Sensitivity analysis of simulating radionuclide decay transport of low-level radioactive waste in nearshore environment

Tunnel disposal at nearshore environment is one of potential final disposal sites proposed for the low-level waste in Taiwan. The proposed tunnel disposal facilities were located roughly 800 m away from the shoreline and 50 m below sea levels. Geosphere parameters, sea levels, and infiltration rates are critical to radionuclide decay transports in far-field environments. A radionuclide decay chain of 4N+2, including Pu-238 → U-234 → Th-230 → Ra-226, was simulated by the 3-D HYDROFEOCHE5.6 numerical model for sensitivity analysis of far-field radionuclide decay transport for 100,000 years with steady flow and transient transport. Hydraulic conductivities, diffusion coefficients, and dispersivities were selected for sensitivity analysis of geosphere parameters. With 10 times higher of hydraulic conductivities than reference values, the Pu-238 concentrations are higher and peak concentration appear earlier at shorelines. However, die Ra-226 concentrations simulated with 10 times higher/lower hydraulic conductivities are all lower than those with reference values at shorelines. Higher hydraulic conductivities will favor migration of mother radionuclides of Pu-238, U-234, and Th-230 before decaying to Ra-226 at shorelines, while 10 times lower of hydraulic conductivities will hinder migrations of all radionuclides considered in this study. Sensitivities of diffusion coefficients on radionuclide decay transport are not significant in this study. Cases with 10 times higher/lower dispersivities than reference values deliver higher/lower concentrations for all radionuclides at shorelines. Considering radionuclide decay transport in association with changes of geosphere parameters are much more complicated than single species simulations. Changes of sea levels and infiltration rates are designed to investigate changes boundary conditions on radionuclide decay transport through changes of fields. Reliable site investigations are crucial to reliabilities of far-field radionuclide transport simulations. (author)