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Schoepite type phases, (U0 3 · xH 2 0(s)), have been reported as corrosion products of spent fuel in long-term leaching experiments under oxidizing conditions. This paper describes the transformation of such a phase into two different phases: (a) becquerelite, CaU 6 0 19 · 11H 2 0(S), and (b) compreignacite, K 2 U 6 0 19• 11H20(S).It has been proven that the uranyl hydroxide solid transforms spontaneously into these two phases (a) and (b), at room temperature after a short contact time in the presence of Ca 2+ and/or K + ions, respectively. Analysis of the solution data indicates that equilibrium was reached and also, the predominance of the hydrolysis reaction vs. the chloride complexation of the uranyl ion at pH values higher than 4.5, even at the high chloride concentration. The solubility products of the two phases were determined in l m CaCl 2 and l m KCl solutions, respectively. The values were extrapolated to the reference state 1=0.The effect of calcium and potassium concentrations in natural waters on the formation of possible secondary phases of uranium(VI) is assessed in the light of these new data.

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In this study, the dissolution of unirradiated U0 2 (s) under oxidizing conditions (5% 0 2 /N 2 ) in 1 Μ NaCl was investigated at neutral pH. A decrease in pH and subsequent return to neutrality led to the formation of a yellow solid phase precipitated on the U0 2 (s) surface, closely resembling schoepite, U0 2 (0H) 2 • H 2 0(S), (chemically equivalent to U0 3 · 2H 2 0(s)). The uranium concentrations measured in solutions matched schoepite solubility. These solubility data have been compared with the values obtained from parallel experiments on the solubility of synthetic schoepite in the same ionic medium. Both approaches gave similar results. The morphology of the solids has also been compared by means of Scanning Electron Microscopy.The obtained solubility data have been evaluated using the computer program C-LETAGROP with the thermodynamic constants being calculated by means of the Specific Interaction Theory for the corresponding ionic medium, 1 Μ NaCl. The solubility constants, logÄT", for the reaction U0 3 · 2H 2 0(S) + 2H + ^ UOr + 3H 2 0 obtained at 1 Μ NaCl and Τ = 25 °C are: 5.92 ± 0.08 (schoepite on U0 2 (s)), and 5.57 ± 0.08 (synthetic schoepite). The extrapolated values to the infinite dilution standard state are 5.73 ± 0.28 and 5.38 ± 0.20, respectively.

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