Accelerated Glass Reaction Under PCT Conditions
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ACCELERATED GLASS REACTION UNDER PCT CONDITIONS
W. L. Ebert, J. K.Bates, E. C. Buck, and C. R. Bradley Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439-4837. ABSTRACT
Static leach tests similar to the PCT were performed for times up to two years to assess the long-term reaction behavior of high-level nuclear waste glasses similar to those expected to be produced at the Defense Waste Processing Facility. These tests show the reaction rate to decrease with the reaction time from an initially high rate to a low rate, but then to accelerate to a higher rate after reaction times of about one year, depending on the glass surface area/leachant volume ratio (SAN) used. The solution concentrations of soluble glass components increase as the reaction is accelerated, while the release of other glass components into solution is controlled by secondary phases which form during the reaction. The net result is that the transformation of glass to stable phases is accelerated while the solution becomes enriched insoluble components that are not effectively contained in secondary phases. The rate becomes linear intime after the acceleration and may be similar to the initial forward rate. Acurrent model of glass reaction predicts that the glass reaction will be accelerated upon the formation of secondary phases which lower the silicic acid solution concentration. These tests show the total silicon concentration to increase upon acceleration of the reaction, however, which may be due to the slightly higher pH that is attained with the acceleration. The sudden change in the reaction rate is likely due to secondary phase formation. INTRODUCTION The long-term behavior of high-level nuclear waste glasses to be emplaced in
a geologic repository must be predicted on the basis of observations made inshortterm laboratory experiments and cannot be simple extrapolations of experimental data [1,2]. The underlying basis of many models of glass corrosion is that the ratedetermining step for glass corrosion is the hydrolysis of the bond between a network silicon and an -OSi(OH)3 group to release silicic acid [3]. The glass reaction rate is modeled using a reaction affinity approach inwhich the rate is expressed as a function of the difference between the silicic acid concentration in solution and a "saturation" concentration. According to this model, the reaction slows as the silicic acid concentration approaches "saturation" with respect to the glass and eventually proceeds at a low rate [2-7]. Simulations have shown that leachate solutions may become saturated with respect to secondary phases during the reaction, and that these phases may control the solution chemistry at long reaction times such that the solution composition does not attain "saturation" conditions and the reaction rate remains high [4,6]. The long-term reaction rates predicted by computer simulations may be different depending on the assemblage of secondary phases formed [8]. In order to address the issue of long-term glass reaction rates, tests were per
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