Incorporation and phase separation of Cl in alkaline earth aluminosilicate glasses

Pyrochemical reprocessing of spent nuclear fuels may lead to the generation of chloride containing wastes. ³⁶Cl wastes may also arise from the treatment of irradiated graphite. Such wastes will have limited solubility in the borosilicates currently used for waste vitrification. Despite requiring higher processing temperatures aluminosilicate glasses show promise for this application. In a series of alkaline earth aluminosilicate glasses we demonstrate that chloride solubility is related to the alkaline earth species as follows Sr > Sr + Ba > Ba > Ca > Mg, with the strontium aluminosilicate glass accommodating up to 5.92 at% Cl. Typical chloride retention rates are ∼80% of the batched chloride content at 1400 °C. It has also been observed that, when Cl is present in the glass in excess, phase separation firstly occurs as formation of non-Cl crystals (mainly alkaline earth aluminosilicates, with a minority of aluminates); a segregated chloride layer is only formed at higher chlorine loadings. This indicates that chlorine solubility in glass is not only controlled by the capacity of glass network to accommodate Cl⁻ but also by the stability of glass network after Cl⁻ incorporation. In addition, increased incorporation of Cl⁻ in glass results in steadily decreased glass densities and glass transition temperatures.