Molecular Dynamics Study On The Microstructure And Physicochemical Properties Of LiF-NaF-KF Eutectic Molten Salt

Molten salt reactor is one of the six major technical routes for future nuclear energy development.Among them,LiF-NaF-KF（also known asFLiNaK）eutectic system is used as an excellent coolant and solvent for molten salt reactor（MSR）.Dissolving nuclear fuel in the system,can effectively simplify the structure of the nuclear reactor.At the same time,as a carrier,it can improve the physical and chemical properties of the system co-melt,improve the energy efficiency,and make the reactor operate in a safer environment.Therefore,the study of this system has received extensive attention from researchers.It`s structure and physicochemical properties are the key issues to be solved in the process of its applicationIn this paper,molecular dynamics method with polarizable core-shell model and WBK potential are used to study the the density,microstructure,diffusion coefficient,conductivity and viscosity ofFLiNaK molten salt and compare the results with the experimental values,the main conclusions are as follows:（1）The molten salt density calculated by molecular dynamics is consistent with measured values.With the increase of temperature,the free volume of the system increase,resulting in a decrease in density.（2）The molten salt of LiF,NaF,KF andFLiNaK eutectic molten salt system is composed with simple Li⁺,Na⁺,K⁺,F^-ions,the structure is short-range ordered,long-range disordered,and anion and cation are alternately arranged in ten system.The structure of molten salt was not significantly affected by the increase of temperature,the coordination structure between anions and cations was basically stable,and the coordination number changed slightly.The average coordination distance of theF^-was determined to be 3.12（?）,and the average first coordination distance of the coordinating cations[r_（Li+）,r_（Na+）,r_（K+）]were 1.84（?）,2.20（?）,2.54（?）.At the same time,it is clear that the coordination number ofF-ion and alkali metal cation increases with the increase of the radius of alkali metal cation,and the order of size is N_（F-K）>N_（F-Na）>N_{（F-Li）。}（3）In LiF,NaF,and KF,the diffusion coefficients of anions and cations all increase with the increase of temperature,and greater the ion mass,smaller the diffusion coefficients.The order of diffusion coefficients in LiF-NaF-KF eutectic molten salt isF^->K⁺>Na⁺>Li⁺.（4）The conductivity of LiF,NaF,KF andFLiNaK eutectic molten salt system increases with the increase of temperature,which is consistent with the measured values,and has a certain deviation compared with the measured values.It shows that the Nernst equation considering only the diffusion constant needs to add the correction term of the external electric field to be suitable for the calculation of conductivity.（5）The viscosity of LiF,NaF,KF andFLiNaK eutectic molten salt system decreases nonlinearly with the increase of temperature,and the viscosity of molten salt is related to the radius of alkali metal ions.The calculated value can accurately express the qualitative relationship between temperature,alkali metal ion radius and viscosity.The calculated viscosity values has a negative deviation from the measured values.（6）The surface tension of LiF,NaF,KF andFLiNaK eutectic molten salt systems has a good linear relationship with temperature.The surface tension is mainly affected by the Coulomb force of anions and cations in the system.The greater the Coulomb force,the tighter the bond between anions and cations,and the greater the surface tension of the system.