The Study On Soil-water Characteristics And Nuclides Migration Law Of The Vadose Zone In A Gobi Region Of Gansu Province,China

The development of nuclear power is key for China to improve its energy structure,guarantee its energy supply,adhere to its carbon emission reduction plan,and implement national security prevention and control.After more than 40 years of effort,China has become one of the world's leaders in nuclear power.With the rapid development of nuclear power in China,a large amount of radioactive nuclear waste has been produced,and this has placed great pressure on the ecological environment.The disposal of nuclear waste in an economic,safe,and effective manner has thus become a subject of wide social concern.To improve the utilization of nuclear fuel and reduce the volume of nuclear waste,China has implemented a closed nuclear fuel cycle policy.First,the spent fuel is reprocessed,including quenching,concentration,and separation,and then the available nuclides are separated and recovered.For non-recyclable nuclear waste,near-surface treatment and deep geological treatment are carried out according to the radiation level.During transportation,temporary storage,reprocessing,and near-surface disposal of nuclear wastes,absolute safety cannot be guaranteed.Once nuclear leakage occurs,radioactive pollutants first enter the vadose zone and migrate to groundwater along the vadose zone.The nuclides mainly exist in the form of ions,complex ions,molecules,and colloids in water.When nuclides make contact with and migrate within the vadose zone,the vadose zone can effectively block and delay the migration of nuclides to groundwater under physical,chemical,and biological action and make some nuclides adsorb on the organic matter and surface of soil particles,thereby slowing the rapidity with which nuclides diffuse to the biosphere.Because vadose zones differ greatly in their nuclide-blocking ability,elucidating the composition of the vadose zone,soil–water characteristics,and nuclide migration law is essential for site selection and the subsequent construction of nuclear waste-related plants to ensure that the vadose zone fulfills its function of protecting groundwater in the event of nuclear accidents.In this paper,the composition,structure,and unsaturated soil–water characteristics of the soil in the vadose zone of a preselected site for a spent fuel reprocessing plant in a Gobi Region of Gansu Province,China,were studied.The migration and accumulation behavior of the nuclides ³H,⁶⁰Co,and ¹³⁷Cs in the vadose zone were simulated based on the hydrogeological conditions and unsaturated hydraulic parameters in the vadose zone.Protective measures for the preselected site were proposed based on current protective measures for nuclear waste-related plants as well as the characteristics of the vadose zone and nuclide migration law in the study area,and these measures were evaluated and verified by numerical simulation.The results of this research have important implications for suitability evaluation,risk assessment,the development of emergency measures,and the emergency rescue of this spent fuel plant and plants with similar soils.The main research findings are detailed below.(1)Field data and the meteorological,hydrological,and geological conditions of the preselected plant site and area in the vicinity were analyzed and summarized in detail to evaluate the suitability of the preselected plant area and provide data for the numerical simulation of nuclide migration in the vadose zone.The results indicate that the preselected plant site has a typical continental arid climate,annual rainfall is rare,the groundwater flow rate is slow,variation in the water level is small,the stratum is stable,the thickness of the Quaternary vadose zone is moderate,and no dense population is present in the vicinity.The location of the preselected plant site has several hydrogeologically,climatically,and geographically favorable properties that make it suitable for the spent fuel reprocessing plant.(2)Laboratory tests revealed the composition and basic physical properties of soil samples in the vadose zone in the preselected plant site and provided insight into the formation mechanism,soil composition,structure,and basic physical properties of the vadose zone.The results indicated that the vadose zone of the preselected plant site is characterized by Quaternary alluvial pluvial soil,including a Holocene alluvial pluvial layer and upper Pleistocene alluvial pluvial layer;the particle size range of the soil is large,including clay group,silt group,sand group,gravel group,and gravel with poor homogeneity;the entire vadose zone is weakly alkaline,which has chlorite saline soil and weakly saline soil,with an average dry density of 1.798 g/cm³and 1.815 g/cm³,respectively,and the average porosity is 37.8%and 36.1%,respectively.(3)The saturated vertical permeability coefficient test and water migration test of the vadose zone were carried out in the preselected plant area.In addition to the indoor filter paper test,the soil–water characteristics of the vadose zone were studied to determine the unsaturated hydraulic parameters,summarize the water migration law of the vadose zone,and provide basic data for the establishment and verification of the numerical model.The results indicate that the double-ring method has higher confidence for measuring the saturated vertical permeability coefficient of the vadose zone compared with the single-ring method and the improved IAM method,and the improved IAM method is not suitable for the stratum with high gravel content;the saturated vertical permeability coefficients of the Holocene alluvial pluvial layer and upper Pleistocene alluvial pluvial layer in the vadose zone are quite different,with average values of 6.962 m/d and 0.240 m/d,respectively.Because of the large variation in soil particle size,the curves in the water content and matrix suction in the vadose zone undergo various stages during the natural dehumidification process,which can be explained by the"capillary bundle model."When the water content of the vadose zone changes from the saturated state to the natural state,the matrix suction is in the capillary stage.Fine-grained soil can effectively improve the water-holding capacity of the Gobi vadose zone.The matrix suction of Upper Pleistocene alluvial diluvium was significantly higher compared with Holocene alluvial diluvium as the clay and silt content increased and when the water content was low,and the difference in matrix suction increased as the water content decreased.(4)The full-range soil–water characteristic curve data of the soil in the vadose zone of the preselected plant area were obtained based on the field water migration test and the indoor test using the improved filter paper method.The VG model was used to fit the soil–water characteristic curve,and the model parameters were obtained.The following conclusions were obtained in the process of fitting the soil–water characteristic curve:the improved filter paper method can effectively measure the matrix suction of soil with high sand content and weak structure;the lsqcurvefit function is simple and highly accurate and can be used to fit and solve the mathematical model of the soil–water characteristic curve;the VG model can accurately describe the soil–water characteristic curve of the Gobi vadose zone in Gansu Province;and the unsaturated soil–water parameters of vadose zone soil in the preselected plant area were similar to those of sandy loam and loam soil relative to the official reference value of Hydrus.(5)Based on the meteorological data and unsaturated hydraulic parameters of the vadose zone in the preselected plant site,the migration and accumulation behavior of the nuclides ³H,⁶⁰Co,and ¹³⁷Cs in the vadose zone under different rainfall intensities were simulated by HYDRUS-1D software.The results of the simulation,which were conducted over a period of 1000 years,indicated that only the nuclide ³H migrates to the phreatic water level,and the nuclides ⁶⁰Co and ¹³⁷Cs remained in the vadose zone at all simulated times.As the intensity of rainfall increased over long timescales,the peak value of the ³H radioactivity concentration at the groundwater table gradually increased,and the vertical migration distance of the retained nuclides ⁶⁰Co and ¹³⁷Cs in the vadose zone greatly increased.For the nuclides retained in the vadose zone,the initial concentration mainly affects the value of the peak radioactivity concentration of nuclides in the vadose zone,and the distribution coefficient mainly affects the location of the peak radioactivity concentration and the maximum migration distance in the simulation period.(6)Three protective measures are proposed based on the specificity of the vadose zone,the nuclide migration law in the vadose zone of the preselected plant site,and the current international common protective measures for the plant area related to nuclear waste:isolation of surface water seepage,replacement of the clay layer in the vadose zone,and the setting of dewatering wells,which were verified and evaluated by numerical simulations.The simulation results showed that the three protective measures could effectively block and delay the migration of nuclides to groundwater during and after emergencies,thereby reducing the amount of nuclear waste liquid that infiltrates per unit time at the initial stage of leakage,lengthening the time required for the groundwater radioactivity concentration to reach the pollution standard,suppressing the nuclide radioactivity concentration peak value at the groundwater surface,and reducing the migration distance of the retained nuclides in the vadose zone.