Interaction of Water and Compacted Sodium-Bentonite in Simulated Nuclear Waste Disposal Conditions

Abstract

ABSTRACTAlteration of compacted sodium-bentonite (Volclay MX-80) caused by groundwater in simulated repository conditions for high level radioactive waste, was studied in an experiment where bentonite (wrapped by a copper cylinder) was let to react with two types (A,B) of synthetic granitic groundwater that are distinguished by their initial concentration of potassium and chloride. The reaction took place in ambient conditions at a temperature of 75 °C and proceeded during several time intervals up to 36 months.At the end of each time interval the water was chemically analysed for determination of possible changes in composition. Chemical and mineralogical changes in the bentonite were investigated by using NH4Cl extractions, XRD and microprobe (SEM, EDS) analyses and were studied as a function of the reaction time (months) as well as of the distance (mm) from the contact front with water.Sodium ions were found to migrate out from the bentonite while being replaced by other cations such as calcium, magnesium and to some extent, particularly during the reaction of the bentonite with water B, by potassium. No clear evidence was found for the fixation of potassium ions in the interlayer position of montmorillonite clay and the transformation to illite. The main mineralogical change in the bentonite was from Na- to Ca-rich montmorillonite. Secondary processes were the dissolution-precipitation of sulphur compounds, dissolution of gypsum and carbonates and the dissolution-precipitation of copper compounds.