Theoretical Investigation Of Thermodynamic Properties Of Tungsten-based Plasma-Facing Material And Its Interaction With Hydrogen

With the development of human society,the issues of energy shortage and environmental pollution are becoming more and more serious.Controlled thermonuclear fusion is believed to be one of the most promising solutions to these issues,and its application relies on the development of key materials,especially plasma-facing materials(PFMs).Due to the excellent physical properties such as high melting point,high thermal conductivity,low physical sputtering yield and low hydrogen(H)retention,tungsten(W)is considered to be the most promising PFM.As PFM,W will inevitably be exposed to high-flux H isotopes,helium ions,and high-energy neutrons,as well as various high-intensity heat flows during service,which have a great impact on its structural and thermodynamic properties,and the performance of PFMs under these conditions is a crucial issue for the ultimate application of nuclear fusion energy.In this dissertation,we carry out a series of theoretical research on the thermodynamic properties of W in nuclear fusion environment,which are summarized as follows:The first chapter mainly reviews the researches on W-based PFM in nuclear fusion reactors.In chapter two,we briefly introduce the tight-binding(TB)potential model and the Kubo method used in this dissertation.In chapter three,by combining the Kubo method with the TB potential model,we study the electron transport properties of W metal.The results show that the method is particularly suitable for the calculation of the electron transport properties at high-temperatures or in defective systems.We calculate the electron thermal conductivity of W at various point-defect systems and find that the presence of point defects significantly degrades the thermal transport property of W.In chapter four,we present a new linear scaling algorithm for large-scale TB calculations.The method is based on the idea of 'divide and conquer',in which a system is divided into subsystems and each subsystem is calculated separately.We apply this method to the W metallic system and show that this method yields the same results as those obtained from the exact diagonalization of TB Hamiltonian matrix of whole system.This method has the advantages of linear scaling complexity,less memory consumption,and high parallel efficiency,making it suitable for the large-scale TB simulations,which lays the foundation for subsequent research.In chapter five,we consider the electronic excitation effect and study the thermodynamic properties of W metal under non-equilibrium conditions(electronic temperature is much higher than ionic temperature).Under intense electronic excitation,the metallic bond weakens gradually as the electronic temperature increases,leading to the thermal expansion of W lattice and further weakens the interactions between W atoms.Thus the bulk modulus,the Young's modulus and the point defects formation energy of W system will decrease obviously.Meanwhile,the diffusion rate of point defects(vacancy and self-interstitial atoms)will increases rapidly with the increase of electronic temperature.Furthermore,the high electronic temperatures will lead to a significant drop in the resistance for neutron irradiation of W material.In addition,intense thermal expansion induced by an instantaneous high electronic temperature will result in an abnormal melting of the system,in which the interior melts while the surface region does not.In chapter six,based on first-principles calculations,we study the interaction of H atoms with the small-angle tilt grain boundary(GB)in W and the interactions between W,Be,and H.The results show that the small-angle GB can capture a large amount of H atoms,resulting in the direct formation of H bubbles.Multiple Be atoms can dissolve in W mono vacancy to form nBe-Vw(n=1-10)complexes which can decrease the formation energy of vacancy nearby,thus promote the growth of nBe-Vw complexes and dissolve more Be atoms.The presence of the nBe-Vw complexes significantly reduces the retention of H in W vacancy.The Be atoms can also dissolve in the small-angle GB of W.We demonstrate that a small amount of Be atoms dissolve in the GB has little influence on the H retention and the formation of H bubbles,while a large amount of Be atoms will promote the growth of cavities in the GB region.Finally,in chapter seven,we summarize the main conclusions of this thesis,and present a brief outlook for the future studies on W-based PFM.