Abstract
Tungsten (W) is currently deemed the main candidate for the plasma-facing armor material of the first wall of future fusion reactors, such as DEMO. Advantages of W include a high melting point, high thermal conductivity, low tritium retention, and low erosion yield. However, was an accident to occur, air ingress into the vacuum vessel could occur and the temperature of the first wall could reach 1200K to 1450K due to nuclear decay heat. In the absence of cooling, the temperature remains in that range for several weeks. At these temperatures, the radioactive tungsten oxidizes and then volatilizes. Smart W alloys are therefore being developed. Smart alloys are supposed to preserve properties of W during plasma operation while suppressing tungsten oxide formation in case of an accident. This study focuses on investigations of thin film smart alloys produced by magnetron sputtering. These alloys provide an idealistic system with a homogeneous distribution of the elements W, chromium (Cr), and yttrium (Y) on an atomic scale. The recommended composition is W with 12 weight % of Cr and 0.5 weight % of Y. Passivation and a suppression of WO3 sublimation is shown. For the first time, the mechanisms yielding the improved oxidation resistance are analyzed in detail. A protective Cr2O3 layer forms at the surface. The different stages of the oxidation processes up to the failure of the protective function are analyzed for the first time. Using 18O as a tracer, it is shown for the first time that the oxide growth occurs at the surface of the protective oxide. The Cr is continuously replenished from the bulk of the sample, including the Cr-rich phase which forms during exposure at 1273K. A homogenous distribution of yttria within the W-matrix, which is preserved during oxidation, is a peculiarity of the analyzed alloy. Further, an Y-enriched nucleation site is found at the interface between metal and oxide. This nucleation sites are deemed to be crucial for the improved oxidation resistance.
Highlights
Fusion reactions are the power source of the sun
The plasma is magnetically confined in the vacuum vessel of a fusion reactor; the loads on the first wall of future fusion power plants such as DEMO will be much higher compared to current experimental devices: heat and particle loads of around 0.5 MW m−2 and 2 × 1021 m−2 s−1 are foreseen to reach the first wall of the vessel [2]
Smart alloys concept comprises the advantageous properties of W during plasma operation [17,18] coupled with suppressed tungsten oxide formation during an accident [5]: during normal operation, the plasma preferentially sputters the alloying elements with a lower mass compared to W [19], leaving a depleted zone of almost pure W as plasma-facing surface
Summary
Fusion reactions are the power source of the sun. The process relies on a nuclear reaction, fusing nuclei to convert mass into kinetic energy. Smart alloys concept comprises the advantageous properties of W during plasma operation [17,18] coupled with suppressed tungsten oxide formation during an accident [5]: during normal operation, the plasma preferentially sputters the alloying elements with a lower mass compared to W [19], leaving a depleted zone of almost pure W as plasma-facing surface. There is a lack on the understanding of the detailed mechanisms yielding the oxidation resistance of smart alloys. In this paper these mechanisms are addressed in detail for the first time. Further details on the microscopic properties are investigated: the diffusion processes yielding the formation of the protective oxide are analyzed and the phase formation, including its consequences, is studied. A particular focus is on Y which is not detected in measurements on a microscopic scale
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