An explosion occurred when restarting an hydrocracker in a petrochemical industry. The leakage gas is caused by the failure of the air cooler due to corrosion of a component under high pressure. The released gas ignited due to an unknown ignition source resulting in vapour cloud explosion that was followed by a fire. Result of loss power, emergency procedures is activated and all the connecting pipes to the tank storage were shut-off. 30 t hydrogen, 150 t of light hydrocarbons (C1 to C4), 5,5 t pentane and hydrogen sulphide are involved. 8 people received light injuries, but no effect was reported outside the establishment. The cost of the material damages has been estimated in about 50,5 M$.