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. The 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, the researchers in the group of professor LIU Changsong in Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences (CAS) have revealed the micro-mechanisms of bubbles and precipitates formation in W and 3C-SiC by carrying out a series of theoretical simulations. The research results have been published in Nucl. Fusion 57 (2017) 016006, Nucl. Fusion 57 (2017) 086006, and Nucl. Fusion 57 (2017) 066031.
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 6. The vacancy-formation energies close to vacancy-helium complex are substantially decreased (Figure 1). 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 provide an explanation for bubble formation in W even if there is no displacement damage. The works have been published in Nuclear Fusion, entitled with ¡°Bubble growth from clustered hydrogen and helium atoms in tungsten under a fusion environment¡± (Nucl. Fusion 57 (2017) 016006).
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 2). The aggregation of H atoms can reduce the stability of their closest lattice atoms, which may also result in the cascade phenomenon and the bubble formation. The work has also been published in Nuclear Fusion, entitled with ¡°The effect of irradiation-induced point defects on energetics and kinetics of hydrogen in 3C-SiC in a fusion environment¡± (Nucl. Fusion 57 (2017) 066031).
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 3). Os is observed to strongly promote the formation of vacancy-rhenium clusters, while Ta can suppress the formation of vacancy-rhenium and vacancy-osmium clusters. The results of these theoretical studies have been confirmed by the researchers in Oxford University (Xu et al., Acta Mater. 124 (2017) 71). The work has been published in Nuclear Fusion, entitled with ¡°Clustering of transmutation elements tantalum, rhenium and osmium in tungsten in a fusion environment¡± (Nucl. Fusion 57 (2017) 086006).
The works are done in cooperation with the scientists from Institute of Plasma Physics and Institute of Modern Physics, CAS, and supported by the National Magnetic Confinement Fusion Program, the National Natural Science Foundation of China.
Figure 1. The stability of the vacancy closest to the hydrogen or helium clusters in W.(Image by YOU Yuwei and SUN Jingjing )
Figure 2. The formation of hydrogen molecules in vacancy clusters in 3C-SiC. (Image by YOU Yuwei and SUN Jingjing )
Figure 3. The segregation of tantalum, rhenium and osmium in W. (Image by YOU Yuwei and SUN Jingjing )
Key Laboratory of Materials Physics, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences,
E-mail: email@example.com (C.S. Liu)