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A flat arid unmanned Gobi in northwest Gansu, China, has been recognized as a good location for a planned spent-fuel reprocessing plant site. To better assess its applicability and safety as a proposed site, it is important to understand the composition and characteristics of the local vadose zone. Laboratory tests were carried out to analyze the basic properties of the soil in the unsaturated layer, such as particle size, structure, chemical composition and mineralogical characteristics. The permeability coefficients at different depths were measured by three methods, including the single ring, double ring, and inverse auger methods. Four groups of probes were buried at depths of 0.2–4 m in the vadose zone to detect changes in matrix suction with water content in the dehydration state and to determine the field soil-water characteristics curve. Based on the measured hydrological and geological parameters, the migration of intermediate-level nuclides H-3, Co-60, and Cs-137 from the vadose zone into groundwater was simulated by HYDRUS-1D under different rainfall intensities. The simulation results showed that the vadose zone in the study area has a strong retention effect, in which the Co-60 and Cs-137 nuclides were retained in the vadose zone during the 1000-year simulation period. The radioactivity and distribution coefficients of the nuclides were the main factors affecting the time to reach the pollution standard at the groundwater level on a short time scale. Rainfall significantly increased nuclide migration speed on a large time scale. Finally, two engineering protection measures derived from weakening the driving force and increasing the migration path were proposed and were effective in a numerical simulation. • Vadose zone in a planned spent fuel reprocessing plant site was investigated. • Soil-water characteristic curve was fitted by field test and filter paper method. • Migration of H-3, Co-60, and Cs-137 in the vadose zone was simulated. [ABSTRACT FROM AUTHOR]

• The regional-level coupled vadose zone-groundwater nuclide migration model was established. • The temporal and spatial evolution of 3H, 99Tc, 137Cs, and 79Se was investigated. • The human health risks caused by 3H, 99Tc, 137Cs, and 79Se were assessed. 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. [ABSTRACT FROM AUTHOR]