In an electric steel mill at 7:10 pm, the supervising operator of a melting furnace (70-tonne capacity) noticed blue flames from the surveillance camera images, interpreted as a sign of the presence of water in the furnace. He closed the safety hatch in front of the glass pane separating the control booth from the furnace enclosure and proceeded by requesting the evacuation of all personnel in the area. A powerful explosion occurred a few moments later upon contact between water and molten metal.

During the afternoon, a water leak had been observed on two cooling return tubes from the furnace crown; one was changed and in order to remedy the second, it was decided to activate the emergency return circuit. Since the water valve on this backup circuit had not been opened, a cooling system malfunction occurred and caused perforation of a tube and, along with it, water inflow into the furnace. Two employees sustained slight concussions and were taken to hospital; they were both released during the evening. The material damage was significant: collapse of doors on the “dog house” (furnace enclosure), broken crown suspension tie rods, projection inside the building of a portion of the smoke recovery device. The cost of this material damage was estimated at 1.64 million euros, with operational losses totalling 630,000 euros.

During its investigation work, the hazardous installations inspectorate recorded the following: an inadequate organisation of maintenance work specific to water supply pipes feeding the furnace crown (procedures, management of technical personnel, monitoring, etc.), onsite instrumentation unable to efficiently control either furnace crown cooling or water circuit function (no temperature or pressure variation measurement), and the lack of instrumentation on the backup cooling circuit. The operator commissioned an investigation by an independent body in order to elucidate the causes of this accident as well as determine both the technical and organisational measures to adopt for avoiding a repeat accident. The investigation also provided specifications for: backup circuit instrumentation, maintenance organisation overhaul, implementation of a hydrogen detector, and completion of a study on cooling circuit instrumentation to improve efficiency monitoring.