In-situ Electrochemical Characterization of Grouted Radioactive Waste
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IN-SITU ELECTROCHEMICAL CHARACTERIZATION OF GROUTED RADIOACTIVE WASTE ALBERT A. KRUGER*, JINGYAN GU** AND MICHA TOMKIEWICZ** *Westinghouse Hanford Company, MSIN R4-03, Richland, Washington 99352 **Department of Physics, Brooklyn College, Brooklyn, New York 11210 ABSTRACT This work is based upon the notion that a grid of electrodes will be incorporated in the slurry of radioactive waste and grout-forming solids. These electrodes will remain in the hardened grout, and will serve to monitor the ionic concentration of the liquids contained within the matrix, the structural integrity of the matrix, and the rates of out-diffusion of the electrolytes. Ultimately, we will investigate four techniques: (1) ionic conductivity, (2) electrochemical potential, (3) cyclic voltammetry, and (4) impedance measurements. Eventually, the results will be correlated with present quality verification methods, which include recovering samples and following the Toxicity Characteristic Leach Procedure (or other such accepted standard tests). This paper presents the preliminary results of a recently initiated effort. INTRODUCTION At the Hanford Site, twenty eight double-shell tanks (DST), and one hundred forty nine single-shell tanks (SST), are used for storage of radioactive liquid and sludge wastes that are retrievable by pumping and other methods. A fundamental goal of the Westinghouse Hanford Company is to end the current storage practice for liquid wastes and to dispose permanently of the waste in the DSTs. Residual liquid wastes will be pumped from the solids in SSTs and transferred to DSTs. The low-level radioactive fractions of these wastes will be immobilized in a cemetitious grout at the Hanford Grout Processing Facility, and disposed of in concrete vaults of the Grout Disposal Facility. Prior to closing each vault, post-curing verification will show that the final product meets the performance requirements. Any long-term disposal system of radioactive waste will require monitoring to warn against structural deterioration and/or leach of the radioactive or hazardous components into the environment. Ultimately, a grid of long-lasting electrodes will be embedded into the solid grout. We make the fundamental assumption that only solid grout with some electrolyte in the pores can give rise to leaching. As a result of local flooding of the pores of the solid, a sudden decrease in resistivity and change in potential will be monitored. Leaching will cause a further decrease in resistivity. The spatial distribution of these changes will allow us to determine the effective diffusion rate of the electrolyte. This presentation will report our preliminary findings in applying experimentally the following techniques: DC Conductivity: The migration dominated DC conductivity of an unobstructed electrolyte is proportional to the sum of the products of the ion concentrations, their valence, and their mobility: a= F~t. z.C
(1
j=l J J J
where a is the conductivity, z is the ionic charge, g its mobility, and C the ionic concentration. In a porous me
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