| Being the most promising candidate of plasma-facing materials in future fusion reactors,tungsten faces challenges from extreme service conditions.Exposing tungsten to hydrogen(isotopes)plasma and 14.1 MeV fusion neutrons will cause retention of fusion fuel and blistering/flaking on tungsten surface,therefore raise severe concerns regarding safety and stability of fusion reactors.Focusing on hydrogen retention and bubbling problems in tungsten,and employing multiscale modelling techniques,the present study revealed central atomic mechanisms underlying hydrogen self-trapping and hydrogen trapping in nanovoids in tungsten.Physical models were established to provide explanations and accurate predictions toward related experiments.In the present study,an object kinetic Monte Carlo software package was developed to model hydrogen diffusion,trapping,and desorption in tungsten under fusion environments.Acceleration algorithms were developed and implemented to enable a spatial scale around micrometers and a time scale around hours.Using this software,we calculated sink strength for different traps.Comparing our simulation results with analytical theories,we revealed origins of discrepancies between them,and proposed theoretical corrections.More accurate predictions of defect evolution under fusion environments become accessible with these corrections,which certainly assists further quantitative studies of hydrogen trapping,clustering,bubble nucleation and growth at defects.Using first principles calculations,we show,for the first time,the formation of platelet hydrogen self-clusters along {001} surfaces via hydrogen self-trapping in tungsten.Such hydrogen self-trapping behavior can be quantitatively understood by the competition between long-ranged elastic attraction and local electronic repulsion among hydrogen.Further analysis showed hydrogen self-trapping is kinetically feasible and thermodynamically stable above a critical hydrogen concentration.Based on this critical hydrogen concentration,we predicted the hydrogen irradiation condition required for the formation of hydrogen self-clusters.We proposed a possible mechanism for the hydrogen bubble nucleation via hydrogen self-trapping.Results of the present work provide mechanistic insights and quantitative models towards understanding of plasma-induced hydrogen bubble formation in plasma-facing tungsten.Focusing on tungsten as a model BCC system,the present study explicitly demonstrated sequential adsorption of hydrogen adatoms on Wigner-Seitz squares of nanovoids with distinct energy levels.Interaction between hydrogen adatoms on nanovoid surfaces is shown to be dominated by pairwise power law repulsion.A predictive model was established for quantitative determination of configurations and energetics of hydrogen adatoms in nanovoids.This model,further combined with the equation of state of hydrogen gas,enables prediction of hydrogen molecule formation in nanovoids.Multiscale simulations based on our model were then performed,showing excellent agreement with recent thermal desorption experiments.This work clarifies fundamental physics and provides a full-scale predictive model for hydrogen trapping and bubbling in nanovoids,offering long-sought mechanistic insights crucial for understanding hydrogen-induced damages in structural materials. |
