The influence of diffusant concentration, c 0 , on effective diffusion coefficients, D e , was examined under steady-state conditions in dense clays for c 0 ranging from ~10" l° to 10~6 mol/1. Two clays, Avonlea bentonite and Lake Agassiz clay, were used, and D e values were measured for I" and Cs + using a through-diffusion method. The clays were compacted to a dry density of approximately 1.35 Mg/m 3 and saturated with a synthetic groundwater solution having an ionic strength of 0.22 mol/1. The D e values for both I" and Cs + were about 2x10" 11 m 2 /s and, in accordance with Fick's law, were not markedly affected by c 0 . The results also indicate that surface diffusion is not an important transport mechanism for Cs + . Apparent diffusion coefficients, D a , were also obtained from the experiments and were approximately 7 χ 10" 10 m 2 /s for I" and 4 χ 10 ~1 3 m 2 /s for Cs + . The lower D a value for Cs + is attributed to its greater sorption on the clay.
Compacted bentonitic clay/sand mixtures are being considered for use as buffer materials in the Canadian concept for nuclear fuel waste disposal. This paper describes a laboratory study of the swelling pressures that develop in statically compacted, air-dry specimens of mixtures of sodium bentonite and silica sand as they are saturated with double-distilled, deionized water. The results are interpreted with the aid of scanning electron microscope observations of the soils' structures.It is shown that the sand acts as an inert filler material, and swelling pressures are controlled by a parameter termed the effective clay dry density, γC, defined as the ratio of the mass of clay to the combined volume of the claq plus voids in the mixture. A threshold value of γC exists below which swelling pressures can be expected to be isotropic. Above the threshold value of γC, pressures parallel to the axis of compaction can be expected to be greatgr than those perpendicular to it. This is related to a change in soil fabric as γC is increased above the threshold value. For the Canadian disposal concept, γC would probably be below the limiting value and swelling pressures of 2.5 MPS or less are expected. The swelling pressures are likely to be isotropic within a saturated buffer mass.
Unique variations in ATS deposition patterns were seen between lenses with atomic force microscopy. The application of both HPS and MPS removed most visible surface deposits.
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