Around 1:40 am, an explosion in a supermarket’s petrol station lifted the concrete paving, which wound up splitting into many fragments and falling onto the tanks. The plates on 5 inspection chambers leading to tanks compartments manholes were blasted 40 to 60 m northward. A recreational vehicle parked on the lot was also damaged, but its occupants were unhurt. In 1992, this filling station was the site of a pollution incident contaminating Dijon’s southern water table, which supplied a portion of the metropolitan area (50,000 population). The installation included two 100cu.m tanks: one with two 50cu.m compartments for unleaded gas 95 and 98, and another with 3 compartments (one measuring 20cu.m for premium grade, the other two 40cu.m for diesel).

The measures requested related to soil and groundwater pollution, setting-up of a safety zone, tank drainage and neutralisation, tanks placed in safe operating mode, and site surveillance. The elements available made it possible to conclude to the absence of any pollution risk (no tank leak, etc.). The damage observed stemmed from the explosion of 4-8cu.m of an explosive atmosphere composition lying between the lower flammable limit and stoichiometry, i.e. less than 0.5 litres of liquid fuel. The presence of this quantity of fuel was due both to the release of vapours at the level of joints or a stuffing box (that leaked when transferring unleaded 95/98 on 30th December) and to the vaporization of liquid fuel that had spread during sampling for analyses in the unleaded 95, 98 and premium grade tanks on 22nd December. The ignition source was of an electrical origin, as electrostatic, mechanical, thermal and malicious causes could all be excluded. The most likely hypothesis was that of a spark associated with an overvoltage appearing in a ground line located 130 m from the tanks subsequent to a current leak produced by accidental contact between a medium voltage power line cable phase and the ground. This malfunction resulted in a loss of electrical power (hotel, etc.) and damage (breakdown of the lightning rod, etc.). The absence of filling material in the space between the tanks and the concrete paving, as a result of various modifications, facilitated the accumulation of an explosive atmosphere. This cavity, even filled with an inert material, should have been considered as an occasional explosive atmosphere. The station rebuilding project (€1 million) planned on using electrical equipment adapted to the overvoltage situation and covering the pit and tanks with a layer of gravel and asphalt mix. The cost of securing the site amounted to €500,000, while operating losses were also assessed at €500,000.