Correcting Cylinder Verification Neutron Count Rates for Variations due to UF6 Distribution

Neutron-based measurement of the 235U mass and enrichment in UF6 product cylinders at uranium enrichment plants have the potential to improve the ability of the IAEA to verify operator declarations by measuring the entire contents of large 30B cylinders. The current study concerns potential improvements to an experimentally tested 3He neutron detection system found to be capable of simultaneously determining 235U mass and enrichment, respectively using the Singles and Doubles rates. It has been established that UF6 distribution variations (modelled using the ‘X-factor’) have a significant impact on count rates, and may have been a factor limiting the accuracy of experimentally calculated quantities. The ability to account for UF6 distribution is demonstrated experimentally here using a correction based on the Singles ratio measured using neutron detectors at two positions around the cylinder, which was found to significantly improve the accuracy of 235U mass and enrichment calculations. This Singles ratio correction method was then optimized by simulating an additional polyethylene moderated 3He tube at various positions to find the location where the Singles ratio from an additional 3He tube to the main 3He detector modules at the bottom of the cylinder best indicates X-factor. The resulting improvements in accuracy are substantial for both the simulated Singles-based 235U mass and Doubles-based enrichment measurements. Simulations throughout this study include the full range of X-factors and cover 30B cylinders at enrichments ranging 1.5% through 4.9%.