A Study of Magnox Waste Glass Under Conditions of High Temperature, Very Deep, Geological Disposal
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$6WXG\RI0DJQR[:DVWH*ODVV8QGHU&RQGLWLRQVRI +LJK7HPSHUDWXUH9HU\'HHS*HRORJLFDO'LVSRVDO Neil C. Hyatt,* Kathleen J. Taylor, Fergus G.F. Gibb and William E. Lee. Immobilisation Science Laboratory, Department of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield, S1 3JD. UK. $%675$&7 The behaviour of a simulant Magnox waste glass was investigated under the likely conditions of high temperature, very deep, geological disposal, YL]. 760oC and 0.15GPa. Under these conditions partial crystallisation of the glass is observed with the formation of LnBSiO5 (Ln = Y, La, Pr, Ce, Nd, Sm and Gd), LiNaZrSi6O15 and a palladium (ruthenium) telluride phase. The significance of these findings is discussed in the context of the high temperature, very deep, geological disposal scheme. ,1752'8&7,21 High temperature, very deep, geological disposal is a realistic alternative to the conventional mined repository concept for the disposal of High Level nuclear Waste (HLW) [1, 2]. This scheme uses large diameter boreholes drilled into suitable rock (e.g. granite) to depths in excess of 4km. HLW packages are lowered into the borehole which is then backfilled with host rock and sealed. The composition and quantity of HLW in each package are carefully calculated to provide sufficient energy to heat the waste package and the surrounding rock to 750 – 900oC at the required depth. This results in partial melting of the host rock which, subsequently, recrystallises as the thermal output of the HLW material diminishes over time, sealing the waste package in an envelope of durable mineral phases. Any fractures in the rock surrounding the HLW packages are sealed by recrystallisation, annealing or hydration reactions, preventing the access of hydrous fluids to the HLW. Furthermore, at depths of several kilometres, hydrous fluids are physically and chemically isolated from near surface groundwaters, effectively inhibiting transport of radionuclides to the biosphere. This scheme therefore maximises the geological barrier between the HLW material and the biosphere, in contrast to the mined repository concept, which, by comparison, is geologically shallow. Since the geological barrier is the only barrier which can be assured over the timescale for HLW disposal (>104 - 106 years) the high temperature, very deep, disposal scheme presents an inherently safer alternative to conventional repository disposal. The heat output of the HLW packages may be optimised in a number of ways, including codisposal of spent nuclear fuel and vitrified high level waste material [1, 2]. In order to assess the suitability of the very deep, high temperature, disposal strategy for such a co-disposal scenario it is necessary to examine the behaviour of high level waste glasses and spent fuel material under such conditions. Accordingly, we have investigated the behaviour of a simulant Magnox high level waste glass under likely conditions of high temperature, very deep, geological disposal.
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Heat treatment of Magnox waste glass, at a temp
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