Simulation Study Of Irradiation Effects Of Vanadium Alloy And Beryllium Materials For Fusion Reactors

Fusion energy compared to traditional energy sources, which has rich raw-materials, reaction safe and controlled, clean, economical energy with nearly infinite resource, is considered as a ultimate scheme to solve mankind’s future energy. To successfully achieve fusion energy in fusion devices, plasma-facing materials and structural materials under long-term high temperature and14MeV neutron irradiation must be able to maintain high thermo-mechanical stability, low-activation properties, and excellent thermodynamic properties, especially resistance to neutron irradiation performance. For microcosmic behavior, the neutron-material interactions include dispacement damage caused by the elastic collision and H/He impurities produced by nuclear transmutation reactions. For macroscopic behavior, the materials appear swelling, creep, blistering, hardening and embrittlement phenomena. This is a comprensive and complicted issue of physics, materials science, and chemical. Until now large amounts of work have been carried out for key materials of fusion reator and accumulated a great number of experimental data; however, the microscopic mechanism of effect of neutron irradiation damage is still unclear and need intensive studies. Therefore, elucidating the physical process and microscopic mechanism during neutron irradiating ma terials is one of important steps to realize the use of fusion power in the future.Low-activation vanadlium alloys have considered as structural materials of future fusion reactor, wihle low-Z beryllium has been used as plasma-facing materials in fusion reactor. Thus intensive studies for them are helpful for revealing the evolution laws of the materials from microscopic to macroscopic, privding the basis for design and improvement of fusion materials. In this thesis, we choose two key fusion materials, i.e., vanadium/vanadlium alloys, and beryllium solid, as research topics. Density functional theory caculations have studied (1) retention, diffusion and aggregation beahaior of H, He, O, C impurities;(2) interaction bewteen H/He impurities and vacancy defects, stability and dissociation mechanism of H-vacancy and He-vacancy complex clusters;(3) physical mechanism of H bubble and He bubble formations. In addition, we simulated macroscopic service behavior of first wall under operation of fusion reactor using finite element method.Vanadium alloys as structural materials in fusion environment, large amounts of H and He impurities in bulk produced by transmutation reactions have a serious effect on the performance of the material. Using random solid solution model and first-principles methods we studied occpuaying and diffusion behavior of point defects (H, He, self-defect and vacancy) in vanadium, V-4Cr-4Ti and V-5Cr-5Ti alloys; H-H, He-He, He-vacancy and self-defects-self-defect interactions. Since He-vacancy exists a strong attractive interaction, we believe that vacancy privde a location for H and He aggregation, and the incorporation of Ti and Cr alloy elements can restrain H and He diffusion. Secondly, to explain the relationship between He-vacancy interactions and He bubble formation, first-principles calculations studied diffusion behavior of He and vacancy, stability of He, vacancies and of He-vacancy clusters. He high diffusion rate and low formation energy at vacancy is physical origin of He aggregating to vacancy. Based on first-principles results, we evaluate He diffusion rate in vanadium using empirical methods. Finally, to understand vacancy trapping for H in vanadium, we computed the interactions of H-vacancy and H-H, and the stability of H-vacancy clusters using first-principles methods. Lower electron density at vacancy explains why H is easier to be trapped by vacancy. And then we discuss why single vacancy can accommodate multiple H and nucleation mechanism of H bubble.For plasma-facing materials beryllium in fusion environment, H and He impurities from direct plasma bombardment and transmutation reactions seriously influence on the performance of the material. To understand H/He-Be interactions and elucidate the physical origin of H bubble and He bubble formation, we investigated energetics and diffusion behavior of H, He, O, and C impurities in hcp beryllium, H-vacancy and He-vacancy interactions, and H or He trapping at vacancy using first-principles methods. Overall, the O solution in bulk is an exothermic process, while the solution of H, He and C is an endothermic process. We found that the presence of vacancy markedly decreased solution energy of H or He, and a momovacancy can trapp up to5H or12He atoms. This gives an explanation for why H and He bubbles were experimentally observed at vacancy defects in materials. Theoretical results provide an elementary physical picture for H/He aggregation and blister formation in the early stage of irradiation damage.Finally, to assess the macroscopic thermodynamic behavior of first wall materials under normal operation for International Thermonuclear Experimental Reactor (ITER), we considered thermal deposition in first wall from plasma surface heating and neutron body heating, and simulated the combined effects of plasma heating and neutron heating loads in first wal by temperature and stress analysis using finite element method. The results show that high thermal stress exists in the interface of Be layer and CuCrZr layer, we suggest adding an effective buffer layer between two layers to reduce the thermal stress.