Recently, the researchers from Institute of Solid State Physics (ISSP), Chinese Academy of Sciences have made new progress on the hydrogen and helium behaviors of tungsten as plasma facing materials by computation simulation, in collaboration with the researchers from Institute of Plasma Physics and Institute of Modern Physics, Chinese Academy of Sciences. The relevant results have been published on Nuclear Fusion and Journal of Nuclear Materials (Nucl. Fusion, 2013, 53, 073049 and J. Nucl. Mater. 2013, 433, 357-363)
The behavior of helium in metals is particularly significant in fusion research due to the He-induced degradation of materials. A small amount of impurities introduced either by intentional alloying or by transmutation reactions, will interact with He and lead the microstructure and mechanical properties of materials to change. The researchers of ISSP presented the results of first-principles calculations on the interactions of He with impurities and He diffusion around them in tungsten (W), including the interstitials Be, C, N, O, and substitutional solutes Re, Ta, Tc, Nb, V, Os, Ti, Si, Zr, Y and Sc. They found that the trapping radii of interstitial atoms on He are much larger than those of substitutional solutes. The binding energies between the substitutional impurities and He increase linearly with the relative charge densities at the He occupation site, indicating that He atoms easily aggregate at the low charge density site (see Figure 1). The sequence of diffusion energy barriers of He around the possible alloying elements is Ti > V > Os > Ta > Re. Their results suggest that Ta might be chosen as a relatively suitable alloying element compared with other possible ones.
In addition, the impurity role for hydrogen bubble nucleation has also been investigated. A new mechanism of hydrogen bubble nucleation is proposed (see Figure 2): the interstitial oxygen atom traps multiple hydrogen atoms inducing the appearance of some unstable lattice sites nearby, where the initial vacancy can be created to form vacancy每oxygen每hydrogen complex, whose formation energy is so low that abundant vacancy每oxygen每hydrogen complexes could survive and thus the hydrogen bubble nucleates. This mechanism could provide a sound explanation for the hydrogen bubble nucleation in tungsten (with quite low vacancy concentration) exposed to low-energy (far lower than displacement threshold energy) deuterium ions irradiation.
This work was supported by the National Magnetic Confinement Fusion Program (Grant Nos.: 2011GB108004), the National Natural Science Foundation of China (Nos.: 91026002 and 91126002) and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos.: KJCX2-YW-N35 and XDA03010303), and by the Center for Computation Science, Hefei Institutes of Physical Sciences.
Figure 1. Relationship between the binding energy and the relative charge density . The inset shows the plot of binding energy versus the relative charge density for the interstitial solutes C, N, O and Be.
Figure 2. One possible mechanism of bubble nucleation by hydrogen accumulation around the oxygen impurity: (a) The hydrogen atoms (small white ball) do not aggregate to form clusters in perfect tungsten; (b) Multiple hydrogen atoms accumulate around the interstitial oxygen atom (small red ball), inducing the appearance of unstable lattice sites (large yellow balls) and (c) A vacancy is formed on a unstable lattice site, and then multiple hydrogen atoms are trapped in this vacancy to form stable V每O每Hn complex; The tungsten atoms are represented by large balls.