Fig. 1. The punching-loop at the early stage of nucleation and growth of helium clusters in bulk W

Fig. 2. The nucleation and growth of helium clusters at grain boundary ∑3[211](110) in W: (a) 0.043 ns, 8 He, 1 SIA; (b) 0.120 ns, 24 He, 6 SIAs; (c) 0.125 ns, 24 He, 6 SIAs; (d) 0.466 ns, 93 He, 21 SIAs; (e) 0.469 ns, 94 He, 22 SIAs; (f) 0.470 ns, 94 He, 22 SIAs

Fig. 3. The nucleation and growth of helium clusters at grain boundary ∑9[110](411) in W: (a) 0.02 ns, 3 He, 1 SIA; (b) 0.1 ns, 19 He, 7 SIAs; (c) 0.5 ns, 99 He, 23 SIAs; (d) 1 ns, 199 He, 44 SIAs; (e) 2 ns, 399 He, 121 SIAs; (f) 2 ns

Fig. 4. Calculation of the migration barrier for a W crowdion defect in bulk W

Fig. 5. Calculation of the migration barrier for a W crowdion defect at grain boundary ∑3[211](110) in W

Fig. 6. Calculation of the migration barrier for a W crowdion defect at grain boundary ∑9[110](411) in W

Fig. 7. (a) The radius and pressure of the He bubble as a function of simulation time in bulk W; (b) the radius and pressure of the He bubble as a function of simulation time at at grain boundary ∑3[211](110); (c) the radius and pressure of the He bubble as a function of simulation time at at grain boundary ∑9[110](411)