Microstructure and Leaching Characteristics of a Technetium Containing Metal Waste Form
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behavior of technetium is of particular importance from a disposal point of view for this waste form due to its long half life, 2.14E5 years, and its mobility in groundwater. To address these
concerns a limited number of spiked metal waste forms were produced containing Tc. These surrogate waste forms were then studied using scanning electron microscopy (SEM) and selected leaching tests. INTRODUCTION Argonne National Laboratory is currently developing an electrometallurgical treatment for spent fuel to stabilize it prior to disposal in a geological repository. This process yields a uranium
product, a ceramic waste form and a metal waste formn[,2].
The ceramic waste form, a
sodalite/glass composite, contains the active fission products, i.e., alkali, alkaline earth, rare
earth, transuranic, and halide elements. The metal waste form, a stainless steel/ zirconium alloy (85:15), contains the noble metals and some actinides. The noble metals for this process include: Tc, Ru, Rh, Nb, Pd, Ag, Zr and Te. These noble metals are those that are not oxidized while being processed in the molten salt electrorefiner. The focus of this study was to evaluate the metal waste form in regard to its behavior during long term immersion tests and to examine the microstructure. More specifically, three separate goals were to be accomplished by this work. First, the release rate for technetium was to be determined. Second, the effect of major and minor changes in the composition of the metal waste form on the release rates of technetium and uranium was to be established. Third, the microstructure of the alloy was to be examined. The first goal was accomplished by performing immersion tests on alloys consisting of only stainless steel, zirconium and technetium. The second goal was accomplished by testing specimens that had varying amounts of zirconium, noble metals and technetium. The emphasis in the immersion tests was on technetium due to its importance in the realm of performance assessment of a geologic repository. Technetium is a long lived fission product and has tremendous mobility in water. The test method, the long-term pulsed flow test, was performed for time periods of over one year in length. This method employs a monolithic sample submerged at 90 TC with sampling
953 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
occurring on a periodic basis of once every 1-4 weeks; an aliquot of leachate is taken, analyzed and replaced with fresh solution. Many of the tests discussed in this paper are still ongoing. The results to date are substantial and allow for certain conclusions regarding the durability of the metal waste to be made. This paper presents those results and conclusions. EXPERIMENTAL Sample Preparation The metal alloy samples were cast in yttria crucibles under an argon atmosphere using a muffle furnace. The casting temperature was 1650 'C. The argon atmosphere was used so as to ensure that the technetium would not oxidize and volatize. The composition of the various samples is stated in the t
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