Bubble Dynamics Study Of Bubble Generator In Molten Salt Reactor

Molten salt reactor is one of the fourth-generation potential nuclear reactorsthat is of great concern in the world with good development prospects.The molten salt reactor generates a large amount of gaseous radionuclides such as tritium,xenon and krypton during operation.Tritium has strong permeability in metal.In the high temperature environment of molten salt reactor,it is easy to enter the environment,pollute the environment and threaten human health.Xenon and krypton are neutron poisons,which will affect the reactivity of the reactor.Therefore,the removal of these gaseous radionuclides in the molten salt reactor is of great significance for the safe and stable operation of the molten salt reactor.The method of bubbling and degassing is mainly adopted to remove tritium,xenon and krypton in the molten salt reactor internationally.A venturi bubble generator is used to pump the carrier gas into the molten salt circuit by bubbling,and then radionuclides enter the carrier bubble through diffusion and mass transfer.Finally,the carrier bubbles are separated from the molten salt.The venturi bubble generator is an important part of the degassing system.The size and particle size distribution of the generated bubbles directly affects the degassing effect.The degassing effect of the carrier particle size of 0.5 mm is considered to be the best.However,the gas-liquid two-phase flow field in the bubbler is very complicated,the description of the dynamic mechanism for each stage requires improvement.In this paper,the process of kinetic visualization experiments and simulation calculations（CFD）were carried out for the process of carrier gas entering the molten salt circuit through the bubble generator,namely the formation and shedding of bubbles,the movement and fragmentation of bubbles,and the coalescence of bubbles.The main research contents and conclusions are as follows:1)The formation and shedding of bubbles on vertical wall.Firstly,in the hydrostatic system,the formation and shedding characteristics of bubbles on the vertical wall were studied for different orifice sizes and air flow rates.The results showed that when the air flow rate is constant,the larger the orifice size is,the smaller the volume of the bubble is formed.The pressure fluctuation at the orifice can directly reflect the three stages:nucleation stage,stable growth stage and necking stage.Secondly,after the buffer volume was introduced for the first time in the calculation model,the simulation results of the bubble formation period,formation time,waiting time,formation volume and other parameters are in good agreement with the experiment;while the buffer volume is not introduced,the simulation calculation results of the return flow in the intake pipe of venture bubble throat were lower than the experimental values.This proved the necessity and correctness of introducing the buffer volume in the computational model of this subject.2)Movement and fragmentation of the bubble in the throat and expansion section of the venturi bubble generator.Firstly,the movement and fragmentation of bubbles in the throat and expansion sections with different water flow rate and gas content of 0.2%were studied by visual experiments.By analyzing the relationship between bubble fragmentation and bubble velocity distribution,it is pointed out that the bubble velocity gradient determines the severity of fragmentation,the bubble-crushing region shrinks toward the junction of the throat and the expansion segment as the water flow rate increases.Secondly,through the numerical calculation method,the fragmentation of single bubble in the throat and expansion section in high temperature molten salt environment was studied.The deformation and fracture of argon bubble were caused by turbulent flow energy,viscous shear force and interface instability.The viscous shear forces cause a velocity gradient around the bubble,causing the bubble to deform and shred.3)Behavior of coaxial bubble rise and coalescence.The preliminary research results showed that the moving velocity of the upper bubble is almost unaffected by the lower bubble,and the velocity of the lower bubble is obviously increased due to the influence of the wake of the upper bubble.The relative velocity of the two bubbles increased first and then decreases in the process of approaching.Before the two bubbles were coalescence,the liquid film would undergo a process of thickness reduction,shrinkage deformation,and finally breakup in the middle portion of the liquid film.By simulating the coalescence behavior of coaxial argon bubbles in high-temperature molten salt,it was found that argon bubbles(E₀=0.444,M0=5.8×10^-11)with a diameter of 2 mm were more difficult to coalescence.The vortex on both sides of the upper bubble would prevent the lower bubble from moving up.