Experimental Study On Explosive Characteristics Of Liquid Lithium Coolant Interaction

Controlled nuclear fusion,providing a potential,inexhaustible source of clean energy,is the main choice to solve the future energy problem of mankind.The first wall component is particularly critical in magnetic confinement tokamak device.Liquid lithium,which could bear high neutron flux,surface thermal load and could significantly improve plasma performance,is an important approach for plasma facing materials in the future.However,the large-scale application of liquid lithium in tokamak device also has some security risks.Liquid lithium coolant interaction is the key factor for the safety and reliability of fusion reactor.The process of high temperature liquid lithium coolant interaction is very complicated.The key physical process and the mechanism of explosion are not clear.The energy conversion model has not yet been established,which greatly restricts the analysis and evaluation of the accident consequences.In this paper,the liquid lithium coolant interaction is studied by means of experimental research and theoretical analysis from the perspective of security design and risk analysis.It provides scientific basis for safety analysis,accident prevention and operation control of liquid lithium.The main research contents and conclusions are as follows:(1)A visual experiment is carried out to study lithium droplets coolant interaction.The typical phenomena of interaction include peak pressure of explosion and peak pressure of chemical reaction.The peak pressure of explosion is great and the mechanical load generated by shock wave is the important threat to the structural integrity of the device.The influence of initial lithium temperature,initial coolant temperature and initial lithium mass has been studied.The liquid lithium could be fragmented after direct contact with the coolant when the initial temperature of the liquid lithium is equal to or greater than 300?.The contact area increases exponentially and eventually forms a violent explosion,which mainly divided into four stages: formation of gas film,propagation,explosion and droplet dispersion.The fragmentation of lithium droplet occurred in the stage of propagation is the key foundation for violent explosion.(2)Combining the experimental result of lithium droplet coolant interaction,the pressure evaluation methods of the lithium droplet coolant interaction are established separately based on shock wave theory,Back-Propagation neural network and genetic algorithm optimized BP neural network.The relationship between the peak pressure of explosion and the initial temperature of liquid lithium,initial temperature of coolant and initial mass of liquid lithium is studied.The calculation results are in good agreement with the experimental results.The feasibility and effectiveness of the rapid assessment method for liquid lithium coolant interaction under different accident conditions are verified.(3)The chemical reaction and fragmentation mechanism of liquid lithium coolant interaction are analyzed based on the coarse mixing theory for steam explosion of fuel coolant interaction.The hydrogen gas produced by the chemical reaction is dissolved in molten liquid lithium.The rapid cooling of the lithium droplet surface causes the excessive saturation of dissolved hydrogen in the thermal boundary layer,resulting in the pressurizing of the thermal boundary layer on the surface of the molten lithium droplet.The liquid lithium in the thermal boundary layer is fragmented by the quick release of dissolved hydrogen.The tiny particles of lithium are stripped off under high pressure gases and into the surrounding steam-water mixture.The lithium particles formed by fragmentation give the interaction area an instant increase,and the heat released by rapid chemical reaction leads to an increase in energy release.The physical explosion provides the reaction condition for the rapid chemical explosion,which eventually results in a violent explosion.The mechanical energy release model for thermal-chemical explosion of liquid lithium coolant interaction in unit mass is established based on coarse mixing theory and analysis of the fragmentation mechanism for lithium droplet.The calculation results of peak pressure are in good agreement with experimental results of lithium droplet coolant interaction.The model reflects the energy release and explosion intensity of interaction for different initial lithium temperature,initial lithium mass and initial coolant temperature.(4)The confirmatory experiment of lithium column coolant interaction is conducted.The explosive law of lithium column experiment is consistent with the explosion rule of the small lithium droplet coolant interaction and is mainly divided into five stages: coarse mixing,formation of gas film,propagation,explosion and droplet dispersion.However,the explosion of larger scale experiment of lithium column is more intense,and the peak pressure of explosion is far greater than chemical reaction.The mechanical energy release model for thermal-chemical explosion is in good agreement with the experimental result.It can be extrapolated from experiments of lithium droplet coolant interaction to experiments of lithium column coolant interaction.It can be more effective to carry out the assessment for the explosion strength of liquid lithium coolant interaction in different scale and accident conditions,providing mechanism model for safety assessment and calculation analysis program of fusion device.