Year
2022
Abstract
Neutron coincidence counting was chosen as the preferred method for assay of diluted surplus plutonium in Criticality Control Overpacks (CCOs) produced at the Savannah River Site. However, a significant portion of the material consists of scrap oxide containing low-Z elemental impurities, which interact with alpha particles to create additional neutrons. As a result, these impure items tend to give erroneously high readings when analyzed with a standard calibration curve. These biases can result in measurements falling outside required bounds for both Material Control & Accountability and Waste Acceptance Criteria.
A known-multiplication analysis has been implemented for verification measurements on these items. This method assumes that the multiplication can be predicted for a given item based solely on its isotopic ratios. This allows for the singles and doubles coincidence rates to be solved for the plutonium mass and (?, n) emission rate. Due to the consistent geometry and composition of each CCO, the multiplication is particularly well behaved across a range of isotopics, mass loadings, and impurity contents. Calibration standards have been made to empirically determine the multiplication in the CCOs, and working standards have been created to determine the accuracy and precision of the method.