First Principles Study Of Tritium Diffusion Mechanism In Solid Breeder Materials

At present,the shortage of tritium is one of key issues for the use of nuclear fusion energy.In order to produce tritium,it is expedient to generate it in situ from the transmutation of lithium（Li）.The lithium-containing breeder materials which could produce tritium by neutron irradiation mainly include liquid and solid materials.In contrast with the liquid materials,the solid breeder materials have received considerable attention and have become a hot topic in recent years because of their favorable properties such as low chemical reactivity and easy extraction of tritium.The solid breeder materials are generally ternary lithium-containing ceramic materials such as Li₂ZrO₃,LiAlO₂,and Li₂TiO₃.The diffusion and release behavior of tritium in the solid breeder materials,which finally decide the tritium productivity,are the most important factors for the application of materials.However,the diffusion process of tritium in breeder materials is rather complicated,including tritium adsorption in defects,diffusion in crystals,reaction with surface etc.,where the diffusion mechanism is still unclear.In this work,a systematic study of the diffusion behavior of a tritium atom in the solid breeder materials,such as Li₂ZrO₃,Li₂TiO₃ and Li₃TaO₄,is investigated using the first principles method.The diffusion behaviors including tritium diffusing inside the perfect crystal,inside the lithium vacancy and in the defect complexes are studied in detail,while the activation energy and diffusion coefficient are determined accordingly.Defect models with different lithium vacancies of Li₂ZrO₃,Li₂TiO₃ and Li₃TaO₄crystals are built up and calculated firstly.The obtained lattice parameters after structure optimization are consistent with the related experimental results,which prove the reliability of our models and method.Moreover,the formation energy of several types of Li vacancies with different charge state is studied and it is found that the most stable state is a Li vacancy with one electron.This indicates that the intrinsic Li defect of the studied breeder materials is a negatively charged defect,which is consistent with previous results.Based on this,the tritium adsorption energy in various Li defects is calculated and analyzed.Each Li vacancy is constituted an octahedral configuration by six neighboring oxygen（O）atoms with dangling bonds,which resulting in six adsorption sites inside the vacancy.And the calculations show that the most stable defect model of a tritium adsorbed in the Li vacancy is electrically neutral.Further analyzing the adsorption energy difference of each adsorption site,it is related to the mean Li-O bond length around the vacancy in Li₂Zr O₃ and Li₂TiO₃,while in Li₃TaO₄,it is not only related to the Li-O bond length but also to the Ta-O interaction around the vacancy.In contrast with Helium（He）atom,the Li vacancy in studied breeder materials has a stronger adsorption capacity for tritium,which shows that the total energy of the system with an adsorbed tritium atom could be reduced by 4.0 eV.The adsorption capacity for He is relatively weak due to the low chemical activity of He atom.Thus He is predicted to migrate easily from the solid breeder materials.The diffusion behavior of tritium inside Li vacancies of Li₂TiO₃ and Li₃TaO₄ crystals is explored after the study of the adsorption properties.The activation energy（namely diffusion energy barrier）for tritium diffusing inside the Li vacancy of Li₂TiO₃ crystal is less than 0.85 eV,while in that of Li₃TaO₄ crystal,it is no more than 0.72 eV.Based on the analysis of the local density of states,it can be clarified that the activation energy is depending on the T-O interactions.Generally,the activation energy is higher with longer diffusion path and vice versa.The further calculations present that the maximum diffusion coefficient at 600K of tritium diffusing in the Li vacancies is 1.98×10^-11m²/s in Li₂TiO₃ and 4.77×10^-10m²/s in Li₃TaO₄ crystal.This fact implies that tritium is easier to diffuse inside the Li vacancy of the Li₃TaO₄ crystal.The diffusion behavior of tritium in the perfect and defect model of Li₂TiO₃ and Li₃TaO₄ crystals are studied as well.When tritium diffuses in the Li₂TiO₃ crystal with a Li vacancy,the minimum activation energy was 0.76 eV in various diffusion pathways.And the minimum activation energy was 0.51 eV in the corresponding Li₃TaO₄ crystals.During the diffusion process,the activation energy is highest when tritium atom escapes from the trap of Li vacancy into the crystal.Therefore,it is predicted that the presence of Li vacancy is a major factor affecting the tritium diffusion in the studied breeder materials.A completed simulation model is finally set up to explain the tritium diffusion behavior in the studied breeder materials with Li vacancy.Such model mainly includes three processes:tritium diffusion inside the vacancy,tritium escaping from the trap of vacancy,tritium diffusion in the crystal.The proposed theoretical model can provide theoretical foundation for predicting new solid breeder materials,and is beneficial to the application of related breeder materials.