Viability of UF6 Cylinder Verification using the Hybrid Enrichment Verification Array

In recent years, the International Atomic Energy Agency (IAEA) has pursued innovative techniques and an integrated suite of safeguards measures to address the verification challenges posed by advanced centrifuge technologies and the growth in separative work unit capacity at modern centrifuge enrichment plants. These measures would include permanently installed, unattended instruments capable of performing the routine and repetitive measurements previously performed by inspectors. Among the unattended instruments currently being explored by the IAEA is an Unattended Cylinder Verification Station (UCVS) that could provide independent verification of the declared relative enrichment, 235U mass and total uranium mass of 100 percent of the declared cylinders moving through the plant, as well as the application and verification of a Non-destructive Assay Fingerprint to preserve verification knowledge on the contents of each cylinder throughout its life in the facility. As IAEAs vision for a UCVS has evolved, Pacific Northwest National Laboratory (PNNL) has been developing the Hybrid Enrichment Verification Array (HEVA) method as a candidate nondestructive assay method for inclusion in the UCVS. HEVA utilizes an array of sodium iodide spectrometers (NaI(Tl)) to simultaneously measure the direct gamma-ray signature from 235U and via high-energy gamma rays induced by neutrons in specially designed collimators and the sodium iodide, the total neutron emission rate from the cylinder. Modeling and multiple field campaigns have indicated that HEVA is capable of assaying relative cylinder enrichment with a precision comparable to or perhaps better than todays high-resolution handheld devices, without the need for manual wall-thickness corrections. In addition, the HEVA method interrogates the full volume of the cylinder, thereby offering the IAEA a new capability to assay the absolute 235U mass in the cylinder, and much-improved sensitivity to substituted or removed material. By hybridizing the two complementary radiation signatures, HEVA is also capable of detecting off-normal 234U/235U ratios, or 232U that would indicate the presence of UF6 material with non-natural origins. This paper describes HEVA signatures and analysis methods, a notional UCVS design based on HEVA detector modules, and preliminary HEVA viability findings in the context of IAEAs preliminary UCVS performance targets. Unresolved technical and implementation questions, and the path forward, are also discussed.