Abstract
ABSTRACTThe solubility of a crystalline unirradiated UO2(s) has been studied under reducing conditions at 25°C in three different ionic media: 0.008 mol dm–3NaCIO4, 1 and 5 mol dm-3NaCl+. The species responsible for the solubility in perchlorate medium are U(OH)3and U(OH)4, with the stability constants log β13= -0.4(±0.2) and log β14= -5.7(±0.1), respectively. The solubility in both1! and 5 M chloride media is explained at pH values higher than 6 by the species U(OH)4with log β14= -5.5(±0.2) while at lower pH values an abnormal behaviour is observed, with higher uranium concentrations in solution and a proton independent solubility for pH values lower than 4. The XPS observations of the reacted solids do not show the presence of solid surface phases other than uranium dioxide, with an upper oxidation state of UO2.1.
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