Measurements of UF₆ Cylinders with Portable Instruments

The applicability of portable instruments for rapid nondestructive verification of the enrichment of UF₆ in cylinders has been tested on a large number of Types-30 and -5A cylinders. Three basic techniques were used: (a) gamma-ray counting with Nal, combined with ultrasonic measurement of cylinder wall thickness, (b) passive-neutron counting, and (c) active-neutron interrogation with thermal neutrons from a radioactive neutron source. The accuracy of the gamma-ray method was ∼5% (1σ) for Type-30 cylinders of UF₆ and 2% for highly enriched UF₆ in Type-5A cylinders; however, the method occasionally failed for Type-30 cylinders because of background from nonvolatile daughters of ²³⁸U plated on the cylinder walls. The standard deviation of enrichments of 110 Type-30 cylinders, derived from passive-neutron counting data by assuming a constant ²³⁵U/²³⁴U ratio, is The response of the active system increases almost linearly with enrichment up to ∼2.5% ²³⁵U and then saturates at ∼4%.