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Uranium ore concentrates (UOCs), the product of uranium mining and milling, are primarily comprised of uranium oxide (U 3 O 8  and UO 2 ) or peroxide (UO 4 ·4H 2 O and UO 4 ·2H 2 O) compounds. Following production, UOCs are typically placed in storage until they are converted to uranium hexafluoride (UF 6 ) 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 [(UO 2 ) 4 O(OH) 6 ](H 2 O) 5  and/or schoepite [(UO 2 ) 4 O(OH) 6 ](H 2 O) 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 H 2 O during storage. Lastly, a drying/calcining study of hardened U 3 O 8  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 U 3 O 8  to its original condition.

Understanding uranium oxide hardening during prolonged storage