Accelerated Glass Reaction Under PCT Conditions

Abstract

ABSTRACTStatic 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 in soluble components that are not effectively contained in secondary phases. The rate becomes linear in time after the acceleration and may be similar to the initial forward rate. A current 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.