Dissolution of UK Vitrified High-Level Radioactive Waste Containing Zinc and Calcium

In the UK, vitrified High-Level-Waste (HLW) from the reprocessing of spent nuclear fuel is destined for disposal in a deep geological facility. A new zinc and calcium modified alkali-alumina-borosilicate base glass has been implemented at the Sellafield Ltd. site in anticipation for the immobilisation of post-operational clean-out waste streams alongside standard reprocessed fuel waste. At present, the dissolution behaviour and underlying corrosion mechanisms of zinc and calcium containing HLW glass have not been fully explored, particularly in conditions relevant to geological disposal, which is of importance to assist the development of the disposal safety case. This body of work comprises fundamental studies to underpin the impact of zinc and calcium addition to the alkali-alumina-borosilicate base glass by assessing the structure-durability relationship. Studies were also developed to investigate the longer-term durability of waste glasses in conditions representative of geological disposal in the UK, incorporating potential groundwater solutions and an alternate corrosion scenario, demonstrated by the introduction of the Cyclic-Vapour-Static (CVS) methodology. The CVS provides, for the first time, a constrained integrated vapour-static groundwater dissolution test that aims to capture the anticipated initial vapour/glass interaction (altering the glass surface) and subsequent groundwater contact in a repository setting. Such a methodology can advance product consistency tests and further inform the safety case for HLW glass formulations destined for geological disposal. Post dissolution characterisation of glass alteration layers from tests were conducted by nuclear magnetic resonance spectroscopy, scanning electron microscopy and X-ray diffraction to identify the role of zinc and calcium on the alteration layer formation, results which are supported by geochemical modelling. Irrespective of the experimental methodology and compositional complexity of the glasses studied, this work demonstrated the detrimental and positive impact of zinc and calcium additions, respectively, on the chemical durability at later stages of accelerated glass corrosion, the major result being that analogous Stage III ‘rate resumption’ behaviour was observed for all zinc containing glasses, linked to the precipitation of Zn-phyllosilicate clays.