Simulation Study Of The Linear Mercury Diffusion Pump For The Exhaust Gas Pumping System In Fusion Reactor

The fuel cycle is an important part of the fusion reactor.The traditional fuel cycle architecture used by ITER has the problem of high tritium inventory.In the design of DEMO,considering the security and economy,the Direct Internal Recycling(DIR)concept is proposed to reduce the tritium inventory as much as possible.As the technical implementation of the exhaust gas pumping system of DIR in fusion reactor,the KALPUREX(Karlsruhe liquid metal based pumping process for fusion reactor exhaust gases)is proposed by Day et al.of KIT(Karlsruher Institut fur Technologie).One of the important improvements is the use of continuous linear mercury diffusion pump(LMDP)as the main pump instead of the traditional cryogenic pump.The LMDP adopts a rectangular pump body design and a linear nozzle arrangement,which can better match the pumping port of the fusion reactor.At the same time,mercury,which is compatible with tritium,is used as the working fluid.In order to support the design and optimization of the LMDP,the numerical simulation study based on the Direct Simulation Monte Carlo(DSMC)method is carried out in this thesis.According to the LMDP structure in the early experiments,the DSMC simulation model is established by using the dsmcFoam solver in the OpenFOAM platform.Considering that the mass flow at the inlet is the controlled variable in the experiment,a fixed mass-flow-rate boundary model is developed.By monitoring the mass flow rate at a given position and feedback controlling the bulk velocity of the pumped gas at the inlet(assuming the gas at the inlet is in thermal equilibrium),a given mass flow rate at the inlet of the LMDP can be obtained in the simulation.On the other hand,in view of the significant difference in the molecular number density of the pumped gas and mercury vapor stream in the simulation system,based on the traditional no time counter(NTC)sampling method,a collision sampling correction formula of the binary molecule system is developed.Using the correction formula,FN(the number of real gas molecules represented by a single simulated particle)can be set for different gases,which effectively improves the calculation efficiency.In order to ensure the reasonableness of the calculation results,FN1/FN2≤3 is required when using the correction formula(where the subscript 1 represents mercury vapor,and 2 represents different types of pumped gas).When the pressure of air at the monitoring position in the pump is in the range of 1×10-2～2×10-2 Pa,the pumping speed obtained by the simulation is consistent with the experiment.The deviation of the simulation results at higher and lower pressures may be caused by the incomplete diffusion of the air at the inlet in the experiment.Based on the DSMC simulation model established in this thesis,the pumping performance of the LMDP for the air and hydrogen(as a represent of the hydrogen isotope),helium,neon,and argon,which may be contained in the exhaust gas of fusion reactor,is simulated using freestream inlet condition when the inlet pressure is in the range of 0.01～1 Pa.For the lowest inlet pressure,the LMDP has the highest pumping speed,hydrogen～63 m3/s,helium～52 m3/s,neon～27 m3/s,air～20 m3/s,and argon～16 m3/s.The simulation results is consistent with the theoretically predicted pumping speed which is inversely proportional to the square root of the molar mass of the pumped gas.As the pressure of the pumped gas increases,the pumping speed first decreases and then increases,while the working state of the LMDP changes from a diffusion pump to a vapor jet pump.When the inlet pressure is at the lowest,the mercury back-streaming rate reaches its maximum,about 10-2 mg/(min·cm2).As the pressure of the pumped gas increases,the back-streaming rate continues to decrease,resulting from that the increase in the number density of pumped gas molecules hinders the diffusion of mercury particles above the first-stage nozzle to the inlet.