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Nuclear forensics of uranium conversion: investigations of environmentally altered uranium compounds
Pastoor, Kevin J.
Pastoor, Kevin J.
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2021
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2023-04-14
Abstract
Uranium conversion, the chemical process used to convert uranium ore concentrates (UOCs) to uranium hexafluoride (UF6), is an essential step in the nuclear fuel cycle. All uranium destined for applications requiring isotopic enrichment must be converted to UF6. Understanding the chemistry of uranium compounds coupled to the conversion process is important to both the nuclear forensics community and nuclear industry. To address a critical knowledge gap, an emerging area of research is investigating environmentally driven changes in fuel cycle relevant uranium compounds. This dissertation focuses on UOCs and uranium tetrafluoride (UF4), important compounds coupled to the uranium conversion process.
The initial work focused on an emerging hardening phenomenon observed in UOCs stored for prolonged periods. Characterization of several free-flowing and hardened UOC samples revealed the hardened material had undergone hydration and oxidation evidenced by elevated moisture content and the presence of various uranium compounds, particularly metaschoepite [(UO2)4O(OH)6](H2O)5 and schoepite [(UO2)4O(OH)6](H2O)6, not found in the parent material. Drying and calcination was shown to be a means of remediating un-processable, hardened UOCs, and identified a compound of potential nuclear forensics interest, dehydrated schoepite (UO2)4O(OH)6. A controlled aging study of three UOC chemical forms found they were stable for up to 9 months in low to moderate (<40%) relative humidity (RH) but higher RH (>67%) drove changes in chemical speciation, primarily forming metaschoepite. Overall, this work determined the hardening phenomenon stems from a chemical transformation in UOCs wherein H2O, either liquid or vapor, is an essential reactant.
Moving forward in the uranium conversion process, the next effort investigated UF4. A controlled aging study found UF4 was stable for up to 9 months for a wide range of environmentally relevant conditions (20 and 35 °C, and ≤75% RH). However, exposure to higher RH conditions (>90%) drove changes in chemical speciation. Specifically, UF4 hydrate formed within 30 days for UF4 aged at 20 °C and 95% RH and within 180 days for UF4 aged at 35 °C and 91% RH. Investigation of the surface composition of unaged and aged UF4 samples revealed degradation of the surface consistent with the degradation observed for the bulk UF4. This work determined UF4 may persist in the environment for several months unless exposed to very high RH conditions, driving formation of UF4 hydrates.
Finally, UF4·2.5H2O, recently identified as a relevant compound for nuclear forensics, nuclear fuel cycle science, and environmental concerns, was structurally characterized using density functional theory (DFT) and neutron powder diffraction. Complete elucidation of the structure revealed an extensive hydrogen bonding network involving water-fluorine and water-water interactions. Low temperature experiments and thermal analysis showed UF4·2.5H2O is thermally stable from 10 to 358 K, undergoes dehydration at higher temperatures and is nearly dehydrated at 473 K. This work determined UF4·2.5H2O is thermally stable at environmentally relevant temperatures, indicating it may persist in the environment.
Overall, this work demonstrates the importance of understanding environmentally driven changes in relevant nuclear materials. It has shown that uranium compounds, including compounds previously considered stable, may undergo changes in chemical speciation when exposed to certain environmental conditions. These chemical speciation changes can be problematic for the nuclear industry, but the resulting compounds may be useful to the nuclear forensics and nuclear nonproliferation community as indicators of UOCs and UF4 exposed to very high RH or liquid H2O during storage. Future work should consider the role of minor impurities, additional uranium compounds, and other fuel cycle relevant nuclear materials.
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