| The interactions between plasma and wall material(PMI)is one of the key fundamental issues for the fusion energy application using TOKAMAK devices.Under the exposure of high flux helium(He)ions,nanosized gas bubbles will be expected to form in the first wall material in TOKAMAK,while leading to the significant change of thermodynamic properties of wall material.Due to the outstanding thermophysical properties and low H isotope retention and so on,tungsten(W)and their alloys are currently considered as the primary candidate wall material in TOKAMAK.In this paper by using molecular dynamic(MD)simulations,the W microstructure evolution and its influence on thermal conductivity of W under the He ions exposure are studied,focused on the microscale mechanisms of nanosized gas bubble evolution in W and its influence on lattice thermal conductivity(LTC)of W in these non-equilibrium dynamic processes.The contents and conclusions in this thesis are as follows:1.The dynamic He migration and aggregation with the microscale defect of Frenkel pairs production at the early stage of bubble evolution in W bulk are investigated.The He concentration and temperature effect on the distribution of He clusters with small size and the evolution of Frenkel pairs production are analyzed.The results show that He migration and aggregation with Frenkel pairs production are significantly dependent on the temperature.The rise of temperature promotes the increase of He cluster size and the Frenkel pairs production,and the He cluster size tends to obey the Gaussian distribution at the elevated temperature,however the effect of He concentration may reach saturation when it increases to an enough large value.The ratio of He/V in bubble is crucial for most of properties of bubble,it is found that the He/N ratio is ranging from 1.5 to 4 using the He implantation models,and in general decreases with the increase of temperature.Also,it is observed that the trapping of He atoms may occur under the interaction of two nearby bubbles,thereby resulting in the significant descend of He/V ratio in the small He cluster in W bulk.2.From atomic scale the He/V ratio in nanosized He bubble based on mechanic equilibrium and bubble coalescence phenomena in metallic W material are simulated.The influence of temperature and bubble size on equilibrium He/V ratio of gas bubble in W are investigated.The equilibrium He/V ratio in nanosized bubble in W generally decreases with the increase of temperature and bubble size,ranging from 1.6-1.9.The analysis of nanosized bubble coalescence indicates that the occurrence of bubble coalescence in W within typical MD evolution time scale is not only dependent on the elevated temperature,but also the high He/V ratio in bubble.Based on the atomic stress calculation,it is found that the significant interaction between two nearby bubbles promotes the initial contact of early stage of bubble coalescence in W,thereafter He atoms can migrate fast across the new formed bubble,but W atoms displaced are relatively limited,therefore it appears the bubble structure keeps basically stable in the following nanoscale evolution time.3.The atomic scale study of influence of nanosized bubble with different configurations on lattice thermal conductivity of W has been carried out.Based on the non-equilibrium momentum transfer model,the stable heat flux in metallic W containing bubble is created and the temperature profiles along the heat flux averaged evolution time.Combined with the classic Fourier law,the calculation shows that the lattice thermal conductivity of W is dependent on the configuration of bubble when the He/V ratio is equal to one.Also,it is found that these bubbles with different configurations tend to form the shell structure in W.With the He atom number increases in bubble,the bubble pressure becomes higher,although the sphere bubble has a higher pressure,which does not cause the larger degradation of lattice thermal conductivity of W.Besides,the nanosized bubble under various pressure conditions and the increase of He/V ratio in bubble may have a non-linear effect on the degradation of lattice thermal conductivity of W.Further study on the W lattice structure reveals that the lattice distortion around the bubble significantly leads to the degradation of W lattice thermal conductivity. |
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