Microstructure Regulation And Damage Performance Repair Of Nuclear Fuel Cladding Materials Under Pulsed Electric Current

Nuclear fuel cladding material is known as the first safety barrier of nuclear reactors,which is related to the safety and economy of nuclear reactor operation.In the service environment,harmful microstructure of multi-type,multi-scale and multi-configuration are formed in the nuclear fuel cladding material,resulting in the degradation of material performance,thereby seriously affecting the operation safety of nuclear reactors.It is urgent to seek an efficient and energy-saving treatment method to repair the degradation performance of nuclear fuel cladding materials by regulating harmful microstructures,which is crucial for the safe and stable operation of nuclear power equipment.This paper focuses on the "pain points" that restrict the service safety of nuclear fuel cladding materials,and solves the key scientific problem of "the regulation mechanism of multi-type,multi-scale and multi-configuration microstructure of nuclear fuel cladding materials under pulsed electric current and its damage performance repair correlation",and accurately regulates irradiation defects,brittle precipitation phases and heterogeneous interfaces,so as to realize the repair of material damage properties.In the irradiated environment of service,the dislocation loops generated in the zirconium alloy cause the material to harden,while the amorphous transformation of the second phase particles in the zirconium alloy affects the corrosion resistance.The generation of high-density dislocation loops and the amorphous transition of the second phase of Zr-4 alloy were induced by ion irradiation,and the pulse current treatment was carried out.The results show that at 450℃,the pulse current treatment accelerates the annihilation of the dislocation loops in the irradiated material,significantly reduces the number density of the dislocation loops,and restores the mechanical properties of the alloy.Pulsed current treatment at 450℃ can cause the atoms in the amorphous second phase to be arranged in an orderly manner,and the structure can achieve rapid crystallization transition,which is about 300℃ lower than conventional heat treatment(750℃).Molecular dynamics is used to simulate the evolution of the dislocation loop,and the results show that the pulse current enhances the local potential energy of the dislocation loops,accelerates the diffusion of atoms in the dislocation loop region to annihilate it.Based on the difference in electrical properties between the microstructure and the matrix,the numerical calculation results show that the free energy introduced by the pulse current breaks the thermodynamic barrier of the crystallization transition of the second phase,significantly reduces the activation energy of the recrystallization transition,thereby repairing the performance degradation caused by irradiation damage.Zirconium alloy absorbs hydrogen to form embrittlement hydrides when the water corrosion reaction occurs during the service process,which is one of the main reasons for the deterioration of the mechanical properties of zirconium alloy.The hydrogen-embrittled material was obtained by hydrogenation of nucleargrade Zr-4 alloy by gaseous hydrogen permeation,which were then subjected to pulsed current treatment.The results show that the pulse current can greatly reduce the large-size hydrides in the zirconium alloy,drive the hydrides from the brittle slat-like 8 phase to the small-size ζ phase,significantly reduce the hydrogen content,and basically restore the mechanical properties of the alloy.However,no dissolution of brittle hydrides was found in the comparative experiment of isothermal heat treatment at 400℃,and its mechanical properties were not restored.A mathematical model is established for the characteristics and morphological changes of strip hydrides,and the current density distribution and free energy introduced by current in the evolution process of microstructure are calculated and analyzed.The calculation results show that the free energy introduced by the pulse current promotes the decomposition of hydride,reduces the hydrogen atom diffusion activation energy,accelerates the hydrogen atom diffusion and desorption,and realizes the repair of the mechanical properties of hydrogen-charged zirconium alloy.Due to its excellent high-temperature oxidation resistance,FeCrAl alloy has become one of the most promising alternative materials for in-service zirconium alloys.However,FeCrAl alloy undergoes spinodal decomposition in the service environment,resulting in chromium enrichment α’ precipitated phase,resulting in embrittlement of material properties,which is directly related to the safe operation of the reactor.The results show that the pulse current can eliminate the brittle α’phase in FeCrAl alloy and significantly reduce the critical temperature of dissolution of the precipitated phase.Theoretical analysis indicates that in terms of thermodynamics,the free energy introduced by pulsed current reduces the thermodynamic barrier of precipitation phase dissolution.In terms of kinetics,under the coupling effect of concentration gradient and electromigration effect,pulse current treatment significantly enhanced the diffusion of atoms in the precipitation phase,accelerated the dissolution of the precipitation phase,and its dissolution efficiency was an order of magnitude higher than that of conventional heat treatment.The poor adhesion of Zr/Cr heterogeneous interface restricts the structural stability of the candidate clad Cr coated zirconium alloy during the hightemperature oxidation process under accident conditions.To solve this problem,the influence of pulsed current on the diffusion behavior of atoms at the Zr/Cr interface was studied.The results show that the pulse current can significantly reduce the formation temperature of the interfacial diffusion layer,and the treatment temperature is reduced by about 300℃ compared with the traditional heat treatment.The reduction of the treatment temperature will effectively avoid the degradation of the zirconium alloy matrix performance.Theoretical analysis points out that pulsed current reduces the activation energy of the interfacial diffusion layer between the coating and the matrix,improves the diffusion coefficient of the interfacial atoms,greatly increases the diffusion rate of atoms,and realizes the high-quality metallurgical combination of the Zr/Cr interface.In summary,this paper is based on the innovative method of pulsed electric current regulation of microstructures,and uses methods such as structural characterization,performance testing,and numerical calculation to deeply study the inherent mechanism of differential regulation of multiple types,scales,and configurations of microstructures by pulsed current and its impact on atomic diffusion behavior.It provides new ideas for the structural design and performance optimization of nuclear fuel cladding materials.