Analysis of an indirect neutron signature for enhanced UF6 cylinder verification

Abstract The International Atomic Energy Agency (IAEA) currently uses handheld gamma-ray spectrometers combined with ultrasonic wall-thickness gauges to verify the declared enrichment of uranium hexafluoride (UF 6 ) cylinders. The current method provides relatively low accuracy for the assay of 235 U enrichment, especially for natural and depleted UF 6 . Furthermore, the current method provides no capability to assay the absolute mass of 235 U in the cylinder due to the localized instrument geometry and limited penetration of the 186-keV gamma-ray signature from 235 U. Also, the current verification process is a time-consuming component of on-site inspections at uranium enrichment plants. Toward the goal of a more-capable cylinder assay method, the Pacific Northwest National Laboratory has developed the hybrid enrichment verification array (HEVA). HEVA measures both the traditional 186-keV direct signature and a non-traditional, high-energy neutron-induced signature (HEVA NT ). HEVA NT enables full-volume assay of UF 6 cylinders by exploiting the relatively larger mean free paths of the neutrons emitted from the UF 6 . In this work, Monte Carlo modeling is used as the basis for characterizing HEVA NT in terms of the individual contributions to HEVA NT from nuclides and hardware components. Monte Carlo modeling is also used to quantify the intrinsic efficiency of HEVA for neutron detection in a cylinder-assay geometry. Modeling predictions are validated against neutron-induced gamma-ray spectra from laboratory measurements and a relatively large population of Type 30B cylinders spanning a range of enrichments. Implications of the analysis and findings on the viability of HEVA for cylinder verification are discussed, such as the resistance of the HEVA NT signature to manipulation by the nearby placement of neutron-conversion materials.