GREEN CDT Solidification Routes for Encapsulation of Radioiodine

In the UK, over 150,000m3 of radioactive waste has been produced to date. Most of this radioactive waste needs conditioning by solidification into a stable wasteform, most commonly by conversion to a ceramic wasteform, vitrification to form a stable glass, or encapsulating it in cement to prevent release to the biosphere. Many waste streams, however, are not compatible with the above conditioning processes. Radioiodine is particularly problematic, as it is present in dissolver off-gas and aqueous waste streams in various chemical forms of iodine, including those that easily volatilise. Recent work has shown significant potential for immobilisation of radioiodine and solid adsorbents by encapsulation in cement. Cementation of radioactive waste is easier, cheaper, and faster than other radioiodine solidification processes such as low-temperature glass sintering and hot isostatic pressing, and avoids issues regarding volatilisation of iodine, or incompatibility with iodine-loaded adsorbents. However, immobilisation behaviour in cements differs significantly from that in other wasteforms (e.g. ceramics, glasses), due to extensive differences between aqueous and solid-state chemistry, and processing routes in each case. There is little information on what parameters are critical to reliable application. This PhD explores solidification routes for encapsulation of radioiodine-loaded solid adsorbents, examining radionuclide-mineral interactions in cement encapsulants, benchmarked against more common wasteforms produced via thermal treatment. It adopts an in-situ characterisation approach (including surface-specific techniques, spectroscopic and microstructural characterisation) to investigate mechanisms and kinetics of radionuclide-mineral interactions, and effects on wasteform performance. Through this we will discover the fundamental processes controlling immobilisation of radioiodine, and design, produce and test novel wasteform formulations with enhanced performance.