In an experimental ultimate waste storage facility, a fire broke out in a test cell in which 8,940 kg of magnesium turnings from a metallurgical company had been stored for a period of 7 days. The emergency services deployed water curtains and smothered the fire by covering the 40 m² of burned materials with a 20 cm thick layer of sand and cement. A laser pyrometer was used to monitor the peripheral temperature of the block affected. The alert was lifted 8 hours after its activation. The consequences of the fire were minor: no one was injured, and there was no water or soil contamination due to the use of sand to extinguish the fire. The heating of magnesium chippings, caused by the oxidation reaction used to stabilise the waste, was at the origin of the accident. This stabilisation, initiated 7 days earlier, involved oxidising the significant organic load of the waste (probably due to cutting oil) by adding a mineral substance. A second cement-based mineral load was then added to the waste; the mixture was then mixed with water and stored in a cell for 7 days. The oxidation reaction caused a 20 °C rise in temperature. The fire started in the lower level of the block when the formwork was being removed. The rise in temperature, caused by the oxidation reaction which was greater at the bottom of the block due to the reduced heat dissipation, caused the magnesium to ignite. This was the first time that such a large amount of magnesium had been globally treated on the site, and the mass effect had not been assessed in the laboratory.