  We report a first-principles density functional theory study on the role of
grain boundary and dislocation loop in H blistering in W. At low temperature,
the {\Sugma}3(111) tilt grain boundary, when combined with a vacancy of
vanishing formation energy, can trap up to nine H atoms per (1x1) unit in (111)
plane. This amount of H weakens the cohesion across the boundary to an extent
that a cleavage along the GB is already exothermic. At high temperature, this
effect can be still significant. For an infinitely large dislocation loop in
(100) plane, four H can be trapped per (1x1) unit even above room temperature,
incurring a decohesion strong enough to break the crystal. Our numerical
results demonstrate unambiguously the grain boundaries and dislocation loops
can serve as precursors of H blistering. In addition, no H2 molecules can be
formed in either environment before fracture of W bonds starts, well explaining
the H blistering in the absence of voids during non-damaging irradiation.
