Dynamic Self-Trapping of Hydrogen in Tungsten: A Defect-Free Path to Reduced Diffusivity and Enhanced Retention

Tungsten is a leading candidate for plasma-facing components in fusion reactors due to its superior thermal and mechanical properties, yet hydrogen isotope retention and blistering remain critical challenges. This study employs large-scale molecular dynamics simulations to investigate hydrogen diffusion in defect-free body-centered cubic tungsten at concentrations (0.5–2 at. %) and temperatures (1400–2900 K) relevant to fusion conditions. Hydrogen atoms were initially positioned in non-equilibrium sites near tungsten lattice atoms to mimic the high-energy state of plasma implantation. These atoms rapidly relax to stable tetrahedral interstitial sites (TIS) within several picoseconds. Arrhenius analysis of mean squared displacements reveals a concentration-dependent increase in apparent activation energy from 1.413 eV to 1.473 eV, accompanied by a compensatory rise in the pre-exponential factor from 2.7 × 10−6 to 6.9 × 10−6 m2/s, reducing effective diffusivity by more than 60%. Wigner–Seitz analysis reveals dynamic self-trapping associated with transient multi-occupancy in the vicinity of TIC, where up to five hydrogen atoms can temporarily accumulate within the same Wigner–Seitz cell. This behavior is driven by H–H repulsion and transient lattice distortions and occurs without the presence of preexisting defects. These findings provide a defect-independent explanation for anomalously high hydrogen retention and blistering in plasma-exposed tungsten, offering new insights for improved tritium inventory modeling and plasma-facing component design in ITER and DEMO.