Verification of Small UF6 Cylinders Using Neutron Self-Interrogation and Multiplicity Counting

To improve international safeguards and material accountancy at uranium enrichment plants, accurate verification of 235 U enrichment and mass in uranium hexafluoride (UF6) storage cylinders is needed. Small UF6 cylinders (1.5” and 5” diameter) are used to store the full range of enrichments from depleted to highly-enriched UF6. For independent verification of these cylinders, a measurement of the 235 U mass and enrichment that does not rely operator declarations is vital. The traditional method for verifying enrichment of UF6 cylinders is to measure the net counts from 235 U 186-keV gamma-ray emissions. The primary disadvantage of this method is that the enrichment on the outer layer (~1-mm) of the UF6 dominates the measurement, and extrapolation to the complete cylinder content requires assuming the sample is homogenous. Also, the deposits on the walls from prior use can perturb the results. Recently, the Passive Neutron Enrichment Meter (PNEM) was developed for simultaneously verifying 235 U mass and enrichment in large (30” and 48” diameter) UF6 cylinders. This paper describes the extension of the self-interrogation measurement technique developed for large cylinders with PNEM to small UF6 cylinders with a well counter called the Mini Epithermal Neutron Multiplicity Counter (Mini ENMC). The Mini ENMC consists of 104 3 He tubes that are arranged in four concentric rings and embedded in polyethylene. The high efficiency and short die-away time of the Mini ENMC provides high measurement precision. The system can also be split in half to accommodate 8” or nonstandard cylinder sizes that may be encountered in the field. The neutron self-interrogation measurement method utilizes the neutrons produced from the 19 F(a,n) 22 Na reaction resulting from 234 U alpha decay in UF6. These neutrons induce fission in 235 U, which produces time-correlated bursts of neutrons that are easily distinguished from random (a,n) neutrons by coincidence electronics. We performed a series of experimental measurements of 1S UF6 cylinders at Los Alamos National Laboratory (LANL) using the Mini ENMC to assess the viability of using the self-interrogation measurement technique to verify the 235 U mass and enrichment in small UF6 cylinders