Heterogeneity effects on nondestructive assay measurements of enrichment in uf₆ cylinders

Measurements were performed using multipl e mechanically cooled high-purity germanium detectors at six positions around standard industrial 30B cylinders (2.2 metric ton) of UF₆ to assess if matrix inhomogeneity is detectable and its impacts on the measured apparent uranium enrichment. Uranium enrichment was calculated with FRAM™. Six of the nine cylinders appeared to be homogeneous as indicated by having similarly accurate apparent measured uranium enrichment, at all positions. However, three cyl i nders appeared to have l ocal i zed i nhomogenei ti es. Thi s was mani fested as ver y l ow apparent enrichments, often <10% of the declared value, on one side of the cylinder. Examination of the spectra suggested both elevated²³⁴ Th-^234m Pa daughter isotopes and reduced²³⁵ U were measured at these locations. It is hypothesized that these heterogeneous cylinders may have experienced asymmetric solar heating, which caused volatile UF₆ to sublime preferentially away from the warmed side. Care should be taken during uranium enrichment verification when applying methods that include gamma-r ays associ at ed wi t h daught er nucl i des t o t he determination of uranium enrichment for cylinders that are stored in sunlight.