The Effects of Temperature on the Leaching Behavior of Cement Waste Forms - The Cement/Sodium Sulfate System
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THE EFFECTS OF TEMPERATURE ON THE LEACHING BEHAVIOR OF CEMENT WASTE FORMS - THE CEMENT/SODIUM SULFATE SYSTEM *
MARK FUHRMANN, RICHARD PIETRZAK, JOHN HEISER III, EENA-MAI FRANZ AND PETER COLOMBO Nuclear Waste Research Group, Brookhaven National Laboratory, Upton, NY, 11973.
ABSTRACT The leaching mechanisms of simulated low-level radioactive waste forms are being determined as support for development of an accelerated leach test. Two approaches are being used: (1) comparison of leaching data with results of a model that describes diffusion from a finite cylinder, and (2) observation of the leaching process at temperatures between 20"C and 65"C. To provide results that can be used for modeling, leaching at elevated temperatures must change neither the leaching mechanism nor the structural controls of leaching such as the porosity. Releases of 137Cs, ASr, calcium, sodium and potassium from portland cement containing sodium sulfate, as a simulated evaporator sludge, have been determined under a variety of experimental conditions. Data from the leach tests were compared to model results for diffusion from the finite cylinder. While most leaching appears to be diffusion controlled, notable exceptions occur. For all samples, activation energies ranging between 6 and 11 Kcal/mole have been calculated from the relationship of the effective diffusion coefficient to increasing temperature, close to the expected value of 5 Kcal/mole for diffusion. INTRODUCTION The leaching behavior of cement-based, simulated low-level waste forms was studied at temperatures between 20"C and 65"C as support for the development of an accelerated leach test. In previous investigations [1,2,3,4,5] a variety of factors, such as elevated temperature and increased leachant volume, were examined for their ability to accelerate leaching. Initially, they were studied individually, later the most useful factors were combined to obtain greater release rates. From this work an accelerated leach test method [6] was developed for low-level radioactive waste forms. The leach test method was designed to minimize experimental artifacts that could be misinterpreted as release mechanisms such as effects of increased ionic concentrations in the leachate. This is particularly important when the data is used for mechanistic interpretations and long-term extrapolations. A computer program associated with the accelerated leach test allows the user to test experimental results against a model for diffusion from a finite cylinder [7,8]. If the model and the data fit within preset criteria then the diffusion model can be used to make projections of releases. Leaching data were analyzed to assess: 1) whether the model for diffusion from a finite cylinder describes leaching from cement based waste forms and 2) whether elevated temperature provides a predictable increase in leach rate. In this paper some of the experimental and modeling work used to validate the test method are presented. Only the work on portland cement containing sodium sulfate is presented here. Results
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