Water - Compacted Na-Bentonite Interaction in Simulated Nuclear Fuel Disposal Conditions: the Role of Accessory Minerals

Abstract

ABSTRACTIn an earlier laboratory study1, ion exchange processes and smectite alteration were investigated through the interaction between compacted Na-bentonite (Volclay MX-80) and simulated granitic groundwater solutions. The change of montmorillonite from Na- to Ca-rich was found to be the major alteration process in the bentonite. In the water, a concentration decrease in Ca, Mg and K, and an increase in Na, HCO3 and SO4 were recorded. The total amount of Ca available in the water, however, was found insufficient to account for the recorded formation of Ca-smectite, and it is therefore assumed that the accessory Ca-bearing minerals in the bentonite provide the fundamental source of these cations.X-ray powder diffraction analyses and microscope observations of the bentonite samples were re-conducted. Quartz, feldspars, pyrite, calcite and traces of gypsum were revealed as the primary accessories. In reacted samples, goethite and siderite are found as the secondary mineral products in association with corroded pyrite grains, while calcite and gypsum tend to disappear. From these results it is assumed that the oxygen present in the water and bentonite pore space promotes the oxidation of pyrite (dissolved) and the precipitation of goethite. The pore water pH decreases and calcite is partly dissolved. Through the dissolution, the bulk amount of Ca ions in addition to those arising by diffusion from the water is provided. Some of the reaction-released bicarbonate and Fe2+ re-precipitate in the bentonite as siderite, while the rest (as also SO4 ions) diffuse into the water.Though the relative oxygen content in the experiment may be considered higher than that of the repository concept for nuclear fuel disposal (interaction in a semi-closed system with high water/bentonite ratio), the near field geochemistry predictions imply limited oxidizing conditions, which will be characterized by the above-described processes in sulphide-bearing bentonite and occur for some time after the sealing of the repository.