Mechanism of Unirradiated UO₂ (S) Dissolution in Nacl and Mgcl₂ Brines at 25°C

Abstract

The dissolution of unirradiated U0 2 (s) has been studied in NaCI and MgCI 2 brines under both reducing and oxidizing conditions.The initial uranium release under reducing conditions has been attributed to the dissolution of an initial oxidized layer. The final uranium concentrations have been modeled by using the PHRQPITZ computer program giving the solubility of the solid phase U0 2 (s).Under oxidizing conditions, the initial release is the sum of the oxidized layer dissolution and the oxidation/dissolution of the U0 2 . The release rates calculated are 1.4-10-5 mol d-1 m-2 in NaCI-brine and 3.6 -10-5 mol d-1 m-2 in MgCI 2 -brine. After the initial release, uranium concentration in the NaCI-brine reaches a constant value, which has been attributed to the formation of a secondary solid phase. In MgCI 2 -brine, the uranium concentration increases slowly indicating, in this case, no control by secondary phase formation.

Conceptual and Mathematical Model for the UO2(s) Dissolution in Brines Under Different Redox Conditions

“…6.0x10" 4.0x10" 6 In the experiments carried out in both reducing and oxidizing conditions, there is an initial increase of the uranium concentration in solution with time. This increase has been attributed to the dissolution of an oxidized layer placed initially on the U0 2 [6] which will be formed even if care is taken during the experimental procedure to keep an oxygen free atmosphere around the solid surface [9,10]. The existence of this oxidized layer on the surface of the solid used in our experiments was proved by means of X-ray photoelectron spectroscopy determinations of the solid surface.…”

“…This decrease is due to the precipitation of U0 2 which it is the thermodynamic stable solid phase at these conditions and the uranium concentration in solution exceeds its solubility. Using the PHRQPITZ code [11], the U0 2 solubility has been calculated to be 3 · 10~7 mol/kg [6] in both brines because the reaction taking place at the experimental pH of both brines during this precipitation step is the following [6]: U(OH) 4 (aq) ο U0 2 (s) + 2 H 2 0 (4) Equation (3) has been fitted to the experimental data. The results obtained are shown in Table 3, and the best fitting of the model to the experimental data can be seen in Fig.…”

“…In previous works, we studied the effect of both reducing and oxidizing conditions on the dissolution of unirradiated uranium dioxide in MgCl 2 and NaCl brines [5,6]. These studies led to the conclusion that under reducing conditions there are two different connected processes that affected the uranium concentration in solution, the dissolution of an oxidized layer (UO 2+x) present initially on the U0 2 surface (as determined by means of X-Ray Photoelectron Spectroscopy measurements), and a subsequent precipitation of U0 2 because the total uranium concentration in solution exceeds the solubility of this phase at the experimental conditions.…”