Kinetics of UO 2 (s) Dissolution in the Presence of Hypochlorite, Chlorite, and Chlorate Solutions
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Kinetics of UO2 (s) Dissolution in the Presence of Hypochlorite, Chlorite, and Chlorate Solutions Rosa Sureda1, Ignasi Casas1, Javier Giménez1, Joan de Pablo1,2 1 Dept. Chemical Engineering, Universitat Politècnica de Catalunya, Avda. Diagonal 647, 08028 Barcelona, Spain. 2 Environmental Technology Area-CTM Centre Tecnològic, Avda. Bases de Manresa 1, 08242 Manresa, Spain. ABSTRACT The influence of hypochlorite, chlorite and chlorate in the UO2 dissolution rate has been studied experimentally using a continuous flow-through reactor. Uranium concentration in each outflow solution was measured as a function of time and dissolution rates were determined once the steady-state was reached. The results obtained show that the influence of the hypochlorite anion concentration on the UO2 dissolution rate can be expressed by the following empirical equation: rdiss = 10-8.7±0.1·[ClO-]0.28±0.04 The dissolution rates obtained in this work were higher than those previously determined in presence of either oxygen or hydrogen peroxide using the same experimental methodology. In contrast, neither chlorate nor chlorite had any significant effect on the UO2 dissolution rates under the experimental conditions of this work. INTRODUCTION Different studies have been carried out concerning the UO2 or spent nuclear fuel dissolution in the presence of the main molecular oxidants produced in the radiolysis of water: O2 [1-6], and H2O2 [7-11]. However the effect of the oxidants produced in the radiolysis of groundwaters with a relatively high chloride concentration and brines has been much less studied, in spite of the fact that these ground waters (brines) are expected in saline geological environments. Radiation can alter the chemistry of the brine, producing many radiolytic products such as Cl2- and some oxychlorides [12,13], in particular ClO2-, ClO3-, and ClO-. The hypochlorite ion (ClO-) was found to be formed after the α irradiation of a NaClbrine due to the chloride ion oxidation [14,15] and the formation of ClO- as a function of initial chloride concentration and alpha activity concentration was also studied [16,17]. It has been demonstrated that the formation of ClO- in solution has an important influence on the chemistry of the actinides, for example, it was observed that the ClO- formed was able to oxidize Am(III) and Pu(IV) in solution [14]. Kim et al. [18] studied the solubility of amorphous schoepite in solutions with and without ClO-, obtaining solubility values two orders of magnitude higher in the presence of the hypochlorite ion with a concentration of 10-3 mol·dm-3. On the other hand, the formation of chlorate by gamma radiolysis from NaCl brines (6 mol·dm-3) at ambient temperature and dose rates between 0.1 and 1 kGy/h has been demonstrated by Kelm and Bohnert [19-21], ClO3- being formed proportionally to the dose. The authors could measure ClO3- concentrations between 8.1·10-5mol·dm-3 and 1.89·10-4 mol·dm-3 at 35ºC and doses between 3335 kGy (403 days) and 9094 kGy (415 days), when the temperature was increased
from 35º
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