The role of alkalis on the incorporation of iodine in simple borosilicate glasses

International audienceThe purpose of this study was to identify incorporation mechanisms and saturation levels of iodine in borosilicate glasses that are simplified versions of compositions used for nuclear waste containment. Several series of glasses with different alkali contents (22 or 35 mol% Na$_2$O, or 22 mol% of a mixture of Na$_2$O + Li$_2$O, K$_2$O or Cs$_2$O) were loaded with iodine (from 1 000 to 10 000 ppm at.) at 1100 °C in a specially-designed closed system. When the incorporation limit of iodine was reached, alkali iodide crystals were observed (e.g. NaI, KI…). Crystal-free pools of glass were studied by Electron Probe Micro-Analysis (EPMA) to measure the incorporation limit of iodine, which ranges from 892 ppm at. for a cesium–bearing glass to 7260 ppm at. for the glass with 35 mol% Na$_2$O. The composition of the crystals was analyzed by Inductively Coupled Plasma spectroscopy (ICP) and it was systematically found that crystals are enriched in the heavy alkali compared to the bulk composition. Correlations exist between the saturation level and the composition of the crystals. It is concluded that high alkali content in general and high concentration of Na$_2$O in particular is favorable for iodine incorporation