Discerning Transuranic Sub-Concealments in Nuclear Waste Barrels Using γ-Ray Spectrometry: A Non-Invasive Assay Strategy To Combat the Threat of Nuclear Diversion

Insufficient inventory control arising from inadequate monitoring procedures can lead to vulnerabilities in nuclear security. In addition, insider threats, by either malicious intent or negligence, can pose a substantial risk by exploiting such deficiencies to perform unlawful actions, such as theft or diversion, which may lead to compromised nuclear security. Interim storage barrels, intended for temporary containment of low-density nuclear waste, require special attention in this regard. In certain scenarios, the inadequacy of the existing waste barrel assay methodologies to identify and correctly measure any undesirable transuranic subconcealment present inside may pose a risk of nuclear diversion. The present work aims to establish an absolute waste barrel assay procedure to mitigate such risks by offering a robust method for the detection and assay of transuranic subconcealments in nuclear waste barrels using high-resolution γ-ray spectrometry. Challenges related to the varying amounts of attenuation experienced by the γ-rays within the subconcealments, low-density matrix, and the steel barrel wall have been addressed by an iterative photopeak efficiency transfer approach. The methodology has been verified for the assay of seven mock-up samples mimicking nuclear diversion attempts using waste barrels, for which conventional barrel scanning would be inadequate and tomographic scanning would be highly time-consuming. Using this methodology, Pu and ²⁴¹Am isotopes have been assayed within <10% of the expected value for the majority, with the measurement uncertainty of <10%. The present method, which is simple and noninvasive, can identify inconsistencies with the labeled inventory and detect potential diversion attempts.