Study On Migration Behavior Of Key Nuclides In Cavern Disposal Repository

Nuclear energy,as animportant clean energy,plays an important role in the world energy industry.Chinese nuclear power industry has also been booming in recent years.With the remarkable achievements of Chinese nuclear power industry,enormous amounts of radioactive wastes are produced and difficult to deal with.Most of the hazards of radioactive wastes are difficult to detect,which have the characteristics of long half-life and high toxicity.The elimination effect is poor by conventional physical,chemical and biological methods.Decay is the main way to reduce the harm of radionuclides,so these wastes will have long-term potential harm to the environment.At present,the nuclear waste geological disposal has been widely recognized and possesses mature engineering and technical experience,among which the rock cave disposal method is highly respected.However,under the accident conditions of the repository,nuclides may leak into the groundwater and migrate,which will have an impact on the environment.Therefore,it is necessary to evaluate the feasibility and safety of the repository in the early stage of the repository construction,in which nuclide migration is the key part.Through the study of radionuclide migration in the early stage of storage construction,the pollution of nuclides to the geological environment is predicted,and the key data are obtained,so as to better prevent and block the migration of nuclides to ecological environment during the construction of the repository,so that the radioactive waste can be completely isolated during the operation of the storage and hundreds of years after it is closed,so as to ensure that the waste disposal is harmless to the environment.As an important repository surrounding rock,granite plays an important role in blocking nuclide migration.The adsorption of nuclides by granite can provide a theoretical reference for the safety evaluation of the disposal repository and can predict the impact of nuclide leakage.Based on the environmental protection supporting project of a nuclear power plant in southern China（a low-medium-level radioactive waste rock cave disposal repository project）,this paper conducts field sampling of the surrounding rock（granite）of the repository,studies the adsorption behavior of nuclides on granite,preliminarily analyzes the adsorption mechanism and obtains the migration characteristics parameters.On this basis,combined with the regional hydrogeology data of the study area,the migration and diffusion model of nuclides in the cave disposal system is established,and the diffusion path was predicted and evaluated,and the migration process of key nuclides was revealed,which provide the basic scientific basis for the safety evaluation of the disposal repository.The main research contents are as follows:1)the adsorption behavior and law of nuclides on granite were studied by batch experiments.According to the changes of contact time,temperature,initial concentration of nuclide solution and types of anion and cation,combined with data analysis method such as adsorption isotherm,kinetic model and characterization method such as XRD,XRF,SEM and FT-IR,the adsorption mechanism was preliminarily analyzed,and the migration characteristic parameters were obtained.2)Through the statistical analysis of the geological data in the area where the proposed repository site is located,the related information such as stratum,geomorphology and geological parameters in the area are clarified,and the geological conceptual model of the site is established by GMS software.3)the migration model of nuclides in the groundwater of pre-selected disposal repository is established.The distribution coefficient obtained by batch experiments and the kinetic and thermodynamic simulation parameters are input into the program.The migration range of nuclides after leakage into groundwater is simulated and predicted by this model,and the migration behavior characteristics of nuclides in the geological environment of the disposal repository are revealed,which provides data,methods and theoretical support for the site selection and evaluation of the disposal repository.The main conclusions are as follows:1)The integrity of the mountain where the entire repository is located is good.The water permeability is extremely weak.Bedrock fissure water provides a way for nuclide migration,and belongs to HCO₃-Na?Ca、HCO₃-Na type water.2)The batch experiments showed that the adsorption time of Cs（Ⅰ）、Sr（Ⅱ）、U（Ⅵ） on granite was different,which was 9 h,24 h and 45 min,respectively.The initial concentration has a great influence on the adsorption,and the adsorption distribution coefficient decreases with the increase of concentration,the effect of temperuature is the same,but it is very small.The adsorption efficiency is closely related to p H value,and it is easier to adsorb in the weak acid-neutral environment.The anions and cations will inhibit the adsorption,of which CO₃^2-、HCO₃^-have the greatest influence.3)The Langmuir model and second-order kinetic model were used to fix experimental data well according dynamic and thermodynamic model analysis,which indicating that the adsorption form belongs to monolayer chemical adsorption.4)It can be seen from GMS numerical simulation that,without considering nuclide decay and the blocking of the nuclide by the surrounding barrier materials,assuming the same concentration and migration time and the radioactive source is released at a constant concentration,Sr reaches the farthest distance,followed by Cs and U.The maximum distance of nuclide is less than 100 m within 500 a,indicating that the groundwater circulation in the project area is slow and the migration path of nuclides is long,which meets the basic requirements of the repository site aelection.5)In the actual project,considering various factors,the range of nuclide pollution will be much smaller than the simulation results,indicating that the results of this simulation are reliable,and the pre-selected site area of the repository can effectively prevent the migration of nuclide to achieve the purpose of disposing low and medium level waste.