The Effects Of Lithium Components On Physical And Chemical Properties Of High Z First Wall Materials In Nuclear Fusion Plant

The tokamak device controls the plasma by generating a magnetic field,thus achieving a controlled nuclear fusion reaction.Plasma contains a variety of high-energy particles,such as neutrons,hydrogen ions and their isotopes,and helium ions.The first wall material of the tokamak device must sustain high-energy neutron irradiation,thermal load,ionizing radiation and the interaction between plasma and it.At present,the first wall materials are mainly selected as high atomic number(Z)metal elements:molybdenum and tungsten.Plasma-wall treatment technology is the key to improve the performance of high temperature plasma,which can inhibit plasma impurities,reduce particle boundary recirculation,and ensure the operation of high-parameter and long-pulse plasma.However,after the long-term using Li coatings on the first wall material of tokamak device,lithium atoms may be diffused along the grain boundary and defects and be solidly soluble in the first wall material.The existence of lithium impurity atoms may have certain influence on the performance of the first wall material,which is worth further investigation.Helium bubble or helium embrittlement is a common problem of the first wall materials,and there is no way to completely solve it.What is the influence of lithium impurity atoms on the behavior of helium atoms?In this paper,the microstructure and electronic structure of lithium atom in the first wall material(molybdenum and tungsten metals)and the interaction between lithium atom and helium atom are studied by using the first-principles calculation.And the influence of lithium and helium impurity atom on the mechanical and thermodynamic properties of molybdenum and tungsten crystal is further discussed.The results show that in molybdenum and tungsten metals,a single lithium atom tend to occupy a substitutional site.The migration barrier of lithium atoms in molybdenum and tungsten crystals is 0.19 eV and 0.40 eV,respectively,indicating that lithium atom is easy to diffuse.The second lithium or helium atom tends to occupy the interstitial sites,and the substitutional lithium has an attractive interaction with the interstitial lithium or helium,which means that the substitutional lithium can be used as a trap to capture other interstitial lithium or helium atoms.Substitutional lithium atoms are easier to trap in tungsten than in molybdenum,but capture range is larger for interstitial helium than for interstitial lithium.Multiple interstitial lithium or helium atoms can be clustered around the substitutional lithium atoms,resulting in increased local lithium or helium concentrations in molybdenum and tungsten crystals.By calculating the electronic structure of molybdenum and tungsten crystals(density of states and charge density diagrams),it can be seen that the crystal lattice of molybdenum and tungsten can be significantly distorted when the impurity atoms of lithium are solidly soluble in molybdenum and tungsten crystals.In order to explore the influences of long-term lithiation on the mechanical properties of the first wall material,the Voigt-Reuss-Hill average scheme was used to calculate the bulk elastic modulus,shear modulus,young's modulus,poisson's ratio and anisotropy factors of molybdenum and tungsten metal systems.The results show that the crystal lattice constants of molybdenum and tungsten metal materials are increased and the bulk elastic modulus,young's modulus and shear modulus are decreased due to the impurity defects of lithium or helium.It can be concluded that the stiffness and hardness of molybdenum and tungsten metal materials decrease,and also show a downward trend with the increase of temperature.For the first wall materials,the impurity defects of lithium or helium are not conducive to withstand the bombardment of high-energy particles and the control of plasma quality and density,and affect their service life.However,the increase of B/G value and poisson's ratio indicates that the toughness of the first wall material is significantly improved.Toughness enhancement can alleviate the irradiation embrittlement,recrystallization embrittlement and low temperature embrittlement of the first wall materials.By constructing the Ashby diagram of young's modulus density ratio(E/?)with the bulk elastic modulus shear modulus ratio(B/G),it shows that the rigidity of molybdenum or tungsten has a negative correlation with its ductility.The cauchy pressure is positive and increases with the increase of the number of lithium or helium atoms,indicating that the metal bond is the main chemical bond in molybdenum and tungsten crystal materials containing lithium and helium impurity atoms.The calculation results of anisotropy factor show that when the substitutional lithium atom captures interstitial helium atoms,the isotropy of molybdenum and tungsten crystals increases.But when more interstitial lithium atoms are captured,the anisotropy of molybdenum and tungsten crystals increases.The elastic wave velocity of molybdenum and tungsten crystals in each specific crystal direction decreases with the accumulation of lithium and helium atoms.The elastic wave velocity of molybdenum crystal is higher than that of tungsten crystal,especially the longitudinal wave velocity.The gibbs free energy(G*),Debye temperature,heat capacity(C_v,C_p),entropy(S),Grüneisen(?),thermal expansion coefficient(?),and other thermodynamic properties of molybdenum or tungsten containing multiple lithium and helium impurity atoms were studied by using the quasi harmonic Debye model.It can be seen from the calculation results that the presence of impurity atoms of lithium and helium leads to the increase of gibbs free energy,entropy,Grüneisen and thermal expansion coefficient of molybdenum and tungsten,the rapid increase of constant pressure heat capacity at low temperature and the decrease of Debye temperature.