Understanding Emissions from the Thermal Treatment of Intermediate Level Radioactive Waste

Practical applications of vitrification and usage of glass matrices has highlighted the importance of understanding these materials to a higher standard. This includes prioritising closing gaps in the past research to make the process safer and more effective for the future. One of the clear areas is the loss of radioactive/volatile components of waste upon heat treatment or vitrification. It is widely known that caesium, iodine and chlorine are all lost to some degree through volatilisation mechanisms at elevated temperatures and therefore understanding the extent of these processes is key to maintaining a total inventory of radioactive material on nuclear licenced sites. During this study we have found that iodine loaded into glass compositions: Mixture Windscale (MW), calcium-zinc (CaZn) and a silica-free version of CaZn, is the most volatile (100% loss at 1200 °C) of the three elements investigated, however, all lost mass upon heat treatment. The volatilisation can be reduced for iodine and caesium by introducing common wastes to the system. Clinoptilolite and Corroded Magnox Sludge (CMS) were studied and the loss of atomic caesium was found to show a 15-21 wt% improvement in retention in both MW and CaZn glass when loaded with up to 30 wt% clinoptilolite, the effect being less pronounced for CMS. Iodine retention also improved, a maximum of 16 - 33 wt% iodine was found in base glasses upon 40 wt% clinoptilolite, CMS again having less of an effect on the retention at the same waste loading (4.5 wt%), theorised to be due to the binding of caesium or iodine to the waste before melting. Atmospheric changes were investigated changing from air to nitrogen which had a positive effect on retention of caesium (+ 8 wt%) and significantly so for iodine (+74 wt%). This is a demonstration of the importance of oxygen in these systems, the two sources of oxygen are from the glass frit/waste and from the atmosphere with the atmospheric oxygen being readily available and able to react before the glass frit melts increasing the proportion of gaseous iodine and to some extent caesium.