A COMPARISON OF GLASS REACTION AT HIGH AND LOW GLASS-SURFACE SOLUTIONVOLUME

Static leach tests have been performed at glass surface area/leachant volume (SA/V) ratios of 10, 340, 2000, and 20 000 m-1 to assess the ef fects of the SA/V on the mechanism and rate of the glass reaction. Tes ts were performed using actinide-doped borosilicate waste glasses [Sav annah River Laboratory (SRL) 131 and SRL 202] to monitor the distribut ion of released radionuclides in tests at different SA/V Solution resu lts show the major effect of the SA/V to be dilution of reaction produ cts. Differences in the pH and silicic acid concentrations attained in tests at different SA/V then affect the reaction rate. Tests at low S A/V maintain leachate pH values similar to the initial leachant, while tests at higher SA/V result in higher leachate pH values being attain ed due to ion-exchange reactions. Transuranics released as the glass c orrodes may exist in the leachate in concentrations far above their so lubility limits by sorbing onto colloids, although the colloids may ev entually settle out of solution. Transuranics also sorb onto the steel reaction vessel. The glass reaction progress can be characterized by three stages: (a) an initial stage where the reaction rate depends on the leachant pH, (b) an intermediate stage where the reaction slows to ward a minimum rate as the leachate solution approaches ''saturation, '' and (c) a long-term stage where the reaction rate may be affected by the formation of secondary phases that control the solution chemist ry. Tests at different SA/V cannot always be compared directly because the dominant reaction step and the observed reaction stage (initial, intermediate, or long-term) may not be the same.