PNNL

Abstract

International Atomic Energy Agency (IAEA) inspectors currently perform periodic inspections at uranium enrichment plants to verify UF6 cylinder enrichment declarations. Measurements are performed with handheld high-resolution sensors on a sampling of cylinders taken to be representative of the facility’s operations. As additional enrichment plans come online to support the expansion of nuclear power, reducing person-days of inspection will take on greater importance. Pacific Northwest National Laboratory (PNNL) is developing a concept to automate the verification of enrichment plant cylinders to enable 100% product-cylinder verification and potentially, mass-balance calculations on the facility as a whole (by also measuring feed and tails cylinders). The Automated Cylinder Enrichment Verification System (ACEVS) would be located at key measurement points and will positively identify each cylinder, measure its mass and enrichment, store the data along with operator inputs in a secure database, and maintain continuity of knowledge on measured cylinders until IAEA inspector arrival. Given the potential for reduced inspector presence, the operational and manpower-reduction benefits of the portal concept are clear. However, it is necessary to assess whether the cylinder portal concept can meet, or potentially improve upon, today’s U-235 enrichment assay performance. PNNL’s ACEVS concept utilizes sensors that could be operated in an unattended mode: moderated He-3 neutron detectors and large NaI(Tl) scintillators for gamma-ray spectroscopy. The medium-resolution NaI(Tl) scintillators are a sacrifice in energy resolution but do provide high collection efficiency for signatures above 1 MeV. The He-3/NaI sensor combination allows the exploitation of additional, more-penetrating signatures than those currently utilized: Neutrons produced from F-19(?,n) reactions (spawned primarily from U-234 alpha emission) and high-energy gamma rays (extending up to 10 MeV) induced by neutrons interacting in the steel cylinder. These signatures are indirect measures of U-235 that require a relatively stable U-234/U-235 ratio in the product material in order to be useful. The hypothesis of this work is that the U-234/U-235 ratio is sufficiently constant, for the specific facility where the automated system is installed, to rely on neutron and high-energy gamma-ray signatures for indirect measurement of U-235. Further, these highly penetrating signatures can be combined with a modified form of NaI-based 185-keV enrichment measurements to meet target uncertainties for the verification of product cylinders, with the additional benefits of full-volume assay of the cylinder and 100% product-cylinder verification (as opposed to today’s sampling-based approach). This paper focuses on the enrichment measurement aspects of the ACEVS concept: neutron and high-energy gamma-ray signatures, the radiation sensors designed to collect those signatures, and proof-of-principle cylinder measurements and analysis. Preliminary analysis indicates that an automated cylinder verification approach has the potential to meet target uncertainty values for 30B products cylinders (5%), assuming ore-based enrichment feed and a facility-specific calibration. Also described is the additional work needed to more definitively assess the concept’s viability, particularly through a better understanding of the U-234/U-235 ratio variability in modern enrichment plants.