D.A. Dixon

A study of the compaction properties of potential clay—sand buffer mixtures for use in nuclear fuel waste disposal

Abstract In-situ emplacement of clay-based buffers in a nuclear fuel waste disposal vault limits the maximum attainable buffer density. This will vary with the composition of the buffer. A study of the maximum attainable densities of candidate Na bentonite/sand and illite/sand buffers is described. The addition of sand significantly increases the achievable compacted density. This increase may be obtained without any decrease in the swelling pressures developed by Na bentonite buffers. Sand decreases the shrinkage potential of the buffer and may also decrease the mass diffusion coefficient. A mixture of 50% sand and 50% clay by mass appears to optimise the physical properties of the buffer.

Swelling capacity and permeability of an unprocessed and a processed bentonitic clay

Abstract Research on the longevity of potential bentonite-based barrier materials is an important part of the Canadian Nuclear Fuel Waste Management Program. Valuable information on the longterm effectiveness of bentonitic barriers can be obtained by examining the properties of unprocessed bentonites from natural deposits. This study compares the swelling capacity (Ps) and hydraulic conductivity (K) of an unprocessed (clay that has not been ground and dried by the supplier) and a processed bentonite from south-central Saskatchewan, Canada. The clay deposit is 75 to 85 Ma old. At a given clay density, the processed bentonite exhibits a greater P3 and a lower K than the unprocessed clay. This can be at least partially attributed to the partial cementation of the unprocessed bentonite particles; this cementation is likely ruptured when the clay is ground during processing. Even though K for the unprocessed clay is higher than that of the processed clay, it is still low enough (

The effect of steam on montmorillonite

Abstract The effect of a steam phase on selected properties of an uncompacted montmorillonite clay was examined. Steam significantly affects the specific volume (SV) of the clay even at temperatures as low as 110°C. For example, compared to an unsteamed sample of clay, the SV decreased by about 25% when the clay was steamed in a closed system at 110°C for 21 days at a moisture content to clay mass ratio of 0.20 g/g. Moreover, the decrease in the SV of partially saturated clay was much greater than when the clay was heated in either an oven-dried or a saturated state. Steaming the clay did not affect its cation-exchange capacity, nor was there any detectable change in the mineralogical composition of the clay after steaming as indicated by X-ray diffraction analysis. Like the unsteamed clay, the steamed clay expanded to approximately 1.7 nm upon solvation with ethylene glycol and it collapsed to 0.98 nm when heated to 350°C. Scanning electron microscopy showed that the aggregate size and the particle morphology of the clay were altered, however, by steaming in a partially saturated state. The decrease in the swelling capacity of montmorillonite clay when exposed to steam has important implications for the use of this clay as a sealing material in some waste disposal strategies.

The Effects of Dry Density and Pore-water Salinity on the Physical and Microbiological Characteristics of Compacted 100% Bentonite

This study examined the conditions required to suppress microbial activity in compacted bentonite, such that microbially influenced corrosion (MIC) of copper waste containers, surrounded by compacted bentonite in a future deep geologic repository, would become insignificant. Experiments were carried out to determine the effects of dry density and pore-water salinity on swelling pressure, water activity (a{sub w}) and the culturable microbial community in compacted bentonite. A dry density {>=} 1.6 g/cm{sup 3} ensures that a{sub w} is 2 MPa. Both conditions suppress microbial culturability below background levels (2.1 x 10{sup 2} Colony-Forming Units/g) in as-purchased bentonite. Under such conditions, cells likely survive as dormant cells or inactive spores, which greatly reduces the possibility of significant MIC. Observations in natural clay-rich environments support these findings. (authors)