A new process for synthesising an organomagnesium compound was being manufactured in a fine chemistry plant when an operator noticed smoke escaping from the 40-litre filter that had been used for 2 hours before filter excess magnesium. When alerted to the situation, the foreman decided to spray down the filter with water to cool it down, but the water vaporised immediately. He then asked his team to step back. A few seconds later, the filter burst and incandescent debris filled the workshop. A piece of this debris reached a 100-litre container of acetone and ignited.

The operators evacuated the premises, and the operator initiated the internal emergency plan and alerted the emergency rescue services. At around 4:30 p.m., he finally alerted the prefecture to initiate the special intervention plan, given the extent of the fire and the smoke in the premises. The 40 firemen arrived on site at 4:50 p.m., and assisted by a chemical emergency squad, set up a 3 km safety perimeter and confined a school. The operator managed to control the fire. Approximately twenty military policemen controlled the vehicular traffic around the site. The factory was secured by a retaining ring, and the outlets to the Mayenne River were closed. Firefighters finished extinguishing the fire around 5:15 p.m. after having protected the above ground acetone storage tanks. The internal emergency plan/special intervention plan was lifted at 6:15 p.m.

This accident scenario had not been foreseen. It was caused by the presence of unstable materials that had self-decomposed in the filter. The filter had been closed, and had no safety device to release the overpressure caused by the decomposition phenomenon, so it exploded. The operator overhauled the steps of the process and replaced the filter by a secured model able to withstand overpressure. The personnel were reminded of the rules for handling containers containing flammable materials.