Investigation Of Hydrogen Behaviors In PFM-W

Fusion energy is the most promising energy source in the future, owing to its features in the following aspects:cleanness, safety, and efficiency. It is therefore considered to be the ultimate remedy for solving the worldwide energy problems. As one of the strategies for realizing controlled fusion reaction, magnetic confinement fusion reactor has proved to be a viable solution, whereas the problems in the engineering technology remain unresolved yet. One of the obstacles is how to choose the materials. Tungsten (W) is known as one of the best plasma-facing materials (PFMs) for the equipment of fusion reactor because of its high melting point, low sputter yield, high thermal conductivity, and low solubility of H isotopes, et al. Meanwhile, graphite is also used as PFMs in the divertor of ITER and EAST, due to its stability at high temperature. Positioned in severe environment in the reactor, PFMs will be subjected to the bombardment of the high-temperature, high-throughput, and high heat load particle flux for long time. The surface of PFMs will suffer chemical and physical sputter, and re-deposition. These processes will lead to the formation of W-C mixed layers on the surface. And these mixed layers will influence the retention of H isotopes and thus change the thermal and mechanical properties of the materials. Besides, the retention of hydrogen isotopes will decrease the concentration of the reaction fuel and the reaction efficiency, which would affect the operation of the fusion reactor. So one work of this thesis is to study the H trapping under the effect of W-C mixed layers. Furthermore, will they cause deterioration of W material just as other metals if a large number of transmutation products, such as H isotopes and He, retent in PFM-W? Therefore, the other work of this thesis is to study the influence of H and He on the crack propagation in W material.Our calculations are performed using Density Function Theory (DFT). The main contents comprise two parts:the retention phenomenon of H isotopes in W-C mixed layers; the influence of H and He on the crack growing in W. Conclusions are as follows:(1) C has low solubility in W, which prefers to be segregated on the W surface. Vacancies can enhance the solution of C in W. This makes C appear somewhat carbide feature. Thus, W-C mixed layers should contain multiple phase components. H retention strongly depends on the phase components in the W-C mixed layers. The solution of C will suppress the retention of H in W no matter whether neighboring vacancies are present, or not. Hydrocarbon precursors, which were observed in desorption experiments, prefer to form by means of H binding to C atoms in C amorphous, or in precipitators in the W-C mixed layers, while not in tungsten carbide phase or in W bulk.(2) When put on the crack front of W bulk, He atom prefers to stay in the center of the crack. The isolated He atom will make no influence on the growth of crack. On the contrary, H atom favors to dissolve in the front of crack. Dissolution of H atom increases the elastic strain energy, and decreases the surface energy released when a new surface forms in the W bulk. What is more, H can weaken the interaction between peripheral W atoms and thus promote the propagation of crack. It is also found that the higher concentration of H atoms, the more easily crack extends.Innovative points of this thesis are using DFT to study the behavior of H in W, one of PFMs:This thesis provides a good explanation to the H trapping under the effect of W-C mixed layers, while previous works didn’t explain why the quantity of H retention and desorbed hydrocarbon precursors are different in the different experimental conditions-, Besides, this thesis analyzed the effect of H and He on the propagation of crack in W bulk through theoretical calculation, while other previous works all focused on the experiments, and didn’t give a reasonable explanation on the experimental results.