A natural analogue of nuclear waste glass in compacted bentonite

Abstract

A marine based argillaceous rock containing volcanic glass shards has been investigated to infer the long-term durability of vitrified nuclear waste in compacted bentonite, which is a candidate for buffer material constituting the engineered barrier system for nuclear waste disposal. Fission track ages indicate that the volcanic glass shards, andesitic scoria, have been buried in the argillaceous rock for about 1 Ma. Neither glass matrix dissolution nor precipitation on the surface was seen under an optical microscope. Little leaching of any element has been recognized by analyses using an electron microprobe analyzer. Secondary ion mass spectrometry analysis, however, indicates significant hydration which may dominantly be a permeation of molecular water. As an indicator of durability of glass against groundwater a normalized mass loss of Si (NL Si) has been evaluated for the volcanic glass based on free energy for hydration. The difference between estimated NL Si of the volcanic glass and that of a simulated waste glass is within one order so that the volcanic glass may be analogous to a waste glass with respect to durability to water. The argillaceous rock is analogous to the compacted bentonite with respect to physical properties such as dry-density, unconfined compression strength, porosity, and hydraulic conductivity. The ambient physical and chemical conditions surrounding the volcanic glass have been also investigated: temperature was in the range from 4 to 30°C due to the burial history of the volcanic glass. Over most of the past 1 Ma the volcanic glass has been in contact with groundwater originating from seawater. Thermodynamic calculations indicate (1) pH (=7.74–7.94) of the groundwater has mainly been controlled by dissolution of carbonate minerals, (2) the redox potential (Eh=−34–−73 mV) of the groundwater has dominantly been controlled by decomposition of organic materials to produce CH 4(g), and (3) activity of aqueous silica of the groundwater was in equilibrium with SiO 2 amorphous. Because of the equilibrium between aqueous silica and SiO 2 amorphous, the volcanic glass did not dissolve during the burial. Vitrified nuclear waste sealed in compacted bentonite, therefore, will not dissolve significantly if buried in an environment as mentioned above.