In:
Radiochimica Acta, Walter de Gruyter GmbH, Vol. 94, No. 9-11 ( 2006-11-1), p. 567-573
Abstract:
This paper describes the effects of aqueous carbonate concentrations on the UO 2 oxidation and dissolution rates under an alpha beam irradiation in a cyclotron. As solid samples, UO 2 colloids were synthesized by nano-particle precipitation. The specific surface area obtained is 85.3 m 2 /g. All aqueous solutions were buffered in the pH range 8 〈 pH 〈 9. The observation of particles under TEM and X-ray diffraction showed well-crystallized UO 2 . Molecular radiolysis products such as H 2 O 2 and carbonate degradation products where analysed. Solution analysis of dissolved uranium and of radiolytically generated H 2 O 2 allowed to deduce dissolution rates of UO 2 as well as H 2 O 2 consumption rates (deduced from the H 2 O 2 concentrations in presence of UO 2 colloids and those in irradiated blank solutions). In absence of carbonate, the dissolution rates of UO 2 are much lower than the H 2 O 2 consumption rates. The difference is explained by the oxidation of uranium(IV) at the UO 2 /solution interface ( e.g. formation of U 3 O 7 ). In contrast, in the presence of carbonate (up to 4×10 -2 mol/L), the dissolution rates of UO 2 are similar to H 2 O 2 consumption rates. In this case the oxidized uranium is complexed with carbonate ions resulting in highly soluble species as indicated by the geochemical model PHREEQC calculations [UO 2 (CO 3 ) 3 4 - ]. The high carbonate concentration did not affect the H 2 O 2 formation/consumption rates. Radiolytic decomposition of carbonate ions and formation of organic radiolytic degradation products were negligible. The results indicate that under the present experimental conditions, UO 2 oxidation by H 2 O 2 and UO 2 dissolution are connected as a sequential reaction: for a given H 2 O 2 concentration, UO 2 oxidation occurs at a constant rate. At low carbonate concentration, the dissolution of oxidized UO 2 is slower than the oxidation rate (hence oxidation is not rate limiting), while at high carbonate concentrations all oxidized UO 2 becomes immediately dissolved and UO 2 oxidation becomes rate limiting for UO 2 dissolution. This mechanism is in agreement with that proposed in the Matrix Alteration Model (MAM).
Type of Medium:
Online Resource
ISSN:
2193-3405
,
0033-8230
DOI:
10.1524/ract.2006.94.9-11.567
Language:
English
Publisher:
Walter de Gruyter GmbH
Publication Date:
2006
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