During the cupola drop in a cast iron foundry by opening a door positioned on the lower part of the cupola, an explosion occurred at a time when 1500 kg of clinker and other impurities remaining at the end of the melting cycle were being dropped into a bin. A portion of the building roof was blown off.

According to the foundry operator, the presence of carbon oxide (CO) formed inside the cupola furnace caused this explosion, as the presence of water in the bin had been eliminated from consideration. Stopping the dust removal extraction unit fan at the end of casting had in fact allowed CO to form due to a lack of oxygen in the cupola. During the cupola drop, the CO concentration mixed with oxygen in the air was capable of reaching the explosive limit and then exploded in contact with melted cast iron.

The operator modified the last part of the melting procedure in order to leave on suction until the cupola drop, thus avoiding CO formation. The Classified Facilities Inspection Authority requested that the operator question the cupola furnace manufacturer regarding both this accident analysis and the validity of the cupola drop procedure.