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This work, with an emphasis on helium irradiation rates appropriate for fusion-plasma conditions, advances the understanding of helium evolution at grain boundaries in W, an important consideration in the understanding of W as a plasma-facing component. Using accelerated molecular dynamics, helium bubble nucleation and growth at a symmetric Σ5[100](310) tilt grain-boundary in W is studied. The simulations reveal that the growth mode associated with bubble growth at the grain-boundary leads to a suppression of the helium supply to the bubble and hence to arrested growth. Such an unconventional bubble growth mode may dominate in materials with a high density of sinks.