Characterizing self-attenuation in UO2 powder gamma spectrometry: development of a matrix-independent assay for mass and enrichment quantification in bulk nuclear material⋆

Accurate nondestructiv e assay (NDA) of bulk nuclear materials lacking a rigid physical form presents significant geometric challenges compared with standard item-form configurations such as sintered pellets, plates, or fuel rods. The quantification of unsealed uranium dioxide UO 2 powder is compromised by variable localized packing densities and severe internal self-attenuation of the primary 185.7 keV 235 U gamma-ray emission. This study investigates these limitations by characterizing UO 2 powder samples (0.4 to 4.0 wt% 235 U enrichment; 1–25 g uranium mass) using both a portable low-resolution NaI(Tl) and a high-resolution HPGe spectrometer. Empirical modeling of the specific count rate against areal density confirmed adherence to the integrated Beer-Lambert law, establishing a minimum physical thickness threshold of 1.0 cm to achieve infinite thickness saturation of 95%. Absolute 185.7 keV count rates below this geometric threshold are highly susceptible to density fluctuations, rendering them unreliable for direct mass or enrichment quantification in non-rigid matrices. To overcome this self-attenuation barrier, the highly penetrating 1001 keV emission from 234m Pa was validated as an internal bulk mass standard. Using the relative activity ratio (185.7 keV/1001 keV) successfully neutralized both structural matrix inconsistencies and discrepancies in intrinsic hardware efficiency. A comparative analysis between the two detection systems yielded a linear transfer function (R 2 > 0.97, slope = 0.68, intercept = 0.51). This methodology provides a detector-agnostic physical signature that significantly enhances bulk uranium verifications and lays the groundwork for accurately quantifying advanced non-item nuclear materials, such as molten salt fuels.