We have investigated the dissolution, segregation and diffusion of hydrogen (H) in a tungsten (W) grain boundary (GB) using a first-principles method in order to understand the GB trapping mechanism of H. Optimal charge density plays an essential role in such a GB trapping mechanism. Dissolution and segregation of H are directly associated with the optimal charge density, which can be reflected by the H solution and segregation energy sequence for the different interstitial sites. To occupy the optimal-charge-density site, H can be easily trapped by the W GB with the solution and segregation energy of −0.23 eV and −1.11 eV, respectively. Kinetically, such a trapping is easier to realize due to the much lower diffusion barrier of 0.13–0.16 eV from the bulk to the GB in comparison with the segregation energy, suggesting that it is quite difficult for the trapped H to escape out of the GB. However, the GB can hold no more than 2 H atoms because the isosurface of optimal charge density almost disappears with the second H atom in, leading to the conclusion that H2 molecule and thus H bubble cannot form in the W GB. Taking into account the lower vacancy formation energy in the GB as compared with the bulk, we propose that the experimentally observed H bubble formation in the W GB should be via a vacancy trapping mechanism.

https://iopscience.iop.org/article/10.1088/0029-5515/50/2/025016/pdf

Investigating behaviours of hydrogen in a tungsten grain boundary by first principles: from dissolution and diffusion to a trapping mechanism

As the most promising plasma-facing material for actual and future nuclear fusion devices, tungsten has to face and withstand a broad variety of severe operational conditions. These comprise high transient thermal loads, neutron irradiation-induced material degradation and transmutation, hydrogen and helium attack at the plasma-facing surface, and thermal fatigue under steady state heat fluxes as part of a plasma-facing component. The characterization and understanding of the material’s behavior under these conditions is essential for finding a grade of tungsten or tungsten alloy compatible with such an extreme environment.

4.17 – Tungsten as a Plasma-Facing Material

Although extensive studies on hydrogen (H) retention in tungsten (W) loaded by ion implantation, plasma exposure and gas charging have been done, reported amounts of H retained in W, and their dependences on the incident flux, fluence and temperature are very inconsistent. The present work aims at the clarification of the H retention mechanisms with critical review of reported data and presentation of recent results. H diffusion coefficient for dissolved H in W is large enough to allow for a deep penetration of H into W bulk. Energetic H loading results in H saturation in the subsurface layers. The thickness of the saturated layers could grow under prolonged loading and the H in the layers remain long or hard to remove. Both the growth of the thickness of the saturated layers and the deep penetration of H into the bulk could result in significantly large H retention in W and dissipate the advantage of W as plasma facing wall.

https://iopscience.iop.org/article/10.1088/0031-8949/2014/T159/014044/pdf

Review of hydrogen retention in tungsten

We investigate the physical origin of H–He interaction in W in terms of optimal charge density by calculating the energetics and diffusion properties using a first-principles method. On the one hand, we show a strong attraction between H and He in W originated from the charge density redistribution due to the presence of He, driving H segregation towards He. This can block the permeation of H into deeper bulk and thus suppress H blistering. On the other hand, we demonstrate that He, rather than H, energetically prefers to occupy the vacancy centre due to its closed-shell structure, which can block H2 formation at the vacancy centre. This is because He causes a redistribution of charge density inside the vacancy to make it 'not optimal' for the formation of H2 molecules, which can be treated as a preliminary nucleation of the H bubbles. We thus propose that H retention and blistering in W can be suppressed by doping the noble gas elements.

https://iopscience.iop.org/article/10.1088/0029-5515/50/11/115010/pdf

T Funabiki

Papers

Hydrogen blister formation and cracking behavior for various tungsten materials

Effect of tungsten microstructure on blister formation by hydrogen and carbon mixed ion beam irradiation

Carbon behavior on tungsten surface after carbon and hydrogen mixed beam irradiation

Effects of carbon impurity in fusion plasmas on erosion of RAF first wall