Clustering of H and He, and their effects on vacancy evolution in tungsten in a fusion environment

Abstract

The behaviours of hydrogen and helium in tungsten are vitally important in fusion research because they can result in the degradation of the material. In the present work, we carry out density-functional theory calculations to investigate the clustering of hydrogen and helium atoms at interstitial sites, vacancy and small vacancy clusters (Vac m , m = 2, 3), and the influence of hydrogen and helium on vacancy evolution in tungsten. We find that hydrogen atoms are extremely difficult to aggregate at interstitial sites to form a stable cluster in tungsten. However, helium atoms are energetically favourable to cluster together in a close-packed arrangement between (1 1 0) planes forming helium monolayer structure, where the helium atoms are not perfectly in one plane. Both hydrogen and helium prefer to aggregate stably in vacancy and small vacancy cluster forming Vac m X n (X = H, He). The concentrations of Vac m H n (m = 1) clusters relative to temperature are evaluated through the law of mass action. The present calculations also show that the emission of a 1 1 1 dumbbell self-interstitial atom (SIA) from He n to form VacHe n and from VacHe n to form Vac 2 He n may take place for n > 5 and n > 9, respectively. According to the present results, we predict that a helium monolayer structure could nucleate for He atom platelet lying on (1 1 0) plane in tungsten, and the helium platelet formation on (1 1 0) plane in molybdenum observed by the experiment may be due to the initial monolayer arrangement of He atoms at interstitial sites. Meanwhile, our results contribute to the understanding for nucleation and the development of the voids and blisters in tungsten that are observed in the experiments.

Hydrogen bubble nucleation by self-clustering: density functional theory and statistical model studies using tungsten as a model system

“…It is well known that in tungsten an interstitial hydrogen atom prefers to occupy the tetrahedral interstitial site (TIS) rather than the octahedral interstitial site (OIS), and the interstitial H-H interaction is generally repulsive. [7][8][9][10][11] These have also been confirmed by our current calculations where the TIS is found to be 0.38 eV more stable than the OIS for a hydrogen atom. Our preliminary calculations also show that all TIS-OIS hydrogen pairs, OIS-OIS hydrogen pairs, and OIS hydrogen self-clusters are energetically unfavorable or unstable compared with the pure TIS ones.…”

“…Based on Eqs. (9)(10)(11)(12), the Gibbs free energy change of the system as a function of the self-cluster size n at different C H and T = 300 K is plotted in Figure 6a. We see that the enthalpy term ∆H decreases monotonically as n increases, with the rate of change in ∆H first accelerating and then gradually stabilized at a constant rate.…”

High-flux He+ irradiation effects on surface damages of tungsten under ITER relevant conditions

“…In our previous study, it was found that He atoms preferred to diffuse and cluster along one certain crystal face at an elevated temperature, leading to surface swelling of He-enriched strips [12]. Helium atoms are energetically favorable to cluster together in a close-packed arrangement between (110) planes forming helium monolayer structure [20]. Charlotte S. Becquart and Christophe Domain have proved that He atoms can be trapped by other He atoms and can explain the formation of He blisters close to the surface of He implanted tungsten [21].…”

Toward a European Network of Positron Laboratories

“…High heat fl uxes are expected in the divertor, apart from neutron, He, H atoms, and ions bombardments. Theory [31] indicates that He and H coimplantation in W can cause similar blistering as in Si. However, tests simulating the fusion environment indicated that exposure of W to deuterium atmosphere but seeded with He could result in suppression of blisters, see, for example, [32,33].…”

“…Factors for this opportunity are: -subjects that found a methodological barrier when using traditional investigations [23], -new challenges for material in manufacturing consumables, transportation, energetics, environmental protection, and healthy societies, -global, expanding outside EU search for positron answers [31], -growing scientifi c infrastructures in 'emerging' countries, -vast EU experience in projecting positron beams [8, 14-18, 40, 46, 48, 49]. …”