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 °C and pH ∼ 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 CeIII and CeIV were obtained by recording the Lα fluorescence emission at two different incident X-ray energies, coinciding with the maximum contrast between CeIII and CeIV fluorescence intensities. The micro-XRF maps revealed that Ce was dominantly present as oxidized species (CeIV) in the original glass. After dissolution from the glass matrix, CeIV was partly reduced and re-immobilized as CeIII at grain boundaries or in the interstitial spaces between the glass particles. The concentration of CeIII 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 CeIV was probably mediated by an external source of reductants, such as Fe(0) from the steel reaction vessel. results, in conjunction with an Eh–pH stability diagram of the CeIII/IV–O–H–CIV–P–SVI–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 PuIV to PuIII 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.