Direct Determination of Transport Parameters in Repository Materials
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DIRECT DETERMINATION OF TRANSPORT PARAMETERS IN REPOSITORY MATERIALS
J.L. CONCA*, M.J. APTED** AND R.C. ARTHUR** * Washington State University Tri-Cities, Richland, WA 99352 ** Intera Sciences, 3609 South Wadsworth Blvd., Denver, CO 80235. ABSTRACT
A new flow technology has been developed that significantly decreases the time required to obtain transport data on saturated and unsaturated porous/fractured media. This technique is based on open-flow centrifugation and was developed to measure steady-state transport properties in most geologic materials within a matter of hours. Centripetal acceleration does not induce artificial effects in samples i.e., fracturing, collapse of interlayer structures, structural dewatering, compaction, chemical changes, etc., that occur with high-pressure methods. Using this technique, hydraulic conductivities (K) and diffusion coefficients (D) for compacted bentonite and four host rocks have been measured and re-interpreted. Based on these new data, K for compacted bentonite is less than 10-14 m/s, a factor of 1000 lower than previous pressure-gradient measurements, providing further assurance that radionuclide transport through bentonite backfill will be diffusion limited. Measured K for mudstone (1.8 x 10-12 m/s) indicates diffusion-limited far-field transport, while advective transport should occur for granite, basalt, and tuff, with expected matrix diffusion coefficients (correlated to measured D values) of 8.3 x 10-13 and 2.5 x 10-12 m2/s for fractured granite and basalt, respectively. INTRODUCTION
Modeling the transport of contaminants in subsurface materials surrounding nuclear and hazardous waste repositories requires knowledge of the material characteristics under saturated conditions. The diffusion coefficient, D, and the hydraulic conductivity, K, are key input parameters to existing and developing models of contaminant release from engineered barrier systems in saturated repository systems [1,2]. Knowledge of D and K as a function of the volumetric water content is also important for unsaturated conditions in a repository that controls recharge into the repository horizon, and during groundwater re-invasion of backfill or the desiccated zone surrounding the waste package after repository closure [3,4]. The diffusion coefficient and the hydraulic conductivity can, however, be difficult to measure in materials such as whole rock, concrete, and compacted bentonite. Standard techniques for measuring K can often lead to mechanical disruption of samples, and spurious values for K can therefore result. Chemical tracer techniques for determining D are compromised by transient chemical retardation effects so that D cannot be unambiguously determined. Therefore, two new methods have been recently developed for measuring D and K in engineered and natural barriers materials for possible radioactive waste disposal:
"*A new ultracentrifuge
technology rapidly measures hydraulic conductivity in highly impermeable materials, down to 10-16 m/s,
"*Electrical conductivity measurements,
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