At a chemical site, a double-walled steel cryogenic tank of anhydrous ammonia (NH3) with a capacity of 11,800 tons burst during filling after a down period for maintenance; 105 tons of NH3 were released into the atmosphere (30 tons from the pressure surge). One employee was seriously injured. The tank had been turned off and drained, yet not inerted, 5 months prior in order to change the two bottom valves which had been leaking slightly. Restart operations then proceeded according to a well-defined process: introduction of 20% cold ammonia water (NH4OH) to protect the tank bottom (over 25 cm), followed by spraying by the tank lid with cold anhydrous NH3 to gradually return the storage chamber to its temperature. After around ten days, the anhydrous NH3 introduced by spraying (30 additional cm at the tank bottom) enabled the temperature to be lowered to -20°C. The decision was thus made to fill the tank normally via the bottom valves. When the valves were opened, the pipes began to shake and a considerable pressure rise in the storage chamber was noticed until the shell of the tank bottom opened and lifted nearly a meter and a half off the ground, causing partial drainage of the contents. Despite the precautions taken, the anhydrous NH3 and the 20% solution of NH4OH mixed too rapidly, and the presence of oil in the tank would have been responsible for the accident. The oil would have formed a separating layer between the ammonia water and the anhydrous NH3, and this separation would have broken upon opening the bottom valves, placing the NH4OH and NH3 in contact with one another too quickly.