A fire broke out at a potash mine about 4:15 am within an underground storage site devoted to toxic industrial waste (50,000 tonnes/year), which had been operating since February 1999 in galleries specifically excavated for this purpose at a depth of 535 m below the ground surface. The waste authorised for storage, identified as not flammable, explosive, gaseous, liquid, volatile, radioactive, biologically toxic, unstable at the mine bottom temperature (35°C) or reactive with salt or water, was placed in “big bags” (capacity: 1 m3), with a double pocket or in metal containers stored in 220-litre metal barrels. The fire spread over a 1,700-m² area and involved 1,800 tonnes of household/industrial waste incineration ash and residue from asbestos removal operations; according to the site operator, it was the residue packing material (big bags, pallets) that caught fire.

Miners were first disturbed by smoke entering the rock salt galleries around 5 am, then the storage site personnel sounded the alarm about 7 am. The fire was located at 7:15 am; 4 teams of miners, all wearing self-breathing apparatuses, relayed one another at the mine bottom assisted by 25 fire-fighters at the surface. A water pipe was installed inside the gallery. Blocked air intakes helped limit spreading of the flames. A press release delivered during that afternoon suggested that the situation would soon be under control. High sulphur dioxide concentrations were measured at the manhole outlet; the population was not asked to remain indoors, yet 3 schools close by were closed out of precaution. The slow combustion of these wastes would last for several days, with hotspots persisting for another 2 months. A court case was brought against unknown persons for deliberate endangerment of others. Conformance of the waste being stored relative to operational specifications was examined, and 4 independent experts were called to verify compliance with safety rules. The impact of smoke released into the environment was also investigated.

Site activity was suspended, 27 employees at the storage facility and 350 miners were made redundant. Since such an occurrence had been deemed unimaginable, the galleries were never equipped with fire detection sensors; 3 hours were required to locate the source of this fire. Since the storage centre was not independent of the mining activity, some of the galleries were connected to salt mining galleries. The operator revised the entire centre’s safety plan. Expert analyses pointed to self-ignition of the wastes (through biological degradation, chemical decomposition, chemical reaction between isolated substances). The centre’s definitive closure, announced for September 2003, led to conducting additional studies, this time focusing on: onsite waste storage safety through building confinement, partial or total waste recovery, long-term mechanical behaviour of the underground facility, environmental and health-related impacts for neighbouring residents and businesses.

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