Sensitivity of radiological impacts of a waste disposal system to radionuclide sorption in the geosphere

The purpose of this study was to evaluate the role of sorption in the geosphere in retarding the release to the surface environment for 31 selected radionuclides, including members of the 238U decay chain, other actinides of possible concern such as Np, Pu, and Am, and fission products identified as part of the instant release inventory from used fuel. An integrated system assessment model was used to simulate the release and transport of contaminants through the reference disposal system and to predict their long-term impacts on human health. In probabilistic assessment, tens of thousands of simulations are performed, each with a different set of randomly assigned parameter values that fall within the limits of a defined probability density function, to give a distribution of consequence estimates over a specified timeframe. We used simulations from such an assessment to examine the change that resulted in the annual effective dose equivalent attributable to a specific radionuclide when zero sorption in the geosphere was assumed. This process was repeated for each radionuclide studied using the highest-consequence simulations. We found that the radiological impacts contributed by 126Sb, 126Sn, and 79Se in the reference disposal system were quite sensitive to reduced sorption in the geosphere. In contrast, the sensitivity analysis confirmed that the contributions to dose from Pu, Np, and other actinides in the reference disposal system are controlled largely by system parameters other than sorption in the geosphere; processes and factors affecting the sorption of these actinides, or their complete lack of sorption, in the geosphere have negligible radiological impact