In-Situ Diffusion Experiment in Sparsely Fractured Granite
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In-Situ Diffusion Experiment In Sparsely Fractured Granite Peter Vilks1, Neil H. Miller1, and Mark Jensen 2 1
AECL, Whiteshell Laboratories, Pinawa, M.B., Canada, R0E 1L0. E-mail: [email protected] Ontario Power Generation, Nuclear Waste Management Division, 700 University Ave, Toronto, Ontario, Canada, M5G 1X6. E-mail: [email protected] 2
ABSTRACT The in-situ diffusion experiment was conducted at AECL’s Underground Research Laboratory (URL) to improve the understanding of diffusive solute transport in sparsely fractured or intact granitic rock (SFR). The experimental program used a comparative series of laboratory and in-situ field experiments to evaluate the ability of laboratory measurements to estimate in-situ rock properties and to explore issues surrounding the influence of stress relaxation, rock texture, porosity, pore geometry, and anisotropy on derived effective diffusion coefficients (De). In-situ experiments yielded iodide De between 1.4 x 10-13 and 1.1 x 10-12 m2/s. Unlike laboratory results, the in-situ De estimates did not exhibit correlation with sample depth or varied stress regime. Laboratory-derived measurements of De, porosity and permeability were found to systematically increase for samples removed from greater depths and higher stress regimes. Laboratory-derived iodide De values consistently trended higher than in-situ values by a factor of 1 to 15, except on the shallowest 240-m Level (σ1 ≈ 30 MPa) where differences were negligible. Laboratory-derived estimates of permeability were consistently higher than in-situ derived values by a factor of 2 to 100. This experimental program provides evidence that laboratory steady-state diffusion experiments are most likely to yield conservative values of De for simulation of diffusive mass transport in SFR. INTRODUCTION
Phase II of the In-situ Diffusion experiment was initiated in 1999 to further develop an understanding of diffusive mass transport in low permeability sparsely fractured or intact granitic rock (SFR)[1]. In particular, the experiment was designed to examine the derivation and uncertainty in estimates of diffusivities for sorbed and non-sorbed ions within highly stressed felsic rock from the Whiteshell Research Area, situated in Manitoba, Canada. The experimental work program, through a series of comparative laboratory and in-situ experiments, explored issues surrounding the influence of stress relaxation [2, 3, 4], mineralogy, texture, stress regime, porosity distribution, pore geometry, anisotropy and temperature on diffusivity estimates and their applicability in predictive modelling of mass transport. The principal objectives of the Insitu Diffusion program were: 1) To confirm and build confidence in the laboratory and in-situ experimental methodology to estimate the effective diffusivity of SFR; 2) To complete a systematic set of laboratory and in-situ diffusion experiments to assess the influence of scale dependence, sample alteration and experimental uncertainty on diffusivity and permeability estimates for SFR;
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