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H₂O self-diffusion coefficient of water-rich MX-80 bentonite gels

Exploration Geophysics Research Group, National Institute of Advanced Industrial Science and Technology (AIST), Higashi 1-1-1 Central 7, Tsukuba, Ibaraki 305-8567, Japan

^* E-mail: nakashima.yoshito{at}aist.go.jp

(Received 28 September 2005; revised 10 January 2006)

MX-80 is a bentonite clay from the Clay Spur bed, Wyoming, USA, consisting of ~80 wt.% Na-rich montmorillonite and is a leading candidate material for use in engineered barriers within underground nuclear waste disposal sites in Switzerland and Sweden. The diffusive invasion of groundwater into the engineered barriers is likely to cause the dry high-density bentonite to swell and evolve into a less-dense, water-rich gel during re-submergence of the shafts and galleries. Data on H₂O diffusivity within water-rich bentonite gels are therefore essential to the safe design of the engineered barriers. To address this need, the self-diffusion coefficient, D, of ¹H₂O molecules in water-rich MX-80 bentonite gels was measured using the pulsed-gradient spin-echo method of ¹H nuclear magnetic resonance spectroscopy at 0.47 T; D was measured over a wide range of temperatures (20.6–70.1°C) and of bentonite volume fractions (0 to 17.2 vol.%). The results showed that D increased markedly to values of bulk water self-diffusivity as the bentonite volume fraction decreased towards 0 vol.%. The activation energy of the diffusion process for the gels was approximately equal to that for bulk water. As a result, the normalized H₂O self-diffusivity, D/D₀, was approximately independent of temperature, where D₀ is D in bulk water. The D/D₀ data for 6.39 vol.% bentonite fraction were successfully fitted to a porous clay gel model that involves unbound water molecules that diffuse in the pore space by avoiding randomly placed discoidal obstacles (clay particles) of a high aspect ratio. The D/D₀ data for MX-80 were similar to data for SWy-2 (Wyoming bentonite from the Newcastle formation) and Kunigel-V1 (bentonite from Yamagata, Japan), suggesting that all three bentonites are equally suitable candidate materials, in terms of water diffusivity performance, for use within engineered barriers.

KEYWORDS: back-filling material, buffer material, Colony mine, MX80, PGSE NMR, porous media, random walk, surface relaxivity, tortuosity