A case study on the soil-water characteristics of the vadose zone and the migration of intermediate-level nuclides in a planned spent-fuel reprocessing plant site

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.