SOLUBILITY EQUILIBRIA IN THE U(VI)-CA-K-CL-H2O SYSTEM - TRANSFORMATION OF SCHOEPITE INTO BECQUERELITE AND COMPREIGNACITE

Schoepite type phases, (UO3.xH(2)O(s)), have been reported as corrosio n products of spent fuel in long-term leaching experiments under oxidi zing conditions. This paper describes the transformation of such a pha se into two different phases: (a) becquerelite, CaU6O19.11H(2)O(s), an d (b) compreignacite, K2U6O19.11H(2)O(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 pre sence of Ca2+ and/or K+ ions, respectively. Analysis of the solution d ata 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 concentra tion. The solubility products of the two phases were determined in 1 m CaCl2 and 1 m KCl solutions, respectively. The values were extrapolat ed to the reference state I = 0. The effect of calcium and potassium c oncentrations in natural waters on the formation of possible secondary phases of uranium(VI) is assessed in the light of these new data.