The mobility of small vacancy/helium complexes in tungsten and its impact on retention in fusion-relevant conditions

Abstract

Tungsten is a promising plasma facing material for fusion reactors. Despite many favorable properties, helium ions incoming from the plasma are known to dramatically affect the microstructure of tungsten, leading to bubble growth, blistering, and/or to the formation of fuzz. In order to develop mitigation strategies, it is essential to understand the atomistic processes that lead to bubble formation and subsequent microstructural changes. In this work, we use large-scale Accelerated Molecular Dynamics simulations to investigate small (N = 1,2) VNHeM vacancy/helium complexes, which serve as the nuclei for larger helium bubble growth, over timescales reaching into the milliseconds under conditions typical of the operation of fusion reactors. These complexes can interconvert between different ILVN+LHeM variants via Frenkel pair nucleation (leading to the creation of a additional vacancy/interstitial pair) and annihilation events; sequences of these events can lead to net migration of these embryonic bubbles. The competition between nucleation and annihilation produces a very complex dependence of the diffusivity on the number of heliums. Finally, through cluster dynamics simulations, we show that diffusion of these complexes provides an efficient pathway for helium release at fluxes expected in fusion reactors, and hence that accounting for the mobility of these complexes is crucial.

Theoretical Model of Helium Bubble Growth and Density in Plasma-Facing Metals

“…This correlation, much like other studies of helium bubble energetics and bubble growth in tungsten [21][22][23][24][25][26][27][28][29][30] , has been useful in developing coarse-grained models of helium transport and surface morphological evolution in tungsten [31][32][33][34][35][36] . However, the correlation in Eq.…”

Effect of W self-implantation and He plasma exposure on early-stage defect and bubble formation in tungsten

“…As these clusters grow they are capable of bootstrapping their own vacancies by forcing tungsten out of their lattice sites [9], with the activation energy of this process decreasing with increasing size of the helium cluster [10]. The mobility of these structures is very sensitive to both the number of bound vacancies and helium atoms, and can change via the spontaneous creation or annihilation of Frenkel pairs [11]. Vacancy formation energies are significantly lower in the vicinity of the sample surface [12] and grain boundaries [13], leading to enhanced cluster formation in these regions.…”

Evaluation of defect formation in helium irradiated Y2O3 doped W-Ti alloys by positron annihilation and nanoindentation

“…According to theoretical model calculations [15,16,18] [16,17]. When the number of He atoms in the clusters increases, the surrounding metal atoms can be displaced from their lattice sites and HenVm defects are created.…”

“…properties of tungsten and result in the surface swelling and embrittlement of the material [8][9][10][11]14]. Theoretical investigations [15][16][17][18] study intensively the mobility of these He-vacancy complexes in tungsten and model self-trapping of He and trap mutation. In particular, the migration properties of the HenVm defects and their interaction with point defects created by irradiation and other defects in the material are of fundamental as well as practical interest.…”