Durability studies of simulated UK high level waste glass
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Durability studies of simulated UK high level waste glass Nor E. Ahmad1,2, Julian R. Jones3 and William E. Lee1,3. 1 Centre for Nuclear Engineering, Department of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, UK. 2 Physics Department, Faculty of Science, Universiti Teknologi Malaysia, UTM Skudai, 81300 Johor, Malaysia. 3 Department of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
ABSTRACT A simulated Magnox glass which is Mg- and Al- rich was subjected to aqueous corrosion in static mode with deionised water at 90 °C for 7-28 days and assessed using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) with Energy X-Ray Dispersive Spectroscopy (EDS) and Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES). XRD revealed both amorphous phase and crystals in the glass structure. The crystals were Ni and Cr rich spinels and ruthenium oxide. After two weeks of incubation in deionised water, the glass surface was covered by a ~11 μm thick Si-rich layer whilst mobile elements and transition metals like Na, B, and Fe were strongly depleted. The likely corrosion mechanism and in particular the role of Mg and Al in the glass structure are discussed. Keywords: high level waste glass, durability, corrosion mechanism INTRODUCTION In the UK, spent nuclear fuel (SNF) is reprocessed to recover uranium and plutonium. The remnant waste is immobilized by vitrification in an alkali borosilicate glass prior to packaging in steel containers, storage and eventual geological disposal 200-1000 m underground within a suitable geological formation. There are two sources of HLW glass compositions in the UK: (a) Magnox which arises from reprocessing of Magnox fuel (Mg- and Al- rich); and (b) Blend that arises from other reprocessing activities within the Thermal Oxide Reprocessing Plant (THORP) at Sellafield [12]. Glass is used largely because of its random network structure which can accommodate most of the waste components [3]. Glass durability is a key parameter. In the underground repository, the wasteforms will eventually be in water environment. Contact with underground water may lead to corrosion which disrupts the glasses ability to retain the radionuclide inventory. When glass is in contact with water, rapid ion exchange process occurs between the cations in the glass e.g. Na+, Ca2+ and H+ in the water. These ions leach out leaving behind silanol bonds (Si-OH) on the glass surface. The water pH increases and a silica-rich region forms near the glass surface. However, when the water pH goes above 9, it attacks Si-O-Si bonds, total dissolution occurs so dissolving the glass. In some cases at saturation, precipitation of a surface layer occurs. The leaching rate depends on the composition of the glass, pH of the solution and the temperature of the environment [4]. The details of glass corrosion mechanisms have also been discussed extensively [4-6].
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Many studies have been done to understand the release of radionuclides from glass wasteforms over
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