Corrosion Mechanism in the Obsidian and its Comparison with the Nuclear Waste Glass for Long-Term Performance Assessment in the Geological Repository

ABSTRACT

Present paper discusses experimental work on obsidian glass to understand corrosion mechanism at variable pressure temperature conditions. For comparison, Advanced Vitrification System, a nuclear waste glass was studied simultaneously under similar conditions. Owing to massive weight loss the glass samples were unable to sustain beyond 4 hour during the experiments. Corrosion experiments on both these glasses were performed under hydrothermal like conditions and observed that the changes in the experimental conditions have major control over the release of sodium and other ions from obsidian and nuclear waste glass, hence effecting corrosion mechanism. X-ray diffraction patterns of the residue obtained after the experiments on obsidian glass revealed appearance of neo-formed minerals, such as quartz, celadonite, halloysite, sodalite, heulandites and wairakite. High degree of corrosion and formation of amorphous residue observed in case of the experiments performed at 300°C over the granules of the nuclear waste glass. The back scattered electron images of the scanning electron microscope indicate distinctive microstructures, associated with the neo-formed minerals, and their formation is linked with the release of sodium, potassium and other ions, causing changes in the pH and conductivity of the leachate. In case of nuclear waste glass back scattered images of the scanning electron microscope indicates formation of multiple alteration layers and secondary alteration products. The surface of the pellet after the experiment shows phyllosilicate with numerous tiny spherules embedded within the structure. These experimental results when extrapolated suggest formation of amorphous silica, quartz and chalcedony as end members in case of obsidian glass whereas, formation of iron rich fayalite and greenalite can be observed in case of nuclear waste glass. The mechanism, conditions of formation of neo-formed minerals and their stability have been discussed in the paper.

Keywords:

Corrosion, obsidian, advanced vitrification system glass, hydrothermal conditions, microstructure, modeling and simulation .