A major ammonia leak occurred on the refrigeration installations during defrosting of the air cooler in the cold storage rooms of a warehouse containing foodstuffs. Trapped in the basement, 2 employees died while trying to escape by a freight elevator (purposely blocked in order to simplify handling?) and 5 others were burned to varying degrees. An expansion plug (diameter: 93 mm, thickness: 4 mm, weight: 191 g), welded 10 years prior, had broken off from the end of a pipe (diameter: 82/89 mm, disused yet still not disassembled) leading into a corridor. The NH3, which was heated (due to the defrosting operation), arriving onto a cold pipe wall created thermal stresses and caused fatigue rupture of the plug. Such a sequence was facilitated by a lack of bond between the metal removed and the base metal. The pipes were inspected so as to eliminate welds and all questionable assemblies. As a result of this accident, defrosting guidelines were modified (limited number of workers present, access to premises prohibited, etc.). Moreover, the emergency exits were renovated and the personnel was equipped with appropriate protective gear.