The Reaction of Synthetic Nuclear Waste Glass in Steam and Hydrothernal Solution
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THE REACTION OF SYNTHETIC NUCLEAR WASTE GLASS IN STEAM AND HYDROTHERNAL SOLUTION W. L. Ebert and J. K. Bates, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439. ABSTRACT Glass monoliths of the WVCM 44, WVCM 50, SRL 165, and SRL 202 compositions were reacted in steam and in hydrothermal liquid at 200"C. The glass reaction resulted in the formation of leached surface layers in both environments. The reaction in steam proceeds at a very low rate until precipitates form, after which the glass reaction proceeds at a greater rate. Precipitates were formed on all glass types reacted in steam. The assemblage of phases formed was unique to each glass type, but several precipitates were common to all glasses, including analcime, gyrolite, and weeksite. Reaction in steam occurs in a thin layer of condensed water which becomes saturated with respect to the observed phases after only a few days of reaction. The reaction in steam is accelerated relative to reaction in hydrothermal liquid In the sense that secondary phases form after a shorter reaction time, that is, after less glass has reacted, because of the smaller effective leachant volume present in the steam environment. A knowledge of the secondary phases which form and their influence on the glass reaction rate is crucial to the modeling effort of the repository program. INTRODUCTION The U.S. Department of Energy is studying the feasibility of locating a repository for the isolation of high-level nuclear waste and spent reactor fuel in the volcanic tuff beds of Yucca Mountain, Nevada. Experiments are being performed in many laboratories to project the behavior of various waste forms in the tuff environment over the thousands of years the waste must be isolated from the biosphere. The primary means of nuclide release will be through the ingress of groundwater and subsequent reaction to free and transport radlonuclides from the waste containers. The tuff geology being studied by the Yucca Mountain Project (YMP) is unique among sites previously studied in that it Is hydrologically unsaturated [1]. The thermal load of the emplaced waste will ensure that liquid water will not contact the waste throughout the 300/1,000-year containment period. During this time, only water vapor and small amounts of pore-trapped water will exist near the waste. At longer times, the repository will cool sufficiently that small amounts of liquid water may accumulate. Since leaching from the waste is a major means of nuclide release and liquid water is required to transport released nuclides away from the waste form, most laboratory experiments are performed with large volumes of water relative to the glass surface area. The glasses are found to degrade at a high rate initially but the rate decreases as the solution becomes more concentrated until a low "final rate" is reached [2]. The initial and final rates have been used to model the reaction of several waste glass compositions. A credible scenario of the repository environment permits small volumes of liquid water to conta
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