Core Physics Study Of Lithium Heat-pipe Cooled Space Reactor

Space Nuclear Reactor has many superior performance,such as wide power coverage,small specific mass under high power conditions,long operating lifetime,good environmental adaptability,and independence on sunlight.Space nuclear reactor has become the most ideal power system for space exploration.As one of the space reactor types,the heat-pipe cooled reactor has the advantages of passive heat transfer technology,compact structure and inherent safety.It has been greatly favored by domestic and foreign experts.The working fluids of heat pipe used in the heat-pipe cooled space reactors are mainly alkali metals such as lithium,sodium and potassium.Owing to the low density and good heat transfer performance of lithium,the lithium heat-pipe cooled space reactor has become one of the main reactor types.Comparing with traditional liquid metals cooled reactors,the lithium heat pipe cooled reactors have significant differences in the core structure,core temperature,core material,coolant system,etc.Therefore,the physical and safety performance of the reactor core is quite different from that of conventional nuclear reactors.At present,researches on lithium heat-pipe cooled reactors are at the stage of conceptual design.Based on the investigation of space nuclear reactor worldwide,we focus on topics including core material selection,launch drop safety analysis and reactivity control design to study the neutronics of the lithium heat-pipe cooled reactor.The main contents for this thesis are as follows:1)Analysis of the key factors and core design of lithium heat pipe cooled space reactor.In this paper,in order to improve reactor safety and neutron economy of the reactor,the analysis focuses on the different types of fuels,structural materials,7Li enrichment,etc.the key parameters of the reactor core of the lithium heat pipe cooled space reactor are studied.Based on that result,a new lithium heat pipe cooled reactor core with a power of 1 MWt and a lifetime of 10 years was designed.The results show that PWC-11 and Nb-1Zr have the best neutron economy and safety properties when the operation temperature of reactor core is less than 1500 K.Moreover,when the operation temperature of reactor core is above 1500 K,the Mo-14Re is the best choice.Increase the concentration of the 7Li in the coolant and the 15N in the UN fuel can enhance the neutron economy of reactor,but the void reactivity of reactor core is slightly increased.2)Drop critical safety characteristics of lithium heat pipe cooled reactor.In order to solve the critical safety issue of space reactors in a launch abort accident,the spectralabsorption materials such as 155Gd,157Gd,151Eu and 149Sm are added to the fuel and smeared outside the reactor vessel,respectively.The effects of the spectral absorption materials on the critical safety characteristics and the physical characteristics of the lithium heat pipe cooled reactor are analyzed.The research shows that 157Gd is the most effective spectral shift absorbing material.Adding 0.5%157Gd to the fuel can effectively improve the critical safety characteristics of the lithium heat pipe cooled reactor,and the value of the excess reactivity penalties in the normal operation is the smallest.3)Reactivity control method of the lithium heat pipe cooled reactor.Two ex-core reactivity control options based on rotating drums and sliding segments in the radial reflector are studied.The effects of the key parameters on the reactivity swing are analyzed.Based on the analysis results,two reactivity control schemes are designed and the physical characteristics of the lithium heat pipe cooled reactor are analyzed based on the optimized reactivity control schemes.The study shows that the increase of the B4C absorber thickness and the 10B content can raise the reactivity swing of the rotating drums,and the value of-the reactivity swing is the highest when the wrap angle of B4C absorber is 120°.The variation of the reactivity is nonlinear with the rotation angle of the rotating drums.The change in the drum rotation angle has no obvious influence on the axial power distribution,while it has a great influence on the radial power distribution.For sliding segments in the radial reflector,the movement near the center of the reactor core can obtain both the great the value of reactivity swing and a shorte moving distance.The analysis of the optimized design shows that the moving distance of sliding segments in the radial reflector is linear with the reactivity,which has a great influence on the axial and radial power distribution of the reactor.The results of this thesis provide references for material selection,safety research and reactive control design of the lithium heat pipe cooled reactor.