Pollution
Humain
Environnement
Economique

During unloading of a furnace used for activating the reaction of zirconium tetrachloride (ZrCl4) on magnesium (Mg), an explosion occurred within a plant material handling zone that manufactures zirconium sponges.

Plant staff present at the time suffered hearing loss without perforation of the eardrum; the resulting material damage was phenomenal: 3,000 m² of plant siding and roofing, made of asbestos cement, were pulverised from the detonation blast; ditch cover gratings were projected 3 to 5 m into the air; the building door was torn off; hinges broken, etc. These damage observations suggest that the explosion represented the equivalent of 1 kg of TNT. The associated costs were estimated at 4.5 million francs.

The hypothesis of liquid magnesium flowing in a ditch that also contained equipment washing water was put forward. The explosion could have resulted from several simultaneous phenomena, including the formation of hydrogen by means of water decomposition, water vapourisation, and grinding of magnesium and its reaction with oxygen from air. An expert appraisal concluded however that only the thermal explosion (water vapourisation) could explain the level of damage recorded: the reactions of magnesium oxidation with water or air were contained; and the magnesium spill in the ditch served to generate hydrogen in a quantity proportional to the spill, with this fuel burning at a rate corresponding to its release at the ditch (given the presence of fire and sparking prior to the explosion).

The measures adopted to reduce the probability of recurrence of such an accident were as follows: creation of a metal retention structure for storing reaction cells after unloading while waiting for them to cool, temperature control by means of a temperature colour indicator before any handling, elimination of all washing channels and ditches in the plant that may contain water.