Chemical durability evaluation of silver phosphate–based glasses designed for the conditioning of radioactive iodine

Highlights: • Iodine-containing phosphate-based glasses are compared in terms of durability. • Phosho-molybdate glasses are more durable than phosphate glasses in pure water. • Nb₂O₅ improves the chemical durability of phospho-molybdate glasses. • Acidic conditions seem detrimental to the durability of these conditioning matrices. Silver phosphate glasses have been investigated for the conditioning of radioactive iodine 129. Their disposal in a deep geological repository requires high chemical durability, and to improve this property, two crosslinking agents were examined here: Nb₂O₅ and Bi₂O₃. In addition, for some of these glasses, 50 % of phosphorus was replaced by molybdenum to assess the role of glass former entities. The effect of these modifications on the glass chemical durability was studied in pure water, at 50°C. Given the very long half-life of iodine 129 (15.7 × 10⁶ years), only long-term alteration tests were carried out, in order to check if a passivating alteration layer could be formed in these conditions. Alteration rates were then calculated following various methods and compared. Results showed that the use of elemental releases in solution alone leads to an underestimation of the alteration rate, as there is no tracer element in the glass composition. For phospho-molybdate glasses, the addition of Nb₂O₅ had a significant effect on their chemical durability, i.e. their ability to form a passivating layer that retains most of the elements that are part of the pristine glass. For phosphorus, the more mobile element, release rates decreased by two orders of magnitude compared to phosphate-only glasses, and this trend depends on the Nb₂O₅ concentration.