| Nuclear power is a clean,low-carbon and high-energy density of strategic energy,which is an important part of modern energy system.In nuclear energy system,nuclear fuel is considered as the cornerstone for nuclear energy development and the "granary" for nuclear energy safety.Performances of nuclear fuel determine the advanced nature,economy and security of nuclear power.After the "311" Fukushima nuclear disaster,Accident Tolerant Fuel(ATF)has become a hot issue to improve the safety of nuclear fuel under severe accident conditions and the economy under normal conditions.Development of ATF is currently considered as a great reform for current nuclear plants during more than 60 years.The UO2-Mo fuel system investigated in this paper is an important candidate fuel in ATF.The mature manufacturing process of such fuel and its relationship between microstructure and performance has not been well mastered.The research focuses on the methods to improve the thermal property of UO2-Mo fuel by microstructure design.Guided by the idea of material genome,a high-throughput powder metallurgy preparation method of ceramic fuel are developed,which is suitable for UO2-Mo fuel with Mo dispersion.UO2-Mo fuel with Mo continuous distribution and radial thermal conductivity enhancement,and UO2-Mo-SiCw fuel are fabricated by spark plasma sintering(SPS).The thermal conductivity,coefficient of thermal expansion,modulus of elasticity,hardness and fracture toughness of the fuel are tested under different preparation conditions.The relationship between fuel preparation technology,microstructure and performance is studied systematically,and the sintering densification behavior of fuel is revealed.The interface characteristics of UO2-Mo fuel are characterized by AES and HRTEM.The thermal resistance of UO2-Mo interface is calculated based on the scattering mismatch model(DMM)and acoustic mismatch model(AMM).The residual stress in the UO2-Mo samples are quantitatively investigated by a residual stress neutron diffractometer(RSND)located at the China Mianyang Research Reactor(CMRR).The fabrciated UO2-Mo fuel pellets with independent intellectual property rights satisfy the requirements of research reactor assessment.The principal research contents and conclusions are as follows:(1)The high-throughput system of multi-component composite materials is designed and developed.The high-throughput system of multi-component composite materials was developed based on the design idea of "continuous filling+ batch forming+integrated sintering".According to the above design principle and scheme,a continuous filling system suitable for UO2-Mo fuel with with the Mo dispersion was integrated,which takes only 20-30 min.It has been applied to the preparation of 100 samples/batch of three-component ceramic samples.The high-throughput preparation system can be extended to the development of multi-component composite materials,which provides favorable conditions for high-throughput analysis and characterization of multi-component composite materials.(2)UO2-Mo fuel with Mo continuous distribution is designed and fabricated.The particle size and content of Mo powder have significant influence on the microstructure and properties of UO2-Mo.nano-Mo is easier to form core-shell UO2@Mo powder with microsphere of UO2 than micro-Mo.The continuity of Mo in UO2 is better in the final state.The thermal conductivity of UO2-nano-Mo is slightly higher and the coefficient of thermal expansion is lower compared to UO2-micro-Mo.With the increase of Mo content,the microstructure of UO2-Mo changes from microcrack-free state to microcrack state,and the number of microcrack and propagation degree increase significantly,and the thermal conductivity increases.The optimal thermal conductivity(1273 K)of UO2-10vol%Mo is 82.6%higher than that of pure UO2.The coordinative effect of internal stress and Mo continuous distribution structure on the thermal expansion of UO2-Mo is significantly higher than that of Mo volume fraction.Turner model predicted the thermal expansion behavior of UO2-Mo closer to the experimental value,indicating that the interaction between the components can not be ignored.The coating time is the crucial factor in coating technology,which determine the coating efficiency,final microstructure and thermal properties.The thermal conductivity of UO2-Mo increases by 6.9%,17.2%,8.95%,8.5%and 4.65%(1273K),respectively,since the coating time is t=10min,0.5h,2h and 4h,and the dispersion distribution of Mo.Such result is consistent with the equivalent thermal conductivity and the characterized microstructure of Mo channel in various coating conditions.Sintering temperature is the crucial process parameter,which determines the relative density of UO2-Mo.And hence the requirements of the reactor for the designed density of UO2-Mo fuel can be sastified by adjusting the sintering temperature.(3)A novel type of UO2-Mo fuel with radial thermal conductivity enhanced is designed and fabricated.A novel type of UO2-Mo with radial thermal conductivity enhanced is designed and fabricated to improve the radial thermal conductivity with the most concerned for the reactor.Mo is distributed in the form of mesh Mo along the radial direction.radial thermal conductivity of UO2-10vol%Mo and UO2-6vol%Mo are more than three times and two times of pure UO2(873-1673 K),respectively.The excellent radial thermal conductivity and the increased axial thermal conductivityof UO2-Mo provide a new method for the design of UO2-Mo fuel with directional thermal conductivity.(4)A ternary system of UO2-Mo-SiCw fuel is designed and fabricated.A ternary system of UO2-Mo-SiCw fuel is designed and fabricated by SPS.In the SPS technics of 1623 K/10 min/30 MPa,the relative density and strength of fuel can satisfy the design requirements of nuclear reactor.The residual stress microcracks are restrained significantly with incorporation of a small amount of SiCw.The effect of SiCw on UO2-Mo microcracks is the combination of deflection and bridging of microcracks to enhance the resistance of micorcracks growth and extend the path of crack growth.The fracture toughness of UO2-4Mo composites with incorporation of 0.5vol%SiCw is 0.91 MPa·m1/2,which increased by 68.5%.The thermal conductivity of UO2-4Mo-xSiCw and UO2-10vol%Mo-xSiCw are 1.5-1.6 times(1273 K)and 1.8-2.1 times(1473-1673 K)of the pure UO2.(5)The interfacial characteristics and thermal resistance of UO2-Mo fuel are investigated.The interface characteristics are investigated by AES and HRTEM.There is no significant interfacial product at the phase interface of UO2-Mo.A group of phase interface orientation relations in UO2-Mo are determined as(220)UO2/(110)Mowith a interface mismatch ratio ?(UO2-Mo)=0.117.According to the relationship between interface structure and orientation,the interface thermal resistance of UO2-Mo is calculated by DMM and AMM theory.The magnitude of the interfacial thermal resistance of UO2-Mo based on AMM theory is the same as that of DMM considering total phonon dispersion,and the magnitude of the interfacial thermal resistance is 10-9m2K/W.(6)The residual stress distribution of UO2-Mo fule is investigated quantitatively.The residual stress induced by coefficient of thermal expansion mismatch of UO2-Mo is calculated by FEM simulation.The residual stress in UO2-Mo caused by the mismatch of coefficient of thermal expansion increases significantly with the increase of Mo content.The average residual stress increased from 22.67 MPa to 111.05 MPa as Mo content increased from 2vol%to 10vol%.The results of FEM residual stress nephogram show that the residual stress has the maximum level at the intersection of UO2 microspheres,where is presumably the source of UO2 microcracks.The microcracks propagate induced by the residual stress,which is consistent with the actual microcracks distribution in the UO2-Mo fuel pellets.The residual stress of UO2-Mo is quantitatively analysed by a RSND located at the CMRR.In UO2-Mo fuel,the state of residual stress acting on UO2 matrix is tensile stress.UO2-10 vol%Mo fuel has the maximum value of 148 ± 15 MPa.The variation trend of residual stress with Mo content is consistent with FEM calculation results.The micro-cracks produced in the high-Mo content composites can be explained by the results of the neutron diffraction investigation.These results could provide significant guidance for the manufacturing and improvement of the operational performance of UO2-Mo composites as a next-generation fuel.In this paper,UO2-Mo fuel pellets are investigated for reactor assessment by material genomic thought.The crucial of technology fabricaion and microstructure regulation,densification behavior,interfacial characteristics and thermal resistance,and residual stress distribution in UO2-Mo fuel were determined.Such results provide the significant data and technical foundation for irradiation assessment and the engineering application.Futhermore,the developed radial theram conductivity enhanced fuel and ternary system UO2 based fuel system show excellent thermal conductivity and application prospects,and are expected to be an optimized candidate fuel system as an ATF. |
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