Understanding uranium oxide hardening during prolonged storage

Abstract

Uranium ore concentrates (UOCs), the product of uranium mining and milling, are primarily comprised of uranium oxide (U3O8 and UO2) or peroxide (UO4·4H2O and UO4·2H2O) compounds. Following production, UOCs are typically placed in storage until they are converted to uranium hexafluoride (UF6) at a uranium conversion facility. In this study, the chemical changes responsible for an interesting hardening phenomenon observed in UOCs stored for prolonged periods was investigated to understand underlying causes. Powder X-ray diffraction and thermogravimetric analysis were used to characterize free-flowing and hardened UOC samples and revealed the hardened material had undergone hydration and oxidation as indicated by increased moisture content and the presence of metaschoepite [(UO2)4O(OH)6](H2O)5 and/or schoepite [(UO2)4O(OH)6](H2O)6. Additionally, an aging study found metaschoepite in UOCs after 3 months exposure to a high relative humidity environment. The same study found agglomerated, but not fully hardened, material in nearly all aged UOCs samples. These results suggest metaschoepite and schoepite are indicative of UOCs exposed to elevated levels of H2O during storage. Lastly, a drying/calcining study of hardened U3O8 material demonstrated a means of remediation and identified an intermediate compound of potential interest, dehydrated schoepite. Dehydrated schoepite results from heating metaschoepite or schoepite between 100 and 300 °C and indicates partial reversal of hardened U3O8 to its original condition.

Effect of Diel Cycling Temperature, Relative Humidity, and Synthetic Route on the Surface Morphology and Hydrolysis of α-U3O8

“…The provenance of nuclear material can be elucidated by signatures from industrial processes such as phase purity, synthetic route, precipitation conditions, thermal history, and the rate of oxidation. − Of additional interest in determining material origin are signatures from temporal processes resulting from changes in chemical speciation due to environmental conditions. Previous U-oxide aging studies have demonstrated the utility of morphologic signatures, quantitative crystallography, thermogravimetric analysis, spectroscopic techniques, isotopic ratios, and predictive modeling in determining material speciation as a result of storage conditions. − Each of these studies confirmed the formation of uranyl hydrate phases due to U-oxide aging.…”

Structural Characterization of Uranium Tetrafluoride Hydrate (UF4·2.5H2O)

“…Improving understanding of the chemical and physical characteristics of nuclear materials remains an important goal in nuclear nonproliferation, nuclear forensics, and nuclear fuel cycle science. − Within these domains, recent focus has been given to characterizing environmentally relevant changes in nuclear materials caused by exposure to atmospheric water vapor, UV light, or precipitation. Recent efforts investigated several fuel cycle-relevant uranium compounds and identified changes in chemical speciation and morphology when these compounds are exposed to various environmental conditions. − Concerning the nuclear fuel cycle, environmentally driven alterations can affect the processability of uranium materials. For nuclear forensics, these chemical and physical changes could be used as an indicator of exposure to particular environmental conditions.…”

“…For nuclear forensics, these chemical and physical changes could be used as an indicator of exposure to particular environmental conditions. Overall, exposure to liquid water or atmospheric water vapor has been repeatedly observed to play a critical role in driving chemical speciation changes in fuel cycle-relevant uranium compounds. − …”