A micro-XAS/XRF and thermodynamic study of CeIII/IV speciation after long-term aqueous alteration of simulated nuclear waste glass: Relevance for predicting Pu behavior?

In the present study, the dissolution and mobilization of Ce introduced in a simulated nuclear waste glass (MW) as a surrogate of Pu was investigated after leaching in pure water over 12 a at 90 degrees C and pH similar to 9.6. The microscopic distribution and oxidation state of Ce in the altered glass were studied using micro-X-ray fluorescence (micro-XRF) mapping techniques and micro-X-ray near-edge absorption spectroscopy (micro-XANES). Distribution maps of Ce-III and Ce-IV were obtained by recording the La fluorescence emission at two different incident X-ray energies, coinciding with the maximum contrast between Ce-III and Ce-IV fluorescence intensities. The micro-XRF maps revealed that Ce was dominantly present as oxidized species (Ce-IV) in the original glass. After dissolution from the glass matrix, Ce-IV was partly reduced and reimmobilized as Ce-III at grain boundaries or in the interstitial spaces between the glass particles. The concentration of Ce-III was found to correlate with the spatial distribution of secondary Mg-clay formed during the aqueous corrosion as the main glass alteration product. Micro-XANES spectra collected at locations representative of both altered and non-altered glass domains confirmed the findings obtained by the redox mapping. Because redox-sensitive elements in the pristine MW glass (Fe, Cr, Se) occur almost exclusively as oxidized species, reduction of Ce-IV was probably mediated by an external source of reductants, such as Fe(0) from the steel reaction vessel. These results, in conjunction with an Eh-pH stability diagram of the Ce-III/IV-O-H-C-IV-P-S-VI-Na-Cl system, indicate that the glass was leached at relatively low oxidation potentials (Eh < 0.2 V). However, the comparison with an equivalent Eh-pH diagram for Pu revealed that at comparable pH and water composition the reduction of (PuV)-V-I to Pu-III would require considerably more reducing conditions (Eh < -0.3 V). It is, therefore, concluded that Ce is not a good chemical analogue of Pu, in spite of its wide use as a surrogate in simulated radioactive waste. (C) 2011 Elsevier Ltd. All rights reserved.