Stability of rare earth di-silicates REE2SI2O7 (REE = Sc, Y, Lu) under simulated degraded solutions caused by canister corrosion products

Document available in extended abstract form only. The Deep Geological Repository (DGR) concept has been proposed for disposal of the high-level radioactive waste (HLRW). This model is supported by many scientific and technical studies that indicate a high degree of safety. However, many efforts are still required to establish the long term effectiveness of the barrier system. Therefore, it is necessary to establish the different factors that can alter the physicochemical properties of multi-barrier system and consequently the stability and effectiveness of the model. The geochemical changes that may occur at long-term under the conditions of the deep geological repositories are linked to the groundwater, temperature gradient, interactions between backfill materials and the corrosion products of the steel canister or the aging of the backfill materials (typically bentonites). It's known that the corrosion process of the metallic canister produces new phases of iron oxides (mainly the magnetite phase) and hydrogen gas. The interaction between corrosion products and bentonite, the clay could undergo permanent changes in its mineralogical composition and structure. Those changes diminish drastically their swelling and cation exchange capacities, which are the main mechanism of HLRW immobilization. However, a chemical interactions between the clay minerals (backfill materials) and rare earth cations, REE3+, (analogues for the most important long lived trivalent radionuclides)3 has been observed when they are treated under subcritical conditions (similar conditions in deep geological repositories after long-term). In this reaction the main products are the crystalline phases of di-silicates REE2Si2O7 which are stable over a wide temperature and pH range. Although many studies have been dedicated to analyze the stability range of clay barrier during the canister corrosion, no studies have been carried out to establish the stability of those secondary phases, REE2Si2O7, responsible at long-term of the HLRW immobilization. Therefore, this communication aim is the study of those Rare Earth Di-silicates under simulated canister corrosion products and subcritical conditions. To achieve this objective, a series of the rare-earth di-silicates REE2Si2O7 (Sc, Y, Lu) were synthesized by sol-gel method.5 All products were obtained in a high degree of purity β-Sc2Si2O7 in 82.75%, β-Lu2Si2O7 in 94.10% and β-Y2Si2O7 in 80.97%. The powder mixtures Fe/REE2Si2O7 (0.3 and 1 w/w) were dispersed in 40 mL of distilled water and placed in a stainless steel reactor. This system was closed and heated at 300 deg. C for 48 h. The metallic iron was provided by Sigma-Aldrich, 99%. After treatment, the solid and liquid samples were recovered by filtration. Solid samples were analyzed with Xray diffraction (XRD) and scanning electron microscopy (SEM-EDX). The Rietveld method was used for XRD quantitative phase analysis of the mixtures. The pH, the conductivity and redox potential of the supernatants were measurements using a pH-meter PC 700 (EUTECH Instruments). Similar free-iron treatments of the di-silicates were performed and used as blank samples. The blank experiments show that the hydrothermal treatments do not affect the crystallinity of the β- REE2Si2O7. Likewise, the X-ray analysis of products after hydrothermal treatment indicates that the structure of the phases of di-silicates in the presence of iron has a high crystallinity. Thus, the composition of the phase β-Sc2Si2O7 in the products were of 83.87% and 82.71% for in mixtures with a weight fraction of iron of 0.3 and 1 respectively. The composition of the phase β-Lu2Si2O7 were 70.14% and 63.33% and the composition of the phase β-Y2Si2O7 were 86.45% and 70.95%. Additionally, the hydrothermal treatments in the metallic iron medium provoke the oxidation of iron and iron oxides are generated. The iron oxide phase generated were hematite (Fe2O3), magnetite (Fe3O4) goethite FeO(OH). All experiments generate fayalite (Fe2SiO4) as a result of chemical interaction of the both components (Fe/REE2Si2O7) under hydrothermal conditions and consequently can be expected a decreasing of the di-silicate phase, since the phase formation of Fe2SiO4 requires a source of silicon. Those results demonstrate that the REE3+ cations (analogues for the HLRW) are effectively immobilized into Rare Earth di-silicates which are chemical stable even in presence of corrosion products of the canister. (authors)