Oriented intra-granular bubble transport due to coupling of pinned bubble growth and dislocation climb

•Explanation of gas transport towards grain boundaries in accidental conditions.•The gas is in over-pressurized bubbles pinned on edge dislocations.•Bubbles grow sending uranium atoms along the dislocations, which climb.•Bubbles move with the dislocations. Coalescence maintain the overpressure.•This process explains observed large fission gas release at high temperature.