THE ROLE OF PE, PH, AND CARBONATE ON THE SOLUBILITY OF UO2 AND URANINITE UNDER NOMINALLY REDUCING CONDITIONS

Experimental data obtained from uranium dioxide solubility studies as a function of pH and under nominally reducing conditions in a 0.008 mo l dm(-3) perchlorate medium and in a 1 mol dm-3 chloride solution are presented. The solubility of extensively characterized uraninite sampl es from Cigar Lake (Canada), Jachymov (Czech Republic), and Oklo (Gabo n) was determined in a solution matching the composition of a groundwa ter associated with granitic terrain. The redox potential of the test solution was monitored throughout the experimental period. The results obtained were modeled using aqueous formation constants compiled by t he NEA, using stability constants corrected to appropriate ionic stren gths. The solubility curves have been adjusted by calculating the valu e of K-s4 (UO2(s) + 2H(2)O double left right arrow U(OH)(4(aq))) that gave the best fit with the experimental data. For a low temperature sy nthetic UO2, a value of logK(s4) of -7.3 was determined, while for ura ninites the best fit was obtained with a value of logK(s4) of -8.5. A wide range of published UO2 solubilities can be reproduced by the avai lable database, where experimental conditions were adequately defined in the original experiments. A lower value of the solubility product o f the uranium dioxide phase defined as fuel in the SKB uranium databas e provides reasonable solubilities for a wide span of experimental res ults at near to neutral pH. Based on the modeling and using the beta(1 ,4) for the U(IV)-OH complexation given by Grenthe et al. (1992a), a l ogK(s0) (UO2(s) + 4H(+) double left right arrow U4+ + 2H(2)O) value of -2.3 +/- 0.2 is proposed. Differences in solubility between natural a nd synthetic samples are attributed to the presence of carbonate in th e experiments performed with uraninites, while differences in solubili ty observed among the natural samples can be correlated to radiation e ffects at atomic scale. Copyright (C) 1998 Elsevier Science Ltd.