Research On The Dynamic Energy Conversion System For Space Nuclear Reactor

The space nuclear reactor now is the potential choice as the future space energy source because of its high energy density and long life.Energy conversion system is one of the most important systems for space nuclear reactor,and its main function is to transfrom the fission power from reactor to the electricity.For the special working environment in space,the space energy conversion system must have high conversion efficiency and compact configuration.Furthemore,several special requirements are limited to the system's design and operation: small mass and volume,high stability,adoptation to the frequent changes of the off-design conditions.The closed Brayton cycle is ideal method to satisfy these requirements and has promising prospect in space.A new closed Brayton cycle is proposed based on improved land-use Brayton cycle system to accommodate the special working environment: Firstly,the working fluid is switched from helium to Noble-Gas binary mixture of helium and xenon,which would lead to a reduction of system thermodynamic property and a promotion of aerodynamic property,lowers the stage number of turbomachine and gains the goal of decreasing the number of compressors and heat exchangers.Secondly,to avoid adding extra cooling system,a bleeding cooling line is set to cool the rotor and the generator.Based on the property analysis of Noble-Gas binary mixture of helium and xenon,thermodynamic analysis and aerodynamic analysis are made at the same time to investigate the influence of fluid's molar ratio changing to the system efficiency and features of the turbo-machines.The results show that as the molar ratio of xenon increasing 10%,the system efficiency decreases by 20% averagely,and the specific work decreases by 24% averagely.On the other hand,the stage numbers of the turbo-machines decrease sharply when the proportion of xenon molar increases.Just adding 10% xenon,the stage number of the compressor decreases to less than 10.For space reactor,it's necessary to change the working fluid into the Noble-Gas binary mixture of helium and xenon.Considering the comprehensive analysis,the best molar ratio of the xenon is 9.6%.According to the thermodynamic analysis,another optimization method: specific power optimization is presented.A comparison is made between the specific power optimization and traditional optimization.Based on the parameters from thermodynamic analysis,engineering designs of the key components(compressor,turbine,ducting,recuperator,gas cooler,radiator)are made to estimate the structure and the mass of the whole system,and analyzes the feasibility of the space Brayton cycle system.Considering the special operating environment of space Brayton cycle system,this paper analyzes the off-design performance of the space Brayton cycle system.The model of space nuclear closed Brayton cycle was established and the available control methods,including inventory control,bleed control and rotating speed control,are proposed.It is found that inventory control can keep the rotor speed constant when the electric load changes in a large scope,bleed control is a better choice when load fluctuated in small range.Rotating speed control is a distinctive control mechanism for space Brayton cycle system: the changing of the rotating speed would cause a deviation from the design point and lower the output power to fit the changes of the electric load.Furthermore,the performance in load shedding condition is also investigated to find the effective controlling measure.The temperatures and pressures would increase sharply in load shedding condition,and the bleed control is an effective way to lower the rotating speed.With the appropriate control,the space Brayton cycle system can adapt all kinds of off-design condition.