Changes to Alkali Ion Content Adjacent to Crystal-Glass Interfaces
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Changes to Alkali Ion Content Adjacent to Crystal-Glass Interfaces Michael J. D. Rushton1, Robin W. Grimes1 and Scott L. Owens2 1 Department of Materials, Imperial College London, London SW7 2AZ, UK 2 Nexia Solutions Ltd., Warrington, Cheshire WA3 6AS, UK ABSTRACT Atomic scale molecular dynamics simulations have been used to predict the location of glass modifying Na, Li and Mg species in a borosilicate Magnox type waste glass adjacent to interfaces with the (100) and (110) surfaces of MgO, CaO and SrO crystals. These simulations show a considerable increase in alkali and alkali earth concentration adjacent to specific interfaces. In particular, there are significant, systematic changes in Na, Li and Mg position and concentration as a function of both the crystal’s terminating surface and composition. INTRODUCTION Despite our best efforts, glass-crystal interfaces occur within “vitrified” nuclear waste [1]. These arise as a result of a lack of reactivity of refractory material in the process feed, or due to crystallisation of complex phases from the vitrification melt. Vitrification of high activity waste from nuclear processes is a desirable treatment route, as glass is a suitable host for high volume fractions of waste of widely varying composition. However these widely varying compositions lead to a greater likelihood of introducing insoluble phases into the glass melt. A better understanding of the structures and processes that occur at the interfaces between these insoluble fractions and the bulk glass during high-temperature processing would greatly aid our ability to optimize feed compositions to the vitrification process and mitigate against any detrimental effects that having a glass-crystal interface might have on, for example, transport of radionuclides. However, due to the multi-element compositions involved and the complexity of their disorder, the atomic structures of glass-crystal interfaces have not been well characterised experimentally nor have they been well modelled. In particular, variations in the composition of a mixed alkali glass, towards the interface, have only been reported in one modelling study [2, 3] and the influence of other additions (such as Mg) have never been described. This study also forms a precursor to examining the interactions between other, more complex crystalline products and the bulk glass melts. The aim of this work is therefore to understand glass ceramic interfaces in the context of vitrified nuclear wasteforms. For this reason, the glass compositions examined are related to the composition used by British Nuclear Fuels Ltd. (BNFL) for vitrification of wastes resulting from reprocessing Magnox fuel. The basic glass composition used during the vitrification of Magnox waste (MW) [4] is given in table 1. It is a borosilicate composition which contains essentially equal amounts (in atomic percent) of sodium and lithium as glass structure modifiers. Magnox fuel assemblies are clad in an alloy of aluminium and magnesium; although efforts are made to remove this cladding du
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