Vapour Phase Hydration of Magnox Waste Glass
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Vapour Phase Hydration of Magnox Waste Glass Neil C. Hyatt,1* William E. Lee,1 Russell J. Hand,1 Michael I. Ojovan,1 Paul K. Abraitis2 and Charlie R. Scales2. 1
Immobilisation Science Laboratory, Department of Engineering Materials, The University of Sheffield, Mappin Street, Sheffield, S1 3JD. UK.
2
BNFL Plc., Sellafield, Seascale, Cumbria, CA20 1PG. UK.
ABSTRACT Vapour phase hydration studies of simulant Magnox waste glass have been undertaken at 200oC, over periods of 5 – 25 days. Electron microscopy studies reveal a thin uniform hydration layer (~10 µm thick) on 5, 10 and 25 day specimens. The formation of isolated surface alteration products occurs between 5 and 10 days at 200oC. The formation of extensive surface alteration products, in the form of a magnesium sodium aluminosilicate, is observed only in the case of the 25 day specimen. The significance of these observations in the context of the composition of Magnox waste glass, is discussed. INTRODUCTION The interaction between high level nuclear waste (HLW) glasses and water vapour may be important under conditions of interim waste storage and repository disposal [1], where a humid environment leads to corrosion of the vitrified waste form by water vapour. In the UK, HLW arising from reprocessing of Magnox and oxide (UO2) nuclear fuel is immobilised in a lithium sodium borosilicate glass matrix [2], contained within a carbon steel canister. The surface temperature of the vitrified product is, initially, in excess of 100oC as a result of radiogenic heating. Therefore, during the initial cooling period of the wasteform, vapour phase hydration of the HLW glass may be an important corrosion mechanism if water were to become trapped in the ullage of the canister. Although highly unlikely, this fault scenario must be considered. The Vapour phase Hydration Test (VHT) was developed as a means of assessing the interaction between nuclear waste glasses and water vapour at elevated temperatures. The application of this test protocol was recently standardised by Jiricka et al [3]. Essentially, the VHT methodology involves subjecting a glass specimen to vapour phase hydration in a sealed vessel at elevated temperature. Sufficient water is present in the vessel to allow a thin film of water to condense on the specimen at the reaction temperature, leading to glass hydration, ion-exchange and network dissolution. As a consequence of these corrosion processes, the thin film of water becomes rapidly saturated in leached species which, at solubility limited concentrations, precipitate as “secondary” alteration phases [4, 5]. Magnox waste glasses, derived from vitrification of the High Activity Liquor (HAL) arising from reprocessing of Magnox nuclear fuel, have a unique composition that is rich in MgO and
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Al2O3 (arising from the fuel cladding material) with only a trace amount of CaO. Table I gives the composition of a full scale simulant Magnox waste glass produced during non-active commissioning trials at the BNFL Waste Vitrification Plant (WVP), Se
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