Characterization of Uranium Particles Produced by Hydrolysis of UF₆Using SEM and SIMS

Abstract

Environmental sampling (ES) is a powerful technique used by safeguards inspectors of the International Atomic Energy Agency and the European Safeguards Office for the detection of undeclared nuclear activities. Since its implementation in the 1990s, ES has proven to be very sensitive and effective. Considering the consequences, the measurements should be carried out under a quality management programme. At the Institute for Reference Materials and Measurements, a new production method is under development for the preparation of reference uranium particles from well-certified UF6, allowing uranium particles with certified isotopic abundances to be prepared that are representative of those found in uranium enrichment facilities. Using an aerosol deposition chamber designed and built for the purpose, particles are formed by the hydrolysis of UF6 and their morphology and (isotopic) composition measured using SEM-EDX and SIMS. The SEM measurements show that by varying the relative humidity of the air in the reaction chamber, the morphology of the particles can be changed. By making a distribution map of the chemical composition of the particles, the relationship between fluorine and uranium as main constituents of the particle could be established. The presence of fluorine is a valuable indicator for the occurrence of nondeclared enrichment activities.

Transition State for the Gas-Phase Reaction of Uranium Hexafluoride with Water

“…4 In that work, researchers found only UO 2 F 2 product for UF 6 these two values and no reaction mechanisms were proposed. Additionally, several groups have studied the particle morphology [5][6][7][8][9][10] and reactor design for the production of particles from aerosols of UF 6 . 11…”

Morphologic and chemical characterization of products from hydrolysis of UF6

“…A glass or metal sample holder with UF 6 was pressurized with heat, allowing the UF 6 to be released as a liquid. Analyses of the products formed indicated that aggregate particle morphologies and size distributions of the settled products were dependent upon relative humidity, temperature of the UF 6 , and time (hours) [14,15]. In this latter study, UF 6 was contained in glass that was mechanically ruptured, allowing the compound to hydrolyze with air in a surrounding enclosure.…”

“…SEM-EDS spectra indicated that the major elemental constituents were uranium, oxygen and fluorine (Supporting Information Figure S In contrast, Pickrell observed formation of ~ 10 μm agglomerates made of discrete spheroids on the order of 0.1 μm in diameter following release of liquid UF 6 to excess water, and suggested that agglomeration of particles over time was likely due to charging [11,12]. Kips reported an increase in the average particle diameter with increased relative humidity [15]. Because the morphologic shapes of the materials deposited onto a carbon conductive surface were consistent with these prior experimental measurements and analyses, we chose not to store these samples under controlled conditions.…”

Uranium aerosols at a nuclear fuel fabrication plant: Characterization using scanning electron microscopy and energy dispersive X-ray spectroscopy

“…The spherical shape and size strongly indicates UO 2 F 2 . Several studies have shown that formation of UO 2 F 2 from UF 6 in a humid environment results in spherical shapes of about ≅0.2-2.0 μm diameter [33][34][35]. The presence of nitrogen is slightly surprising, but could be explained by attachment of nitrogen-rich materials present in the atmosphere.…”