The Effect Of Point Defects On Thermal Conductivity Of Cladding Materials

With the increasing demand for energy in the process of social development,the development and utilization of nuclear energy has a sustained improvement.However,several major accidents in the history of nuclear power plants have made the safe operation of nuclear power plants highly concerned,which further promote the continuous progress of nuclear power technology towards the direction of safer,more reliable and more sustainable.Thermal conductivity,as a physical quantity describing the thermal transport properties of materials,is an important parameter reflecting the conductive ability of heat.The thermal conductivity of fuel cladding materials in a reactor has a great influence on the energy transfer efficiency of the nuclear reactions of nuclear fuel and the overall safety of the reactor.The cladding materials have been continuously in extreme conditions of high temperature and strong irradiation and the thermal conductivity of the cladding is affected by defects caused by the damage of the structure under irradiation.Therefore,the study of thermal transport properties of cladding materials and the effect of defect structure on thermal conductivity are of great significance to the safety of the nuclear reactor.In this paper,the electronic thermal conductivity and phonon thermal conductivity of zirconium cladding materials which are widely used in traditional light water reactor are calculated by applying the first principles electronic structure to the semi-classical Boltzmann transport equation under the relaxation time approximation and molecular dynamics method respectively.Molecular dynamics method is also used to simulate the thermal conductivity of silicon carbide,which is the main candidate cladding material with many excellent properties and wide application prospects for the fourth generation nuclear power plants.Furthermore the thermal conductivity influenced by point defect that is the most common defect structure is analyzed comprehensively.As a semiconductor material,silicon carbide also has a great development prospect in the field of semiconductor.The thermal conductivity of silicon that has a similar structure ofβ-SiC and is the most widely used in the field of semiconductor,is also simulated for providing a reference for the method of thermal conductivity simulation of SiC,which can carry out a more comprehensive and systematic analysis on the influence of different temperatures,various types and concentrations of point defects on thermal conductivity.The simulation can as well explain the effect of various factors on the thermal conductivity from the micro level and deepen the understanding of the mechanism of macroscopic thermal conductivity from microscopic phonon scattering.The contents and achievements of this paper are as follows:（1）The bulk electronic thermal conductivity and phonon thermal conductivity ofα-Zr,which is a widely employed fuel cladding material in light water reactor,and the effects of different concentrations of vacancy defects on the electronic and phonon thermal conductivity at the coolant temperature of 600 K are simulated respectively.With certain relaxation time,the electronic thermal conductivity is calculated by applying the first principles electronic structure to the semi-classical Boltzmann transport equation under the relaxation time approximation.The phonon thermal conductivity is simulated by reverse non-equilibrium molecular dynamics method and the phonon thermal conductivity of bulkα-Zr can be extrapolated from the thermal conductivity of finite size cells to the infinite system according to the size effect relationship between the thermal conductivity and the size of the simulated system.The simulation results present that the total thermal conductivity of the sum of the electronic thermal conductivity and the phonon thermal conductivity is close to the experimental results.Because of the continuous increase of internal defects of cladding materials under the irradiation environment of reactor,the effects of different vacancy concentrations on the electronic and phonon thermal conductivity are investigated and analyzed by applying the two simulation methods above.The thermal conductivity decreases with the increase of the vacancy concentration gradually for both phonon and electronic cases.Vacancies have a more serious effect on phonon thermal conductivity than the effect on electronic thermal conductivity.Through the analysis of thermal resistance,it is found that both the relative additional thermal resistance of electrons and phonons caused by vacancies inα-Zr are proportional to the vacancy concentration considered.Furthermore,considering from microcosmic aspect the inverse relationship between the vacancy concentration and the mean free path of scattering interacted by both phonons and defects or electrons and defects is deduced from this macroscopic proportional relationship.The effects of vacancies on electronic thermal conductivity and phonon thermal conductivity are comprehensively compared and analyzed from the quantitative and microscopic perspective,and the changes of thermal conductivity of zirconium cladding material affected by vacancies are described more detailedly and accurately.（2）As an important semiconductor material and the main candidate material for future fission reactor and fusion reactor,the thermal conductivity of SiC is an extremely significant factor for the energy transfer efficiency and safety of the reactor.The phonon thermal conductivity ofβ-SiC is simulated by reverse non-equilibrium molecular dynamics method and by extrapolation the result of thermal conductivity of bulkβ-SiC is achieved.Since this method is the first time to apply to the thermal conductivity simulation ofβ-SiC,the feasibility and accuracy of this method are illustrated by the auxiliary research on the thermal conductivity of silicon which is the most widely used in industrial semiconductor materials with similar structure toβ-SiC.The influence of each parameter in this method on the simulation result of thermal conductivity is discussed in detail,and the current simulation method of thermal conductivity of SiC is expanded.（3）For the reason that the microstructure of SiC under irradiation mainly consists of a distribution of atomic scale point defects in the regime of reactor operating temperature,the effects of temperatures and types and concentrations of point defects on the phonon thermal conductivity ofβ-SiC are studied systematically by applying the reverse non-equilibrium molecular dynamics method with the consideration of the quantum correction of temperature and the formation energy of defects.The simulation results demonstrate that the thermal conductivity ofβ-SiC decreases with the increase of the concentration of point defects due to the scattering between phonon and defect.The thermal conductivity drops rapidly at low concentration condition and gradually reaches certain value at high concentration condition.The effect of different temperatures on the thermal conductivity decreases with the increase of the defect concentration.When the defect concentration is relatively high,the temperature has no significant effect on the thermal conductivity.By the analysis of thermal resistance,the additional thermal resistance is proportional to the concentration of all types of the point defect considered.Taking the slope of the proportional relationship as impact factor to judge the extent of point defect effect on thermal resistivity（thermal conductivity）,it is found by comparing the impact factor that the point defect type of interstitial of Si _TC and antisite of Si _C reduce the thermal conductivity to a rather considerable extent and destroy the heat conducting property greatly,while the type of interstitial of C_TSi and antisite of C_Si have a relatively small influence on the thermal conductivity.On the basis of relevant previous studies,the relationship that the macroscopic relative additional thermal resistance caused by point defects is essentially the ratio of the mean free path of phonon-defect scattering and phonon-phonon scattering from the microscopic perspective is further revealed.By this relation the effect of temperature and various types and concentrations of point defects on phonon scattering can be quantitatively analyzed the effect of point defect concentration on thermal conductivity of materials.For the first time,the effect of different vacancy concentrations on the thermal conductivity of the cladding material of zirconium is obtained more clearly with in-depth analysis.The influence of different types of point defects on the thermal conductivity of SiC cladding material is considered to comprehensively compare the mean free path of the scattering between phonons and different types of defects.From the microscopic perspective the effect of the scattering between phonons and defects on the thermal conductivity is achieved.Under the high temperature of reactor and in the range of high point defect concentration produced in irradiation environment,the additional thermal resistance and the relative additional thermal resistance of both the cladding materials are proportional to the point defect concentration.Forα-Zr,based on the comparison of the slope of the proportional relationship between the relative additional thermal resistance of phonons and electrons and the vacancy concentration,it is found that vacancies have a more serious effect on phonon thermal conductivity.