Signatures and Methods for the Automated Nondestructive Assay of ${\rm UF}_{6}$ Cylinders at Uranium Enrichment Plants

International Atomic Energy Agency (IAEA) inspectors currently perform periodic inspections at uranium enrichment plants to verify UF₆ cylinder enrichment declarations. Measurements are typically performed with handheld high-resolution sensors on a sampling of cylinders taken to be representative of the facility´s entire cylinder inventory. These measurements are time-consuming, expensive, and assay only a small fraction of the total cylinder volume. An automated nondestructive assay system capable of providing enrichment measurements over the full volume of the cylinder could improve upon current verification practices in terms of manpower and assay accuracy. The 185-keV emission from U-235 is utilized in today´s cylinder measurements, but augmenting this “traditional” signature with more-penetrating “non-traditional” signatures could help to achieve full-volume assay in an automated system. This paper describes the study of non-traditional signatures that include neutrons produced by F-19 (α, n) reactions (spawned primarily from U-234 alpha emission) and the high-energy gamma rays (extending up to 8 MeV) induced by those neutrons when they interact in the cylinder wall and nearby materials. The potential of these non-traditional signatures and assay methods for automated cylinder verification is explored using field measurements on a small population of cylinders ranging from 2.0% to 5% in U-235 enrichment. The standard deviation of the non-traditional high-energy gamma-ray assay approach was 4.7% relative to the declared cylinder enrichments; the standard deviation of the traditional enrichment meter approach using a well-collimated high-resolution spectrometer was 4.3%. The prospect of using the non-traditional high-energy gamma-ray signature in concert with the traditional 185-keV signature to reduce the uncertainty of automated cylinder assay is discussed.