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Modélisation de la durée de vie de composants face au plasma dans les réacteurs à fusion thermonucléaire

Abstract : Since the industrial revolution, global energy consumption has steadily increased. Historically based on the use of fossil fuels (oil, coal and gas), industrial development allowed the economic growth of the world as we know today. However, the intensive use of such fuels is undoubtedly not without consequences on our planet. The current exploitation methods contribute, for instance to global warming, plastic pollution and ocean acidification. The current energy context requires the development of alternative, sustainable and safe energies. The thermonuclear fusion reaction could become one of these new energy sources and may play a major role in the future global energy mix. Plasma facing components must ensure the mechanical integrity of the fusion device internal walls, the extraction of heat and must be compatible with the chemical species present in the plasma to not compromise its exploitation. Critical for the plasma operation and the reactor integrity, these components represent one of the major reactor parts. ITER and WEST divertor components can be exposed to particles fluxes up to 20 MW/m². To withstand such loading, these components are actively cooled. They are made of pure tungsten used as armored material bonded on water cooled pipe in CuCrZr (structural material). Several experimental campaigns have been performed to validate such components technology before their use in tokamak environment. Although this technology fulfills ITER's requirements, damages were highlighted over thermal cycles. Cracks appear in tungsten block up to few tens (up to few hundreds) of thermal cycles at 20 MW/m² and propagate from the exposed surface to the cooling pipe. The appearance of this crack, does not immediately affect the component heat exhaust capability. Nevertheless, this leads to mechanical integrity issues for the machine internal walls and could limit the plasma operation. To optimize the components use, this thesis aims at predicting numerically their lifetime. The time required for the crack opening corresponds to the component lifetime. In literature, several numerical models were developed and identified the major phenomena involved in the component damage process. To improve the prediction of existing numerical tools, this thesis aims at developing a numerical model able to take into account tungsten recrystallization; mentioned in the literature as having significant role on component lifetime. The final numerical model developed (RXMAT) is integrated in the finite elements code named ANSYS. This new numerical tool is fueled by the tungsten recrystallization kinetics studied up to 1800°C and the elastic-viscoplastic constitutive laws of tungsten and recrystallized tungsten identified based on experiments performed from 500°C to 1150°C at several strain rates. For the first time, it is possible to numerically link the evolution of the tungsten recrystallized fraction to a mechanical stress and strain fields. It is shown that RXMAT makes it possible to estimate plastic strains 10 times greater than those obtained in the literature. By doing so, first numerical results also highlighted that further experiments have to be done to study the ductile to fragile transition temperature and fatigue behavior of tungsten. In perspectives, RXMAT can be used to study the component lifetime exposed to non-homogeneous thermal flux, representative of the tokamak environment and also study the impact of tungsten recrystallization kinetics, component geometry and convection parameters on its lifetime.
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Alan Durif. Modélisation de la durée de vie de composants face au plasma dans les réacteurs à fusion thermonucléaire. Autre. Université de Lyon, 2019. Français. ⟨NNT : 2019LYSEE005⟩. ⟨tel-02510425⟩

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