Mobility of Chelated Radionuclides in Engineered Concrete Barriers
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MOBILITY OF CHELATED RADIONUCLIDES IN ENGINEERED CONCRETE BARRIERS
CRAIG A. DICKE and ROBERT W. SMITH Idaho National Engineering Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-2110. ABSTRACT Concrete is a major component in many low-level radioactive waste (LLW) disposal facilities. The use of concrete is widespread because of its physical and structural properties and because it provides geochemical control on metal and radionuclide releases. Organic compounds are often disposed with radionuclides in LLW disposal facilities. Interactions between radionuclides and chelating agents must be evaluated to estimate mobility of radionuclides in concrete vaults. This paper quantifies the effects of two common organic components [citric acid and ethylenediaminetetraacetic acid (EDTA)] on radionuclide mobility in concrete barriers by using equilibrium geochemical calculations. Equilibrium speciation calculations indicate that some radionuclides are chelated in groundwater (pH 7) but are destabilized in the highly alkaline (pH 13) concrete pore fluids. Radionuclides complexed by EDTA and citrate are replaced by calcium in the concrete pore fluids. In addition, the citrate nuclide complex reacts to form uncomplexed citrate in concrete pore fluids. The chemical performance of concrete LLW disposal facilities should not be compromised by small amounts of chelating agents disposed with some radionuclides. However, EDTA may form significant nickel and cobalt complexes above the pH important in the longterm service life of concrete barriers. INTRODUCTION
Decades of weapons production have resulted in a variety of waste streams and disposal practices at U.S. Department of Energy (DOE) sites. These waste streams contain mixtures of LLW, organic chelating agents, and metals. Current environmental restoration and disposal activities that focus on long-term environmentally sound permanent disposal have led to the use of concrete vaults to entomb mixed wastes. This paper quantifies the effects of two common organic components (citrate and EDTA) on radionuclide mobility in concrete barriers by using equilibrium geochemical calculations in order to understand the effects of waste codisposal on radionuclide partitioning. Concrete barriers have many physical properties that are important in waste isolation such as low permeability, high strength, and low diffusion rates. In addition, concrete offers an ideal geochemical environment for immobilization of radionuclides. The high pH of concrete pore fluids promotes precipitation of nuclides as hydrous oxides, and the high internal surface area of cured concrete provides ample adsorption sites. Chemical effectiveness of concrete to isolate radionuclides is shown by the distribution coefficients (Kds) given in Table 1. As shown in this table, radionuclides are effectively partitioned onto the solid phase from solutions in contact with concrete. Disposal of EDTA and citric acid with radionuclides has been reported at several DOE waste sites[l]. Although the concentration of organic compounds
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