In the fusion environment, both the plasma facing material tungsten (W) and structural material cubic silicon carbide (3C-SiC), will be subjected to high-energy neutron. Irradiation of neutron produces plenty of defects and a large amount of H, He and transmutation elements, resulting in the formation of bubbles and precipitates.

These bubbles and precipitates can induce the embrittlement, swelling and hardening of the materials, and finally influence the safe operation of fusion reactors. However, the micro-mechanisms regarding the interaction of H, He and metal elements are not clear.

Recently, Professor LIU Changsong's group at Institute of Solid State Physics, Hefei Institutes of Physical Science of Chinese Academy of Sciences (CAS) revealed the micro-mechanisms of bubbles and precipitates formation in W and 3C-SiC by carrying out a series of theoretical simulations.

They found that He atoms are energetically favorable to segregate in both interstitial sites and vacancies in W forming vacancy-helium complex. The emission of a self-interstitial atom (SIA) from interstitial He clusters to form vacancy-helium complex may take place when He atoms come up to six. The vacancy-formation energies close to vacancy-helium complex are substantially decreased. When He atom number adds up to 10, the emission of a SIA from vacancy-helium complex may take place again.

This cascade phenomenon may eventually lead to the formation of He bubbles. The results, published in Nuclear Fusion, provided an explanation for bubble formation in W even if there is no displacement damage.

As for 3C-SiC, the vacancy clusters produced by high-energy neutron irradiation can also act as the trapping sites for H and lead to the H accumulation. H molecules are found in a single vacancy and vacancy clusters (Figure 1). The aggregation of H atoms can reduce the stability of their closest lattice atoms, which also result in the cascade phenomenon and the bubble formation. This finding was also published in Nuclear Fusion.

In addition, it is theoretically found that the transmutation elements exhibit more complex segregation behavior compared with H and He in W under high-energy neutron irradiations. Osmium (Os) can easily segregate to form clusters even in defect-free W alloys, whereas extremely high tantalum (Ta) and rhenium (Re) concentrations are required for the formation of clusters.

Vacancies greatly facilitate the clustering of Re and Os, while Ta is an exception (Figure 2). Os was observed to strongly promote the formation of vacancy-rhenium clusters, while Ta suppressed the formation of vacancy-rhenium and vacancy-osmium clusters. The results of these theoretical studies,  published in Nuclear Fusion, have been confirmed by the researchers at Oxford University (Xu et al., Acta Mater. 124 (2017) 71).

The studies were done in cooperation with scientists from Institute of Plasma Physics and Institute of Modern Physics of CAS, and they were supported by the National Magnetic Confinement Fusion Program, the National Natural Science Foundation of China.

Figure 1: The formation of hydrogen molecules in vacancy clusters in 3C-SiC. (Image by YOU Yuwei and SUN Jingjing )

Figure 2: The segregation of tantalum, rhenium and osmium in W. (Image by YOU Yuwei and SUN Jingjing)