Fill Height Corrections in Segmented Gamma Non-Destructive Assay for Highly Attenuating Scenarios

Accurate characterization of nuclear material is critical for material accountancy and waste characterization. Segmented Gamma Scanners (SGS) and large gamma box counters are often employed for containerized waste and scrap with gamma-emitting nuclides. Matrix attenuation corrections using a transmission source performed on a segment-by-segment basis adequately account for containers with partially filled material. However, items with high density materials, such as spent resins or sludges, or those packed in large containers are often subjected to large bias situations which render transmission-based attenuation correction impractical. Examples of such containers are concrete-lined drums, lead-lined drums, concrete casks, pipe over-pack containers, and large high-integrity containers made of plastic or steel. For these cases, inaccurate assessments or uncertainties in the matrix fill height can lead to biases in the activity determination, unless appropriate calibrations or corrections are implemented. In this work, we review the impact of fill height variations as related to assay accuracy and variance for the sample scenarios mentioned above, where matrix corrections on partially filled containers are typically performed based on the bulk weight or density. This will be assessed using mathematical modeling and measured data from an SGS. Alternative methods and algorithms for determining fill height and applying appropriate corrections will be presented. In addition, the limitations of these methods due to extreme source-matrix configurations will be discussed.