Using fast neutron signatures for improved UF6 cylinder enrichment measurements.

The authors have investigated the use of fast neutrons—primarily the fast neutron energy spectrum—as a signature for uranium hexafluoride (UF6) nuclear accountancy measurements. Detailed modeling of UF6 storage cylinders and a proposed neutron detection system indicates that the measured neutron energy spectrum is indeed a function of uranium enrichment. Field measurements at the Department of Energy’s Paducah Gaseous Diffusion Plant with a detection system similar to the modeled system provided an opportunity to collect signatures from several storage cylinders containing UF6 with a range of enrichments. Subsequent analysis lends credibility to the modeling results, indicating that enrichment over the range measured (0.72% to 4.95% uranium-235) can be extracted from the measured neutron energy spectrum. These results were scaled to estimate the tradeoff in measurement system size and counting time to achieve a relative enrichment measurement uncertainty of 5%.