| Fusion power is considered as the ultimate solution to the energy crisis in the future due to its advantages, such as clean, safe and controlled, rich raw-materials, etc. In the extreme environment of fusion reactors, the first-wall structural materials will be subjected to high temperature and high neutron loads with high energy (14MeV). Structural materials must be able to maintain high thermo-mechanical stability, low-activation properties, especially excellent resistance to neutron irradiation. Huge number of vacancy defects and impurities (hydrogen (H) and helium (He) will be produced by neutron irradiation. These vacancy defects capture impurities and grow up to form bubbles, which result in material swelling and embrittlement. In this thesis, we mainly focus on impurity behavior in three promising candidate structural materials using first-principles calculations with DFT and pseudopotential method. We hope our theoretical results are helpful for understanding the impurity effects in the early stage of irradiation damage.Silicon carbide (SiC) and its composites have attracted considerable attention for first-wall structural materials, mainly because of their excellent high-temperature strength. The stability of small He-vacancy cluster (HenVam) in 6H-SiC has been investigated for n, m= 1-4. The vacancy and interstitial He binding energies to the clusters are found to be positive in all cases, i.e., all interactions are attractive. The cluster with high He/vacancy ratio prefer to combine with vacancy and rather than He atoms due to He-He repulsion.6H-SiC has a weak capacity to capture He atoms, at most 14 He atoms can be trapped by a 7-atom small void, due to its brittle property. The estimated internal pressure (2.5 GPa) has the same order of magnitude as the experimental value (0.8 GPa).Helium behavior and vacancy defects in bcc vanadium also have been investigated. So far, tremendous effects have been devoted to study of the interactions between He and small size vacancy defects. However, in real materials, the diameter of voids can be as larger as up to nanometer. In this chapter, we focus on two cases (monovacancy and 9-atom void). To understand the microscopic mechanism of multiple He being trapped by vacancy defects, we calculated the trapping energies and internal pressures as function of He atoms in monovacancy and 9-atom void. Our theoretical results demonstrate that vacancy defects provide effective nucleation sites for He bubbles. Compared to monovacancy, small void has stronger trend to capture He atoms. The space expansion of vacancy defects due to capture of huge number of He atoms will lead to high internal pressure in structural materials. The He-He distance constrained in small void is shorter than that in gas-phase Hen cluster. This finding is consistent with the results obtained by radial distribution function.Reduced activation ferritic/martensitic (RAFM) steels are leading candidate structural materials for the first wall of fusion reactors because of their excellent mechanical properties and resistance to swelling under irradiation. The basic composition of RAFM steels are chromium (Cr) and tungsten (W), i.e., Fe-Cr-W alloys. It is found that alloying elements (Cr and W) have significant effects on resistance to irradiation and mechanical properties of RAFM steels. Effects of Cr and W on the stability and diffusion of He atoms in bcc Fe have been investigated. The trends of formation energy of He in tetrahedral interstitial site (T-site) and O-site with different number of Cr and W atoms are non-linear, respectively. It is found that antiferromagnetic Cr-Cr coupling in bcc Fe transforms to ferromagnetic coupling, and the repulsion from first nearest neighbor (Inn) He and W is larger than Cr atoms. Compared to pure Fe host, the number of He atoms can be trapped by monovacancy becomes lower due to addition of Cr and W atoms. Cr and W slightly hamper He trapping in vacancy compared to pure solid Fe.Vanadium alloys are the promising candidate structural materials of the first-wall for future fusion reactors, due to their low-activation and superior high-temperature performance. To investigate behavior of inherent impurities (oxygen, nitrogen, carbon), we calculated energetic and diffusion as well as O-O/N-N/C-C interactions in pure vanadium with body-centered cubic (bcc) structure. Our theoretical results demonstrate that O, N and C atoms prefer to occupy octahedral interstitial site (O-site), and exhibit high migration barriers with 1.23,1.48 and 1.14 eV, respectively, via diffusing between two neighboring O-sites. Such high migration barriers indicate that these impurities are difficult to diffuse inside bulk vanadium. The corresponding diffusion coefficients as function of temperature were estimated by Arrhenius diffusion equation. The O-O/N-N/C-C interactions can be neglected when the distances between two impurities are larger than 3A. |
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