Hydrogen permeation in fusion-relevant materials is of great importance as hydrogen substantially changes material properties such as embrittlement, and hence, strongly reduces the remaining useful life (RUL) of the reactor. At the same time, hydrogen isotopes, tritium in particular, need to be extracted from W divertors as they are the fuel that makes fusion go, not to mention radioactivity. Therefore, controlling hydrogen permeation is crucial in the operation of fusion reactors. So far, hydrogen is known to spread over the entire polycrystalline W via diffusion, noticeably through grain boundaries that work as fast channels, but the grain boundary diffusion may not be a single dominant mechanism for two reasons. First, the energy barrier for hydrogen atoms to climb out of a grain boundary is high. Second, the corresponding heat necessary for activation is likely to come from ion bombardment supplying energy in the form of collision cascades. Consequently, the local atomic arrangement used as a basis for activation energy calculation becomes obsolete. In this talk, we propose a complementary hydrogen permeation mechanism in irradiated W, where hydrogen atoms confined in a grain or along a grain boundary may burst out to neighboring grains upon a primary knock-on atom event in the vicinity of a grain boundary. A PKA not only delivers hydrogen to neighboring atoms but also empties grain boundaries that can further accommodate hydrogen atoms. We present atomistic calculations results that visualize the PKA piercing, mixing, and the resulting cascades around grain boundaries. We also suggest a complex big picture of the hydrogen permeation in W, in which the permeated hydrogen changes the defect morphology that in turn changes the hydrogen diffusion.