High pressure experimental study on iodine solution mechanisms in nuclear waste glasses

Abstract

The 129I is a long-lived isotope, radiotoxic and highly mobile by-product of fission reactors. Its disposal in geological repositories requires matrix at least as durable as current nuclear waste glasses. The volatility of iodine at high temperature is a concern for nuclear waste glass processing, because its evaporation prior to vitrification, results in a low retention rate in the final glass. The present study investigates the use of pressure for immobilizing iodine during vitrification. For three different glass compositions: International Simple Glass (ISG), NH (a depolymerized glass similar to Low Activity Waste glasses, LAW) and boron-free trachytic glass; we conducted the experiments between 0.5 and 2.0 GPa for pressure and 1300–1500 °C for temperature. We determined the iodine solubility in glass compositions representative of the simplified nuclear waste glass simulants, and the influence of iodine on the structure of theses glasses.In comparison to ambient atmosphere experiments, increasing pressure (up to 1.5 GPa) induces an increase in the iodine concentration in ISG and NH glass compositions at ∼1 and ∼2 mol.%, respectively; whereas the iodine concentration remains almost constant for boron-free trachytic glass composition suggesting the influence of boron in the iodine solution mechanism. The glass structure investigated via 23Na NMR showed that sodium environment is modified by the presence of iodine in its surroundings. 11B NMR highlighted the structural changes induced by iodine: polymerized glasses (ISG) tend to depolymerize with iodine dissolution, depolymerized glasses (NH) tend to polymerize with iodine dissolution. Our high-pressure data suggest that both I− and IO3− are the solubilized forms of iodine with a preferential affinity to sodium atoms within the glass structure; and comparable to previous studies conducted at ambient pressure.