The fire was in a polyamide production chemical plant classified SEVESODuring maintenance of an isolated unit producing a polyamide monomer (LACTAME), a fire developed on the ground floor of the unit of hydrogenation of cyclo-dodecadiene (CDA). A fire detector on the unit triggered an alarm 4 min after the fire started and the station operator put the unit in safe conditions and alerted the firefighters of the site. The fire was controlled after a half an hour. The nitrogen inerting device of the reactors was put into service, it then presented malfunctions: difficulties of connection of the hose, leakage on the valve. There was no impact on the environment. Damage to the unit was noted (damaged valves, melting cables), it restarted 13 days later.The analysis of the accident by the plant operator showed that a sensor had failed to give the signal of low level in the CDA storage tank. The low level has allowed hydrogen into the production line, which was in stand-by. The hydrogen escaped from the manual valve at the bottom of the reactor finishing the process . This valve had already been found leaking a week before and tightened. In the presence of an unidentified ignition source, the leaked hydrogen ignited. The fire was further alimented by the opening of the valves of automatic degassing of the reactors, due fire damage. This opening caused the increase of the hydrogen supply of the unit. Late fire detection and too slow pressure drop in the decompression line were also aggravating factors.