| The fusion reaction of Deuterium(D)and Tritium(T)is expected to be one of the key solutions to the ultimate energy problem.In order to achieve this goal,at present,the ITER of the world’s largest magnetic confinement Fusion facility,CFETR,and all kinds of Demonstration reactors are working hard to solve some physical,Engineering and material problems before fusion reactor power generation,this includes the problem of tritium fuel self-sustaining.D is very rich in seawater and easy to extract and almost inexhaustible.However,the content of T in nature is very rare,and the existing methods cannot meet the demand for T for the D-T fusion reactor.So,the D-T fusion power needs to be self-sustaining through neutron transmutation with the lithium-containing breeder in the tritium breeder blanket.Since the 1980s,solid-state tritium breeder materials have become a hot research topic because of its good chemical stability and without magnetohydrodynamics(MHD)effect compared with liquid-state tritium breeder materials.As the core of a fusion reactor,the tritium breeder must possess excellent structure and thermal conductivity to resist the influence of high temperature and irradiation.The blanket of a fusion reactor is irradiated by high-energy particles,including neutrons,γ-ray,and various transmutation elements.After irradiation,the microstructure and thermal properties of tritium breeder material will be changed to some extent.The formation and annihilation of irradiation defects have significant effects on T release and thermal conductivity.Because γ-ray irradiation has only electron excitation process,it produces defects with uniform spatial distribution.Firstly,the isochronous and isothermal annealing of γ-ray irradiated Li2TiO3,Li4SiO4,and Li2TiO3-Li4SiO4 pebbles were studied.To avoid the effect of spatial microstructure,xLi2TiO3-(1-x)Li4SiO4(x=0.25,0.5,0.75)powder samples were also studied.The annihilation rate constants and activation energy of the irradiated defects E’-center and O--center are obtained.The results show that Li2TiO3 has higher radiation damage resistance than Li4SiO4.(2)In order to analyze the effect of irradiation defects on the thermal conductivity of Li2TiO3,the experiments of y-ray irradiation of Li2TiO3 were carried out.When the irradiation dose reaches 200 kGy,the thermal conductivity of Li2TiO3 is decreased by the irradiation of E’-center defects,and the increase of thermal conductivity is mainly caused by the addition of Ti3+.Due to the low irradiation dose,the thermal conductivity is only reduced to a small extent.(3)In order to avoid the influence of foreign particles on the research problem,the 800 keV self-ion O(irradiation dose of 3×1016ions/cm2)was used to irradiate Li2TiO3.The results show that the materials are damaged and appear locally disordered.Positron Annihilation Lifetime Spectroscopy(PALS)combined with first principles has successfully predicted the Lifetime of Li and Ti vacancies.XPS showed that the peak intensity of Ti 2p decreased with the increase of irradiation dose,which was related to the irradiation damage of the Li2TiO3 surface and the carbon pollution caused by the carbon recoil in the vacuum during O injection.TEM results showed that the samples after irradiation have some degree of amorphous,crystal plane spacing increases.The measurement of thermal conductivity by TDTR showed that the thermal conductivity decreased by an order of magnitude after irradiation(before irradiation:4.137 W/m ·K;after irradiation:0.2042 W/m·K).Practical measures must be taken in advance to minimize the potential harm of reduced thermal conductivity.The neutronics-thermohydraulic coupling calculation results show that the maximum service temperature of the breeder is about 1173 K.In order to evaluate the service performance of the tritium breeder.(1)The microstructure and thermal conductivity of tritium breeder were analyzed at 1173 K and He+ 0.1%H2 for 1000 h.The surface morphology and microstructure of Li2TiO3 had no obvious change,while Li4SiO4 had obvious change.XRD and Raman spectra showed that the crystal structure was disturbed.It is proved that the phase decomposition of Li4SiO4 occurs and the content of the second phase Li2SiO3 increases with the increase of annealing time.Electronic Paramagnetic Resonance(EPR)detected no defects associated with unpaired electrons.With the increase of annealing time,the thermal conductivity of Li2TiO3 remained stable and Li4SiO4 decreased obviously,mainly due to the structure change and phase decomposition during annealing.The results show that the thermal conductivity of Li2TiO3 is stable,while Li4SiO4 is unstable at high temperatures.The experiments of Li2TiO3,Li4SiO4,and Li2TiO3-Li4SiO4 under He+0.1%H2 for 400 h were also carried out in order to clarify the relationship between microstructure and thermal conductivity at actual service temperature.Due to short annealing time,the changes of micro structure and thermal conductivity are not obvious.More long-term annealing experiments will be carried out.First-principles density function theory does not rely on any empirical parameters,according to the material’s crystal information file,we can get a variety of material thermodynamic performance data.At present,there are few data on the thermal conductivity of new tritium breeder materials.In addition,different preparation processes of tritium breeder materials result in the different microstructures of materials,even though different thermal performance test methods are used in the same process,such as thermal conductivity test Hot Disk Method,Laser Flash Method,3ω and so on,will also have a significant impact on thermal conductivity and other thermal performance test data.The complexity and uncertainty of experiments bring different research costs to fusion materials scientists.This method was used to evaluate the thermal properties of tritium breeder materials,and it was verified that the thermal conductivity and bulk modulus of Li4SiO4 were increased by adding Al element.The existence of particle-like and wave-like heat transfer mechanisms in tritium breeder materials at higher temperatures successfully explains why the thermal conductivity obtained by the Phonon Gas Model(PGM)is lower than the experimental value.Thermal performance data can provide data support for the design of tritium breeding blankets for fusion reactors in the future.Infrared and Raman data can provide data reference for the study of fusion material fundamental science.As neutron multiplication materials are in contact with tritium breeder in blankets,thermal performance data is also a key component of advanced blanket material. |
