An ammonia leak (NH3) occurred during unloading of a tanker car within a steel mill. The stored quantities of NH3 at this site consist of two 30-m³ tanks (18 tons) supplying the hydrogen production installations. This gas is used as an atmospheric gas for heat treatment ovens within the “finished steel sheet” unit. The reservoirs were coupled, yet could still be isolated by means of manual valves. At the end of 2003, power valves actuated by a palm button were installed as a complement to the sectioning devices designed to reduce reaction time in case of accident. In the hours preceding the leak, the southern tank reached its low point and was isolated form the network by activating the power valve; the emergency intervention report did not make mention of the type of closing device employed. During material transfer, the agents observed that only the northern tank was being supplied and they isolated it using the manual valve in order to fill the two tank capacities evenly. Since the southern tank’s motorized sectioning device was closed, the supply pipe rose in pressure and one of the relief valves positioned on the pipes opened. The operator shut off the transfer pump but the leak continued for another hour, and 50 litres of liquid NH3 were discharged into the atmosphere. The odour nuisance was perceptible at a distance of 500 m from the installation. The POI emergency plan was not enacted, and no casualties were reported. The inspection investigation revealed that subsequent to the works conducted in 2003, material transfer procedures were not updated and did not address the electric sectioning devices. This organizational flaw, which caused the verification of power valve position to be overlooked, is the reason behind the ammonia discharge. The process was then modified: both NH3 reservoirs were to be replaced at the beginning of 2005 and 2006 respectively by hydrogen storage facilities. A draft prefecture decree, preceding the accident, had been transmitted to the prefect for authorization of this modification and, in the interim, reinforce safety measures for NH3 installations