Degradation of International Simple Glass Cracks and Surface
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Degradation of International Simple Glass Cracks and Surface R. K. Chinnam1, C. Hutchison1, D. Pletser1 and W.E. Lee1 1
Centre for Nuclear Engineering (CNE), Department of Materials, Imperial College London, South Kensington Campus, London SW7 2AZ, UK. ABSTRACT Water degradation of glass waste forms has been studied extensively under a variety of conditions including of bulk glass immersed completely in static or dynamic water. In practice, the vitrified nuclear waste cracks as soon as poured into a container because of differences in thermal expansion coefficients. In addition, in repository the canisters may be only partially immersed in water. Later, water condenses on the surface of glass which corrodes releasing ions. In this work experiments have been performed to understand these effects on the degradation of International Simple Glass (ISG). Simulated cracks were found to develop pitting corrosion in the crack openings when tested by immersing ISG in water. Under load, these pits concentrated stress and grew as large planar cracks inside the glass. The condensation of water on glass surfaces leads to formation of pits and growth of calcium silicate crystals. INTRODUCTION Vitrified nuclear waste disposed of in geological repositories will be protected with a multilayer barrier system. These barriers, during the course of time, will get damaged and can leak water into canisters. In such an event, water can come in contact with glass and activate corrosion on the surface and inside already existing cracks. Most previous studies have been dedicated to understanding surface degradation mechanisms of glass [1, 2], while fewer discuss degradation of the cracks [3, 4, 5, 6,7]. Glass corrosion was classified by Hench and Clark [8] into 5 types, based on the corrosion medium pH and the gel layer formation mechanisms. With increase in pH, the glass corrosion mechanisms and the surface gel layer formation were found to vary. At pH > 10, pits were reported to form due to accelerated dissociation of silicate species. Kamizono et al., found congruent dissolution of glass in in-situ burial conditions and speculated that cracks can form grooves or pits [3]. Tournié et al., determined that pits forming at the early stages of glass corrosion will grow and interconnect forming craters, which increasingly become deeper and more abundant. In turn, the craters grow and become interconnected in such a way that they form a continuous altered, opaque corrosion crust [9]. Such exposure of new glass surface can elevate the chances of radionuclide escape. Precise knowledge of the nature of the alteration film that develops on the glass surface in an aqueous environment can be extremely useful when examining the glass behaviour and properties. These properties could directly influence the gel structure, which is highly dependent on the alteration conditions such as pH. Such gel layer properties are of particular importance for long-term behaviour studies of radioactive waste containment glass after it comes into contact with groundwater
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