Irradiation-induced defects and hydrogen retention in fusion reactor plasma-facing materials

Le résumé fourni par la source

Plasma-facing materials in fusion reactors experience extreme conditions as high energy particles such as neutrons and ions interact with the materials. Irradiation damage caused by the energetic particles alters the properties of the plasma-facing materials. One of the most important properties of the plasma-facing materials is the fusion fuel retention. Tritium will be used as one of the fusion fuel components. As a radioactive isotope of hydrogen, it will impose a radiological hazard to the plasma-facing components when being trapped in the material. Therefore minimizing the hydrogen isotope retention is crucial for efficient operation of future fusion reactors. This thesis presents experimental results on defect formation and hydrogen isotope retention in tungsten, the plasma-facing material chosen to be used in the experimental fusion device ITER. In addition, results of hydrogen isotope removal from a novel high entropy alloy are presented. The experimental methods include ion irradiations, deuterium gas loading, elastic recoil detection analysis, positron annihilation spectroscopy, and thermal desorption spectrometry. By combining these methods, we can gain extensive information on trap formation and hydrogen isotope trapping in various types of defects and depth profiles within the first few hundred nanometres. The experimental results show interesting properties with sequential deuterium implantations into tungsten. When the tungsten lattice already contains a small amount of retained deuterium, retention from a subsequent energetic deuterium implantation is greatly increased when compared to a single energetic implantation alone. Depth profile analysis shows that this increase is not only happening around the depth of maximum damage formation but also deeper in the bulk. Deuterium gas loading of self-irradiated tungsten revealed formation of various trap types in tungsten. Within the first few tens of nanometres from the material surface, vacancy clusters were the dominant trap site while mono-vacancies were found to be the most common type in the bulk. Hydrogen isotope exchange effect can be used to enhance the removal of radioactive tritium from plasma-facing materials in fusion reactors. The experimental results show the importance of excess solute isotopes in tungsten to efficiently remove the unwanted isotopes. In addition, hydrogen isotope exchange was done to a high entropy alloy material. The results revealed efficient removal of heavy isotopes near the sample surface but less efficient removal in the bulk. Very high hydrogen isotope retention was found during the experiment which hampers the use of the specific high entropy alloy as a plasma-facing material and shows the need of optimization for fusion operation. The results of the thesis reveal interesting properties of the plasma-facing materials. They can be used to improve estimates from laboratory conditions to simulations of fusion reactor operation. The experimental results show behaviour of defect formation in plasma-facing materials. These results can be used to determine the actual defect formation mechanisms which are relevant in future fusion reactors.

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