OZONE PROMOTES FORMATION OF URANYL PEROXIDE IN HUMID CONDITIONS: IMPLICATIONS FOR URANIUM ALTERATION CHEMISTRY (Invited Presentation)

Studtite and metastudtite are the only known peroxide-bearing minerals. Formation of these uranyl peroxide minerals is normally attributed buildup of peroxide produced by the alpha radiolysis of water over numerous years. Recent reports of studtite formation associated with a depleted uranium projectile and in laboratory studies of the uranyl hydroxide mineral metaschoepite cannot be attributed to the effects of alpha radiolysis as the radioactivity in those systems is too low and the time is too short. Based on an extensive series of experiments and detailed material characterization, we propose that ozone, a trace atmospheric constituent, is important for uranyl peroxide formation in high relative humidity conditions. Ozone has recently been observed to form hydrogen peroxide in water microdroplets, prompting our investigation of formation of studtite by alteration of an existing uranyl hydroxide phase stored at elevated humidity. In this work, the uranyl hydroxide phase metaschoepite was exposed to various controlled storage conditions. Analysis by Raman and infrared spectroscopy and powder X-ray diffraction confirm that uranyl peroxide forms when metaschoepite is housed at high relative humidity for a defined period of time. The alteration rate is greatly increased when additional ozone is introduced into the vessel. These results suggest the importance of ozone in the alteration of stored used nuclear fuel as well as in nuclear forensics applications, as atmospheric ozone levels will vary with location and increase with warming temperatures.