  This manuscript embarks on an inquiry into the influence of helium
implantation on nanocrystalline tungsten, a contender for plasma-facing
components (PFCs) in nuclear fusion reactors. The study underscores the
inevitability of helium retention in tungsten due to the anticipated high flux
of helium atoms from D-T fusion reactions in future reactors such as ITER and
DEMO. This retention, potentially culminating in surface blistering and grain
boundary embrittlement, propels an investigation into the helium's preferential
substitutional configuration over interstitial within tungsten's lattice,
leading to cavity and bubble formation. Atomistic tensile testing simulations,
exploiting a helium-specific interatomic potential, dissect the ramifications
of helium concentration and voids on tungsten's mechanical properties across
grain sizes of 10nm and 15nm. The presence of helium at grain boundaries
instigates detachment, influencing structural behaviors under strain. Notably,
the study reveals a dichotomy in helium's effect: benign at concentrations up
to the critical point, beyond which significant embrittlement and elastic
softening occur, corroborating with theoretical predictions on helium-induced
grain boundary embrittlement. This nuanced exploration delineates the intricate
dance between helium implantation levels, grain size, and the ensuing
mechanical property alterations in nanocrystalline tungsten, casting light on
its viability and endurance as a PFC material under the rigorous conditions
forecasted in fusion reactors.
