Synergistic damage behavior of He ion irradiation and molten salt corrosion in SiC at 750 °C

The Molten Salt Reactor (MSR), as one of the six most promising Generation IV reactors, has attracted worldwide attention owing to its inherent safety, high fuel utilization efficiency, and low nuclear waste production. The structural materials in MSRs are subjected to a combination of challenging environmental conditions, including high temperatures, intense fluoride-salt corrosion, and severe neutron irradiation. Therefore, silicon carbide (SiC) materials and their ceramic-based composites, with the high-temperature strength, chemical inertness, and favorable neutron characteristics, are expected to be applied in the structural components of MSRs. However, the synergistic effect of irradiation and corrosion at high temperatures presents one of the most significant challenges limiting the safe and efficient application of SiC. While the effect of irradiation damage on SiC corrosion in FLiNaK molten salt has been investigated, the underlying mechanisms remain inadequately explored. Additionally, the influence of molten salt corrosion on irradiation defects in SiC materials remains unexplored. Comprehensive research into the interactions is therefore required to gain a deeper understanding.

Recently, a team of nuclear materials scientists led by Jianjian Li at the Shanghai Institute of Applied Physics, Chinese Academy of Sciences, reported for the first time the synergistic damage behavior of He ion irradiation and molten salt corrosion on CVD SiC at 750 °C. This work not only suggests the mechanism of the synergistic damage between irradiation and corrosion on SiC, but also provides effective suggestions and data for the application of SiC materials in MSRs.

The team published their work in Journal of Advanced Ceramics on November 11, 2024.

“In this work, we subjected some of the CVD SiC samples to irradiation at 750 °C under 400 keV He ions at two doses: 2 × 10¹⁶ and 1 × 10¹⁷ ions•cm^-². Subsequently, we conducted further processing on both the irradiated and non-irradiated SiC samples by exposing them to FLiNaK molten salt at 750 °C for 166 h. The synergistic damage behavior of irradiation and molten salt corrosion on SiC at high temperatures was, as a consequence, identified. We have offered a reasonable explanation for this phenomenon，”said Dr. Jianjian Li, the corresponding author of the paper, an associate researcher in the Shanghai Institute of Applied Physics.

“Through TEM, we observed that the corrosion of FLiNaK salt resulted in the formation of a carbon-rich phase with a graphite-rich structure in SiC. This result provides a definitive conclusion to the preceding discussion on this topic,”said Dr. Jianjian Li.

“The unexpected discovery was that Ni impurities in the molten salt react preferentially with Si-Si bonds generated by irradiation, thereby playing a key role in the corrosion process promoted by irradiation,”said Dr. Jianjian Li.

“Following the investigation of the number density and size of He bubbles in the C-rich phase and survived SiC in the vicinity of the corrosion boundary, as well as in the SiC region away from the boundary, it was determined that the vacancies resulting from Si loss during corrosion contributed to the migration and coalescence of He bubbles. The collective findings offer compelling evidence of the synergistic damage behavior of irradiation and corrosion,”said Dr. Jianjian Li.

This work not only provides data that is essential for the safe use of SiC in MSR, but it also serves as a fundamental reference for the development of SiC fiber-reinforced SiC matrix composites (SiC_f/SiC), which can be utilised in MSRs. The subsequent phase of the investigation entails determining the precise composition of the Ni-Si compounds situated at the corrosion boundary. Additionally, it is necessary to ascertain whether other forms of irradiation defects in SiC exhibit synergistic damage behavior with molten salt corrosion.

Other contributors include Jianfeng Zhang from the Shanghai Institute of Applied Physics at Chinese Academy of Sciences and College of Sciences at Shanghai University in Shanghai, China; Lin Zhao, Jinlei Yang, Qiantao Lei, Jun Lin from the Shanghai Institute of Applied Physics at Chinese Academy of Sciences in Shanghai, China; Ya Tang from the College of Sciences at Shanghai University in Shanghai, China.

This work was supported by the National Natural Science Foundation of China (Grant No. 12175301), and the Natural Science Foundation of Shanghai (Grant No. 22ZR1474800).

About Author

Jianjian Li is an associate researcher at the Shanghai Institute of Applied Physics, Chinese Academy of Sciences. He was awarded his PhD in Nuclear Energy Science and Engineering by the Shanghai Institute of Applied Physics, University of Chinese Academy of Sciences, in 2017. His studies are focused on the irradiation behavior of advanced nuclear materials, including silicon carbide and additive manufactured materials. Additionally, he is engaged in the investigation of nuclear technology applications and additive manufactured materials for the nuclear reactor.

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