Prediction of Radioactive Waste Glass Durability by the Hydration Thermodynamic Model: Application to Saturated Reposito
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PREDICTION OF RADIOACTIVE WASTE GLASS DURABILITY BY THE HYDRATION THERMODYNAMIC MODEL: APPLICATION TO SATURATED REPOSITORY ENVIRONMENTS Carol M. Jantzen* and W. Gene Ramsey** *Westinghouse Savannah River Company, Savannah River Site Aiken, SC 29808 **Clemson University, Dept. Ceramic Engineering, Clemson, SC 29631 ABSTRACT The effects of groundwater chemistry on glass durability were The relative examined using the hydration thermodynamic model. durabilities of Si0 2 , obsidian, basalt, nuclear waste glasses, glass were determined in medieval window glasses, and a frit tuffaceous (J-13) groundwater, basaltic (GR-4) groundwater, WIPP-A brine, and Permian Basin brine (PBB-3) using the monolithic MCC-I In the groundwater-dominated MCC-l experiments, durability test. groundwater the interaction of the glasses and the initial (leachant) caused the formation of unique assemblages of secondary The secondary phase formation, in turn, controlled the phases. final groundwater (leachate) pH and ionic strength, I[t). Correlations of the final leachate pH and I[t] with the Si release from the glass indicated that it is the influence of the secondary phase formation on the leachate pH and I[t] that controls the final Since I[t] and the pH of the dissolution rate of the glass. leachates are functions of the precipitation reactions, inclusion of the experimentally determined solution pH in the free energy of hydration model provides for the functional dependence of the Therefore, dissolution rate on the secondary precipitation. superposition of the linear equation for the groundwater and deionized water experiments occurs and the hydration free energy model can be used to compare glass durability in deionized water and in repository groundwaters. INTRODUCTION Determination of glass durability as a function of glass composition is significant to the development of durable glasses for nuclear waste disposal. The long radioactive half-lives of fission products and actinides in high-level nuclear wastes require The that they be isolated from the biosphere for 103 to 105 years. isolation technique selected for defense high-level waste produced at the Savannah River Site (SRS) is immobilization in borosilicate The glass and subsequent emplacement in a geologic repository [1]. intrusion of groundwater into and passage through a repository, is the most likely mechanism by which radionuclides may be removed Thus, it is from the waste glass and carried to the biosphere. important that nuclear waste glasses be stable in the presence of groundwaters for very long periods of time and that the long-term durability can be accurately modeled. To quantify the relative durabilities of nuclear waste glasses as a function of composition, their performance relative to natural and ancient glasses (whose long-term performance is known) was The thermodynamic approach of Newton and Paul investigated [2-8]. [9], which expresses glass durability as a function of glass Mat. Res. Soc. Symp. Proc. Vol. 176. @1990 Materials Research Society
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