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

Danny Perez,Luis Sandoval,Arthur F Voter,Sophie Blondel,Brian D Wirth,Blas P Uberuaga

doi:10.1038/s41598-017-02428-2

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.

Highlights

Nuclear fusion is a promising carbon-neutral source of energy
If even larger clusters (>3 vacancies) were considered to be mobile, the impact would be even greater. These results demonstrate the crucial importance of accounting for the mobility of these complexes in order to adequately predict the evolution of He in the tungsten wall in fusion conditions; and just as important, these results demonstrate that incorrect results can be inferred from only performing molecular dynamics simulations to investigate helium www.nature.com/scientificreports/ Figure 6
The behavior of small VNHeM complexes in tungsten in conditions relevant to nuclear fusion was investigated using Accelerated Molecular Dynamics (AMD) simulations over timescales reaching into the milliseconds

Summary

Introduction

Nuclear fusion is a promising carbon-neutral source of energy. practical schemes for fusion energy production, such as magnetic confinement, place extremely stringent demands on materials, in terms of heat and particle fluxes. We revisit this problem for He in W and characterize the dependence of the mobility on the He and vacancy content of these defects Note that these complexes are relevant to nuclear fission applications where helium is a common fission or transmutation product. ParSplice is a technique to parallelize the generation of long atomistic trajectories in the time domain, leveraging parallel computers to reach very long timescales on relatively small systems As shown below, this allows for the direct observation of the motion of the complexes and for the characterization of their diffusivity. Through a mesoscale cluster-dynamics model, we show that accounting for the mobility of these complexes is critical to the the accurate prediction of the evolution of the first wall in fusion condition

Methods

Results

Discussion

Conclusion