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

Abstract 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.