AIMD Investigation On Structure And Diffusion Properties Of Molten FLiBe Salt Purification System

One of the six IVth-generation reactor candidates,the molten salt reactor?MSR?,has characteristics including high inherent safety,less nuclear waste generation,lesser long-lived minor actinides resulting from fission,and high thorium energy conversion.These properties make MSRs one of the best approaches to solve the energy and environment challenges.Molten FLiBe?2LiF-BeF₂?salts were used as a nuclear fuel solvent or coolant for MSRs,and its physical and chemical properties have an important impact on the safety and economy of the reactor.FLiBe molten salt has been widely studied and applied due to its advantages of high neutron economy,low operating pressure,good radiation resistance,and high compatibility of nuclides?thorium and uranium?.Among them,the compatibility of nuclides and the corrosion of structural materials is strongly related to the purity of molten salts,so the purification of molten salt before entering the reactor is an important guarantee for the critical and safe operation of MSRs.The purification of molten salt adopts multi-phase reaction process of HF/H₂ mixed gas and liquid molten salt,in which the diffusion properties of molten salt and gas-liquid mass transfer are crucial to the purification process and industial scale-up.In this paper,the molecular dynamics simulaition was used to study the correation between network structure and diffusion properties of molten salt basing on the microstructure of molten salt.Then the influence of impurities on the structure and diffusion properties of FLiBe molten salts was investigated,and the diffusion behavior of the reactive gas in the purification of FLiBe was simulated.For the first part of the research,Car-Parrinello molecular dynamics simulation was used to investigate the microstructure and diffusion behavior of LiF-BeF₂ melts.The Becke-Lee-Yang-Parr exchange correlation functional and Goedecker-Teter-Hutter pseudopotential were selected for calculation.Through the analysis of radial distribution function,coordination number,strength of ionic bonds and solvation shell lifetime.The formation mechanism of various ligands in the network of FLiBe molten salts was studied in depth and the quantity ratio was obtained.The self-diffusion coefficient and conductivity were calculated basing on the influence of the network structure on the diffusion behavior.The results show that Be²⁺had a strong complexation ability to form neutral network clusters,and its number decrease with the increase of temperature.Liquid LiF-BeF₂ consist of neutral network clusters and free F^-,Li⁺,BeF₃^-and BeF₄^2-,rather than completely free F^-,Li⁺and Be²⁺.The calculated self-diffusion coefficient and the conductivity based on this microstructure are in good agreement with the experimental results,and the conductivity changes with temperature in accordance with the Arrhenius model,rather than the linear model of infinite dilution solution as the present literature suggests.The mass transfer in the purification process of FLiBe molten salt was studied on the basis of the above simulation.First,the effect of various impurities(SO₄^2-,NO₃^-,Crⁿ⁺,H₂O,Fe³⁺,Ni²⁺)on the structure and diffusion properties of FLiBe molten salt was studied.Then the diffusion behavior of H₂ and HF in impure and pure LiF-BeF₂molten salt was investigated and the diffusion coefficient of this gas were calculated,where H₂ and HF was a reaction gas to remove oxide impurities in the purification process.The structure of the FLiBe molten salt at different temperatures and different impurities contents was analyzed.The results show that both the increase of impurities content and the decrease of reaction temperature can enhance the interaction of Be-F ion pairs which occupy the main coordination structure in FLiBe molten salt.It can also reduce the self-diffusion coefficients of F^-,Li⁺and Be²⁺and the mobility of H₂ and HF in FLiBe molten salt.In addition,it was also found that,HF was easily decomposed into H⁺and F^-ions when HF moves in FLiBe molten salt,resulting in the diffusion coefficient of HF being close to that of F^-.The above two parts research show that the correlation between microstructure and diffusion properties of FLiBe molten salt was revealed,and the diffusion coefficient and conductivity were unified with the experimental results.The influence of impurities on the structure and diffusion of FLiBe molten salt was studied,which provides a basis for the analysis of the reaction process of the molten salt purification system.The diffusion coefficient of HF and H₂ in FLiBe molten salt was obtained,which provides a feasible method for mass transfer data acquisition in the purification process.