Permeability, Swelling and Radionuclide Retardation Properties of Candidate Backfill Materials
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PERMEABILITY, SWELLING AND RADIONUCLIDE RETARDATION PROPERTIES OF CANDIDATE BACKFILL MATERIALS*
J. H. WESTSIK, JR., L. A. BRAY, Pacific Northwest Laboratory**,
F. N. HODGES, AND E. J. WHEELWRIGHT P. 0. Box 999, Richland, Washington
99352
ABSTRACT A backfill placed between a nuclear waste canister and the host geology of a nuclear waste repository can impede the migration of water through the waste package and retard the movement of radionuclides into the geologic formation. Hydraulic conductivities and swelling pressures are being determined as functions of the density of the compacted backfill, temperature, radiation dose, hydraulic head and Bentonite the chemical composition of the permeating fluid. clays and bentonite/sand mixtures have received initial emphasis. Sodium bentonite and calcium bentonite samples 3 compacted to a dry density of 2.1 g•cm had hydraulic In conductivities in the range of 10- 7 to 10-13 cm/s. addition, batch distribution ratios (Rd) for Sr, Cs, Am, Np, I, U and Tc have been measured for a number of candidate permeability and sorption backfill materials. Both initial studies have used a synthetic basaltic ground water.
INTRODUCTION Concepts currently under investigation for the disposal of high-level nuclear wastes include deep emplacement of the wastes in continental geologic formaSuch a disposal method would incorporate a waste package placed within tions. the geologic medium. A backfill barrier, emplaced between the containerized waste and the host rock, can both protect the other engineered barriers and act A as a primary barrier to the release of radionuclides from the waste package. properly designed backfill should 1) retard or exclude the migration of ground water between the host rock and the waste canister system; 2) retard the migration of selected chemical species (corrosive agents and radionuclides) in the ground water; and 3) control the Eh and pH of the ground water within the wastepackage environment [1,2,3]. Candidate backfill materials that have the above attributes have been selected and are being evaluated as part of the Development of Backfill Materials Program sponsored by the Office of Nuclear Waste Isolation under its conTests are being conducted to tract with the U.S. Department of Energy [1]. determine the following for potential backfill materials: permeabilities, sorption capacities, compaction properties, thermal conductivities, and longThis paper presents the term stabilities in thermal and radiation environments. initial results of our investigations into the ability of candidate backfill
* Work performed for the U. S. Department of Energy under Contract DE-AC0676RLO 1830. ** Operated for the U. S. Department of Energy by Battelle Memorial Institute.
330 materials to impede the migration of water, and to slow or eliminate radionuclide movement in the event that water is able to enter the structural barriers and leach the wastes.
RETARDATION OF WATER MIGRATION The ability of the backfill to slow or stop water movement is related to the Obv
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