Effects of using laboratory-measured distribution coefficients for high-concentration I-129 wastes to be disposed in trenches

摘要

At the U.S. Department of Energy Savannah River Site, disposal of radioactive wastes in shallow trenches was simulated with vadose zone (the subsurface region above the water table) models and decoupled aquifer models (Collard, 2001). The vadose zone models provided contaminant fluxes to the aquifer models. Results were used to establish inventory limits, such that operation of the facility would not exceed regulatory limits. Initial simulations used literature-derived soil values to represent the waste's I-129 distribution coefficient (the concentration of I-129 in solids divided by the concentration of I-129 in water). These simulations resulted in a very low allowable inventory of I-129 in trenches. Waste streams from pumping and treating ground water and treating process water produced high-concentration I-129 wastes on activated carbon and resin media. Small quantities of those wastes greatly exceeded the total allowable inventory for all nuclides in a trench. Because the concentration of I-129 in the waste was much higher than in the original ground water, the distribution coefficient (Kd) was thought to be much higher than the Kd for soil. A series of laboratory experiments (Kaplan, et al., 1999 and Kaplan and Serkiz, 2000) measured substantially higher waste-specific Kds. The waste-specific Kd values were used in new simulations that greatly increased the allowable inventories for the trenches. Additional simulations were performed to encompass a wider range of Kds for potential future waste streams. Results from all new simulations were combined to develop a relationship between allowable inventories and Kds. That relationship is a linear log-log plot can be used for future waste streams without requiring additional simulations.