INTRODUCTIONThe release conditions that are considered to be plausible2r3 include (1) a "bathtub" condition in which the waste becomes fully or partially submerged in water that enters the breached container and accumulates to fill the container up to the level of the breach opening, (2) a "wet drip" or "trickle through" condition in which the waste form is exposed to dripping water that enters through the top and exits the bottom of a container with multiple holes, and (3) a "dry" condition in which the waste form is exposed to a humid air environment. Performance assessment calculations assume^ that the needed waste form degradation rate input for these release conditions is bounded by leaching data. This assumption needs to be
RESULTS
GlassThe Unsaturated Test has been applied to glass in both a standard mode (0.075 mL of EJ-13 water injected every 3.5 days with a glass surface area of ~13.5 cm^, presensitized 304L stainless steel) and as modified to study the effects of varying the volume of water contacting the waste, the interval between water injection periods, the ratio of surface area (as-cut or cast) to water volume, and the condition of the stainless steel in contact with the glass. The conditions used in the various experiments and the status of the experiments are shown in Table I. Details of these first sampling period (6.5 weeks), extensive reaction between the metal, glass, and groundwater had occurred resulting in the formation of Fe, Cr, Mn, and Ni silicates and iron oxide-hydroxide reaction products 7 (Fig. 4).In test 7 the 304L steel contained 0.017 wt % carbon, and the extent of sensitization was less than in test 1. The effect of the lesser degree of sensitization is quite evident from the SEM photomicrographs in that only localized reaction of the metal and glass was observed, and the elemental release was less than that measured in test 1.The most extensive glass reaction was found in test 5 (Fig. 2) The bottom surfaces of the UO2 showed more variable coverage of reaction products than the top surface, but the coverage was always far less than noted on the top surface. In some experiments the Teflon™ stand also took on a distinctively yellow color.The reaction products as identified by XRD and SEM/EDS are listed in Table IV
CONCLUSIONSThe Unsaturated Test Method has been applied to both glass and unirradiated UO2 as a way of studying waste form reaction under anticipated repository conditions. For glass, the most significant effect was observed when the volume of water, as controlled by water injection volume and 11 interval period, was such to allow exfoliation of reacted glass to occur.The extent of reaction was also influenced to a lesser extent by the degree of sensitization of the 304L stainless steel in contact with the glass.The normalized releases that are observed are larger than the bounding values currently used in performance assessment calculations.3 This indicates that it is probably inappropriate to assume that glass waste form reaction rates, under some of the plausible ...
