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The high level nuclear waste load is likely to increase in the future containment glasses, rising new questions about their long-term chemical durability. In this study sodium borosilicate compositions with Si substituted for RE (RE = rare earth) are altered under static conditions at fixed pH (8.5). The Na and B leached fractions reached just after the rate drop increase with the RE2O3 content. Part of the RE ions remain in the altered layer which is significantly de-polymerised (IR-ATR and NMR results). Their environment is reorganized and adopts a more centrosymetrical character. The higher leached fractions are attributed to rapid hydrolysis of Si/B-O-RE bonds with respect to Si-O-Si bonds.
Zirconium is an abundant element in nuclear wastes. In this paper, we present structural and crystallization results for a simplified glass composition belonging to the SiO2-Al2O3-B2O3-Na2O-CaO-ZrO2-RE2O3 system (RE = Nd or La) developed to immobilize highly concentrated waste solutions. The effect of varying ZrO2 content on the structure and the crystallization tendency of this glass was studied using a multi-spectroscopic approach. Zr was shown to be located in six-fold coordinated sites whose charge compensation seems preferentially insured by Na+ cations. Whereas a significant decrease of the proportion of BO4 units was observed with ZrO2 content, no effect was detected on the environment of AlO4 units. However, a significant structural evolution of the silicate network occurred due to the formation of Si-O-Zr bonds. Whatever ZrO2 concentration, the crystallization of only a rare earth silicate apatite phase was observed during either slow cooling from the melt or isothermal heat treatment. Whereas nucleation mainly occurred from the surface of the glass without ZrO2, the introduction of zirconium induced apatite crystallization in the bulk. It is proposed that this nucleating effect of ZrO2 is mainly due to changes induced in the neighborhood of Nd3+ cations in glass structure.
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