Oxidation Protection of Tungsten Alloys for Nuclear Fusion Applications

In order to reduce the loss of W and extend its service life in high temperature oxidation environment. The development of self-passivating tungsten alloys and their surface protection technology has attracted widespread attention. Researchers investigated the performance of tungsten armor PFC at HHF to evaluate the choice of starting ITER operation with an all-tungsten shunt at heat currents of 10 and 20 MW.m^-2. The tungsten armor was subjected to 100-5000 cycles of thermal fatigue tests, and in the FE200 test, the change in surface morphology was not significant at a heat flow of 10 MW.m^-2 for 5000 cycles.

However, when the thermal fatigue test was 20 MW.m^-2 with more than 100 cycles, the surface damage of the tungsten alloy armor was very severe with high surface roughness and severe surface melting, and macroscopic cracks were observed. The surface damage of the armor with a thickness of 7.5 mm was even more severe after the thermal fatigue test. In the ITER Converter Test Facility (IDTF), the tungsten armor did not melt during the test, only the shape of the self-cast layer was observed. The self-cast layer was observed on the surface of the tungsten armor under tens of high heat flow cyclic impacts (20 MW.m^-2), which were caused by some uncontrollable defects at the heating water tank connections. Among them, the test facility and the armor thickness are the main reasons for this result.

In addition, researchers investigated the formation mechanism of deep cracks on the surface of tungsten blocks under HHF fatigue tests. The team used the overall ITER-W model used for high pulse experiments at high temperatures, which consists of seven ITER-compliant tungsten blocks connected in series by CuCrZr alloy cooling tubes. The geometry of this finite element model uses the target design of the ITER shunt, and the entire 3D model consists of approximately 19,000 secondary brick elements. It was shown that the surface of the tungsten armor would recrystallize under a cyclic load of 20 MW.m^-2. This decreases the yield stress of tungsten during heating, increases its brittleness and leads to low-cycle fatigue damage. Most during heating and cooling, cracks will grow in the stress concentration zone.

It is evident from the above study that under the influence of high heat flow cyclic fatigue, the surface temperature of tungsten increases rapidly and leads to melting and recrystallization. Finally, high surface roughness and macroscopic cracks are observed along the axial direction. In addition, vertical cracks are also observed in the stress concentration region of the tungsten block due to the decrease in yield stress and the increase in brittleness during heating. This leads to rapid oxidation of the W surface, forming volatile and highly radioactive WO₃. and air will enter the vacuum chamber when a loss of coolant accident (LOCA) occurs. Various studies have shown that alloying can significantly improve the oxidation resistance of tungsten armor, which will provide a safety advantage for ITER. Therefore, the development of self-passivating W alloys has been favored by many researchers.

Reference: Fu T, Cui K, Zhang Y, et al. Oxidation protection of tungsten alloys for nuclear fusion applications: A comprehensive review[J]. Journal of Alloys and Compounds, 2021, 884: 161057.

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