High level nuclear waste glass corrosion in synthetic clay pore solution and retention of actinides in secondary phases

Abstract

The corrosion of the simulated high level waste glass GP WAK1 in synthetic clay pore solution was studied in batch-type experiments at 323 and 363K with special focus on the effect of high carbonate concentration in solution. The corrosion rate after 130 days was <10−4gm−2d−1 – no significant effect of the carbonate was identified. During glass corrosion, crystalline secondary phases (powellite, barite, calcite, anhydrite and clay-like Mg(Ca,Fe)-silicates) were formed. To obtain a molecular level picture of radionuclide speciation within the alteration layer, spectroscopic methods have been applied including grazing incidence X-ray absorption spectroscopy (XAS) to study the structural changes in the coordination of uranyl upon alteration layer formation. The number of equatorial oxygen atoms increases from 4 in the bulk glass to 5 in the alteration layer. Furthermore, reduced coordination symmetry was found. Hectorite, a frequently observed secondary clay mineral within the glass alteration layer, was synthesized in the presence of trivalent f-elements (e.g. Eu) and structurally characterized using time-resolved laser fluorescence spectroscopy. Structural incorporation into the octahedral layer is indicated.