Waste Form Release Calculations for the 2005 Integrated Disposal Facility Performance Assessment

Abstract

SummaryA set of reactive chemical transport calculations was performed with the Subsurface Transport Over Reactive Multiphases (STORM) code to evaluate the long-term performance of a representative lowactivity waste glass in a shallow subsurface disposal system located on the Hanford Site. Twodimensional simulations were run until the waste form release rates reached a quasi-stationary-state, usually after 2,000 to 4,000 yr. The base case for this analysis was four vertically stacked LAWA44 glass waste packages under a recharge rate of 0.9 mm yr -1 . The maximum normalized technetium release rate from LAWA44 glass under a constant recharge rate of 0.9 mm yr -1 was 0.09 Myr -1 . The unit Myr -1 stands for "per million years," indicating the rate at which the technetium, normalized by the amount originally in the four waste packages, would be released per million years. The primary difference between the waste form release simulations for the 2001 Immobilized Low Activity Waste Performance Assessment (ILAW PA) and the simulations described in this document is the number of materials considered. Whereas the ILAW PA considered only LAWABP1 glass, the current IDF PA also describes radionuclide release from three Waste Treatment Plant (WTP) glasses (LAWA44, LAWB45 and LAWC22), two bulk vitrification glasses (six-tank composite and S-109), and three grout waste forms (containing AgI, BaI 2 , and Ba(IO 3 ) 2 ). All WTP and bulk vitrification glasses perform similarly. However, the Tc-99 release from the salt in the cast refractory surrounding the bulk vitrification waste packages is 2 to 170 times higher than the glass release rate and dependent on the water recharge rate. Iodine-129 release from grouted waste forms is highly sensitive to the solubility of the iodine compound contained in the grout. The normalized iodine release rate from grout containing barium iodate is 9.1×10 -1 Myr -1 .v

Corrosion of Metal Inclusions In Bulk Vitrification Waste Packages

“…Due to the probability of oxidizing conditions surrounding the waste packages in the IDF, in the simulations the Tc in the metal inclusions and the glass is conservatively assumed to be released congruently as soluble TcO 4 -. The experimental results and modeling calculations (Bacon and McGrail 2005) show that the metal corrosion rate will, under all conceivable conditions at the IDF, be dominated by the presence of the passivating layer and corrosion products on the metal particles. As a result, the release of Tc from the metal particles at the surfaces of fractures in the glass releases at a rate similar to the Tc present as a soluble salt (McGrail et al 2003;Pierce et al 2005).…”

“…Given the molecular weight and mass density assumed for the metal inclusions in Table 4-8, the metal inclusions were therefore assumed to have a corrosion rate of 2.3 × 10 -8 mol/(m 2 ⋅s). The metal inclusion corrosion rate is two orders of magnitude greater than the glass corrosion rate at a recharge rate of 0.9 mm/y (Bacon and McGrail 2005).…”

“…Metal inclusions sealed inside waste glass are likely protected from corrosion until the waste glass itself corrodes. At a recharge rate of 0.9 mm/y, only 3.8% of the glass has corroded after 4300 years (Bacon and McGrail 2005). Therefore, the corrosion rate of metal inclusions was assumed to be limited by the exposure of the metal inclusions by fractures initially in the glass.…”

“…To estimate the waste form release rates for these PAs, Pacific Northwest National Laboratory (PNNL) has contributed waste form release simulations (Chen et al 1997;Bacon and McGrail 1998;Bacon and McGrail 2001;Bacon et al 2002;Bacon and McGrail 2003;Bacon and McGrail 2005) that are based on data packages quantifying dissolution rates of waste package materials, including ILAW and BV glasses (Mann et al 2001;Pierce et al 2004a;Pierce et al 2004b). The waste form release simulations are performed with the Subsurface Transport Over Reactive Multiphases (STORM) code (Bacon et al 2004).…”

“…This report contains the results of experiments designed to quantify the corrosion rates of metal inclusions from Test ES-32B 1 (AMEC 2005). These experimental results were incorporated into PA simulations recently conducted for BV waste packages (Bacon and McGrail 2005), which were based on experimental data quantifying the corrosion rates of BV glass (Pierce et al 2004b). …”