Molecular Dynamics Studies Of The Local Structures And Transport Properties Of Several Typical Molten Salts

Under the environment of energy crisis,the alternative energy development is an important subject of global scientific research.In the various forms of clean energy,the nuclear energy becomes the key research object of sustainable energy development in the future.During the development of nuclear energy,the fission reactor has been applied in the commercial use in contrast to fusion reactor,and molten salt reactor drew widespread attention for its security and stability.Different molten salts are involved in molten salt reactor.In this paper,the molten salt systems of fuel cycle,spent fuel recovery and coolant cycle are investigated with molecular dynamic simulation method,and the various local structures and transport properties of the salts systems are studied.The fuel of molten salt reactor is made of fissile elements dissolved in basal molten salt,and flows through the primary fuel circuit vessels.The basal salt acts as the solvent and primary coolant.The LiF-BeF₂-ThF₄ molten salt is selected as the representative of the fuel salt,and the local structures and transport properties of the system are investigated as temperature and the fraction of ThF₄.Th and Be are mainly8-and 4-coordinated respectively.The clusters are formed up based on Th-F bonds.As ThF₄ fraction increases or temperature decreases,the isolated clusters tend to polymerize with common F ions.As temperature increases,the average coordination numbers of Th decrease and the ones of Be keep constant;self diffusion coefficients gradually increase and keep an exponential relation with temperature;electric conductivity gradually increases and keeps an linear relation with temperature;viscosity gradually decreases and keeps an exponential relation with temperature.As ThF₄ fraction increases,the transport properties of molten salts gradually decrease.The studies fill up the limited data of fuel salt and are helpful to fuel circult development.The spent fuel was composed of fissile elements and fission product,and the reprocessing of the spent fuel is necessary for the fissile fuel cost and environment protection.The fissile elements are normally recycled during electrochemical reduction process,in which the spent fuel is dissolved in molten c hloride salts and the fissile elements are deposited in the cathode by redox reaction.The LiCl-KCl-MCl₃?M=La,U or Sc?salt systems are selected for spent fuel reprocessing investigation,in which the LiCl/KCl mole fraction is kept constant and the different mole fractions of LaCl₃,UCl₃ or ScCl₃ are added.The local structures and transport properties of the three systems are investigated as temperature and the fraction of trivalent cations.The studies fill up the crucial coefficients of electrochemical reduction process such as electrical conductivity and self-diffusion coefficients.The network composed of M-Cl is found and has a similar feature to the one in LiF-BeF₂-ThF₄ salt.The viscosity is affected by the network and the viscosity follows a nearly linear relation with the degree of network,which suggests the relaxtion of viscosity is essentially the relaxtion of the network.Electrical conducticity of the three systems are nearly equal with same temperature and MCl₃ mole fraction.By investigating the similar ionic molten salts with network structure,it is found the major contribution of electrical conductivity comes from alkali ions.The fact that electrical conductivities are nearly equal related from the nearly equal self-diffusion coefficients of alkali ions in the three systems.The coolant salt flows through the second circuit,in which the heat was transferred from the core to outside for power export.The LiF-NaF salt systems are selected as coolant salt for heat transfer system.The local structures and transport properties are investigated for all components as temperature varies from 1173K to1373K.The main conclusions are listed as follows:as LiF mole fra ction increases,the density and mole volume gradually decrease;the ionic bonds stabilities and lifetime of Li-F and Na-F gradually decrease;the self-diffusion coefficients of Li and Na linearly increase;the viscosity exponentially decreases;the electrical conducticity expontially increases.The systems expand as temperature increases,and the flowability and conductivity become stronger.In conclusion,several typical molten salts systems are studied in this paper and the thermodynamic properties such as density,electrical conducticity,viscosity and self-diffusion coeffcients are investigated.This paper fills up lots of crucial coefficients for molten salt reactor design and development.Besides,the local structure evolutions of molten salts are investigated in the view of atomic scale,and the network composed of multi-valence cations and anions are found when the cations are in high concentrations.Further investigation reveals that the viscosity follows a liner relation to network and the relaxation of viscosity depends on the degree of network.The investigation on the relations of local structure and transport properties can be helpful when dealing with unkonw molten salt systems.