| Summary: | 碩士 === 長庚大學 === 機械工程研究所 === 92 === Metal components in nuclear power plants, after exposing in high dosage neutron radiation for a long time, will cause the material to produce helium. During repair welding of the irradiated components, the diffusion of is affected by heat and shrinkage stresses during cooling. The evolution and growth of helium bubbles may cause weld cracking. According to published research, underwater plasma arc welding (PAW) can produce residual compressive stresses, in the weld zone so it becomes the main subject of this study. Meanwhile, rapid cooling may significantly inhibit the diffusion of helium. Finite Element Method (FEM) is used to simulate the temperature and thermal stress fields during welding. To calculate the growth rate of helium bubbles in the weld heat affected-zone (HAZ) after repair welding, the results of simulation are incorporated in an equation, which evaluate thermal mechanical factors simultaneously. Utilizing this analysis results, one can choose proper welding parameters before repair welding of irradiated materials, which may experience different degrees of radiation exposure, to inhibit the diffusion of the helium effectively
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