TMU and increase the confidence in gamma measurements. In this study, we have used In Situ Object Counting System (ISOCS) software and the ISOCS Uncertainty Estimator (IUE) to understand the TMU for a specific case of a steel pipe-overpack component (POC) containing various masses of Pu-239 material, all contained within a 55-gallon drum geometry. The ISOCS software was run to generate the reference efficiency for the POC, and the IUE software was used to generate uncertainty due to source position, fill height, and hot spots. The overall objective was to estimate and reduce the magnitude of the biggest contributors to TMU with advanced, restrictive knowledge of the geometry and source configuration. The advantages of the TMU reassessment are that no stringent requirement of waste positioning in the drum is needed, thus avoiding excess dose to the workers. Also, now more Pu-239 material can be packed within this type of geometry, which will reduce the overall transportation needs and costs. The overall TMU for Pu-239 waste was estimated to be under 10% for a one or two can source packaging inside the POC.