A case study on the migration of nuclides in the coupled vadose zone-groundwater system at a proposed spent fuel reprocessing site in Gansu, China

Abstract

Nuclear energy represents a relatively novel clean energy source, and potential nuclear contamination must be considered while developing the nuclear industry. In the context of a hypothetical nuclear accident, a nuclide migration model was established based on regional geological and hydrological data to investigate the migration characteristics of the radionuclides 3H, 99Tc(VII), 137Cs, and 79Se(IV) in a proposed spent fuel reprocessing site (SFRS) in Gansu, China. The temporal and spatial evolution of these four nuclides as well as their effects on the environment and local population were discussed in detail. The model of nuclide migration in the vadose zone was coupled to a large-scale regional nuclide migration model of groundwater in the form of boundary conditions. The results under the least favorable conditions showed that nuclides with high adsorption rates (137Cs and 79Se(IV)) remained in the vadose zone and decayed out completely. In contrast, nuclides with low adsorption rates (3H and 99Tc(VII)) penetrated the vadose zone and polluted the groundwater. The vadose zone delayed the arrival of 3H and 99Tc(VII) to the phreatic surface and reduced their concentrations. It was predicted that the concentration of 3H in the groundwater would fall below the threshold after 40 years, and the distance from the leakage area to the leading edge of the contamination plume would be 374 m. 99Tc(VII), which has a slightly higher adsorption rate and a lower decay rate than 3H, spread 833 m after 3000 years when its concentration was below the threshold. 3H and 99Tc(VII) did not migrate to downstream farms to threaten the safety of residential drinking water. This research presents a safety assessment of a nuclear area in Gansu, China, and provides a significant reference for the future construction of nuclear facilities and nuclear contamination control.