AbstractA system has been developed to measure continuously the 235U and 234U enrichment in the UF6 product of a gaseous diffusion plant. The measurements are made on liquid UF6 prior to withdrawal into product cylinders. The 235U enrichment is measured to a relative accuracy of 0.5% at two sigma by counting the 185.7-keV gamma ray from 235U with an NaI detector. The 234U enrichment is measured with a neutron detector counting (α,n) neutrons from UF6. The neutron system can measure either 234 U enrichment directly or 235 U enrichment indirectly (for low-enriched uranium, 2 to 5% 235U, the two enrichments are nearly proportional). The accuracy of the neutron measurement is 2.5% at two sigma. The gamma and neutron measurements are independent and could be used singly if only one isotope were desired. Both are required to measure 235 U and 234 U enrichment. The system is presently installed at the Goodyear Atomic Corporation gaseous diffusion plant in Piketon, Ohio.

A hybrid assay system utilizing both active and passive techniques has been built for the measurement of the plutonium fissile content in fast-breeder-reactor-type fuel pins. A moderated 252Cf source (∼600 µg) is used for the neutron interrogation of the fuel rods, and the fissile content is then determined by counting the high-energy delayed gamma rays resulting from the induced fission reactions. Neutron transport calculations using both Sn and Monte Carlo techniques were used to design the 252Cf neutron tailoring assembly to give an intense fast-neutron irradiation, as well as a high fissile/fertile fission ratio. In addition to the total fissile active assay, pellet-to-pellet uniformity is determined simultaneously by counting the lower energy passive gamma rays from the fuel using the same NaI detectors. The complete assay system, which includes automated fuel-rod handling and data reduction, is being used for in-plant measurements of Fast Flux Test Facility fuel pins.