Year
1985
Abstract
A passive neutron counting method has been developed for measurement of the 235U mass in Model 5A cylinders of UF6. The unique neutronic properties of UF6 containing highly enriched uranium (HEU) permit 235U assay using only passive neutron counting. The sample effectively assays itself by self-interrogation. Shipped from enrichment plants and received at fuel fabrication and conversion facilities, 5A UF6 cylinders hold up to \"V17 kg of 235U each. Field measurements at the Portsmouth Gaseous Diffusion Plant (GDP) showed an average assay accuracy of 6.8% (la) for 44 cylinders with enrichments from 6 to 98% and with a range of fill heights. Further measurements on 38 cylinders containing 97%-enriched material yielded an accuracy of 2.8% (la). Typical counting times for these measurements were less than 5 min. An in—plant instrument for receipts confirmation measurements of 5A UF6 cylinders has been developed for the Savannah River Plant. The Receipts Assay Monitor (RAM) is currently being tested and calibrated. It is designed to confirm declared fissile mass in all incoming 5A cylinders containing HEU in the form of UF6. One of the computer-controlled features is a removable cadmium liner for the sample cavity. The liner allows a sample fill-height correction, which significantly improves assay accuracy.