Absolute calibration methodology for non-uniform uranium and matrix distributions in large barrels of uranium-bearing solid waste

The effective implementation of domestic and international safeguards necessitates verification techniques for Nuclear Materials (NM). Even in the case of very small quantities of NMs, accounting for and analyzing such traces can provide insights into the mass balance of NMs and/or state activities, ensuring consistency in state declarations. This paper proposes and benchmarks an absolute calibration methodology for estimating the uranium-mass content in large-volume barrels (200 L). These barrels may be generated during the operation and decommissioning of bulk handling facilities by accumulating low-density scarab contaminated with NMs. The method relies on the mathematical calibration of a High Purity Germanium (HPGe) detector efficiency against non-uniform uranium and matrix distributions, assuming that the non-uniform distribution can be approximated as a uniform one for low-density matrix materials. The peak efficiency is calculated for different numbers of point sources 1-30 likely distributed inside a simulated barrel while it rotates around the axis of symmetry. The proposed method is benchmarked by comparing the calculated peak efficiency of randomly distributed uranium sources with the homogeneous distribution. Furthermore, a comparison with experimental measurements is conducted to validate the proposed method. Results show that the proposed calibration method considering either random or homogeneous source and matrix distributions in large volume barrels can be used for estimating the uranium mass content in NSW with an accuracy of approximately 12 %.