Chemical and mineralogical aspects of water-bentonite interaction in nuclear fuel disposal conditions

In the field of nuclear fuel disposal, bentonite has been selected as the principal sealing and buffer material for placement around waste canisters, forming both a mechanical and chemical barrier between the radioactive waste and the surrounding groundwater. Ion exchange and mineral alteration processes were investigated in a laboratory study of the long-term interaction between compacted Na-bentonite (Volclay MX-80) and groundwater solutions, conducted under simulated nuclear fuel disposal conditions. The possible alteration of montmorillonite into illite has been a major object of the mineralogical study. However, no analytical evidence was found, that would indicate the formation of this non-expandable clay type. Apparently, the change of montmorillonite from Na- to Ca-rich was found to be the major alteration process in bentonite. In the water, a concentration decrease in Ca, Mg, and K, and an increase in Na, HCO3 and SO4 were recorded. The amount of calcium ions available in the water was considered insufficient to account for the recorded formation of Ca-montmorillonite. It is therefore assumed that the accessory Ca-bearing minerals in bentonite provide the fundamental source of these cations, which exchange with sodium during the alteration process. X-ray powder diffraction (XRD) analyses and optical microscope observations of the initial and reacted bentonite samples were conducted. Quartz, feldspars, pyrite, calcite and minor amounts of gypsum were revealed as the primary accessories. In reacted samples, goethite and siderite were identified as secondary mineral products in association with corroded pyrite grains, while calcite and gypsum were found to disappear. From these results it is assumed that oxygen present in the water and in the bentonite pore space promotes the oxidation reaction of pyrite (dissolved) and the precipitation of goethite. As a result, the pore water pH decreases and calcite is partly dissolved. This dissolution provides a siginificant amount of calcium ions, in addition to those that arise by diffusion from the water. Some of the reaction-released bicarbonate and ferric ions are found to re-precipitate in the bentonite as siderite, while the rest (also as sulphate ions) diffuse into the water. Although the relative oxygen content in the experiment may be considered higher than that of the repository concept for nuclear fuel disposal (due to interaction in a semi-closed system with high water/bentonite ratio), the near field geochemistry predictions imply limited oxidising conditions, which are characterised by the above-described processes in sulphide-bearing bentonite and occur for some time after closure and sealing of the repository.