The three workers faced a terrible calculation. They needed to raise the alarm, but they feared that starting their truck or even using the telephone might ignite the escaping gas. So they set off on foot. By the time they reached help and traffic on the motorway was stopped, it was already too late. The propane cloud found an ignition source -- a car on the adjoining road. Flames raced back to the storage sphere. The refinery processed about 2 million tons of crude oil annually, and the storage area where the disaster unfolded sat in a 145-meter-wide strip between a local road and a motorway boundary fence. Four 1,200-cubic-meter spherical propane tanks, four 2,000-cubic-meter pressure vessels, two horizontal bullet tanks, and ten large floating-roof storage tanks for petrol and kerosene occupied this narrow corridor. The nearest sphere was just 42.4 meters from the motorway.

Fire services arrived but faced a phenomenon few had been trained to handle: a BLEVE, or Boiling Liquid Expanding Vapour Explosion. When fire engulfs a pressurized tank of liquefied gas, the liquid inside boils and the rising pressure eventually ruptures the vessel. The result is a fireball of extraordinary violence. At Feyzin, the leaking sphere exploded while firefighters were attempting to cool the surrounding tanks. The blast killed several firemen outright. Flying debris from the ruptured vessel smashed the support legs of an adjacent sphere, which then toppled and suffered its own BLEVE. Three more spheres collapsed as their inadequately fire-protected support legs gave way. These vessels ruptured but did not explode. Petrol and crude oil tanks caught fire as well. The conflagration took 48 hours to bring under control.

When the final count came in, 18 people were dead and 81 injured. The dead included firefighters who had positioned themselves close to the burning spheres in an attempt to prevent exactly the chain reaction that occurred, and spectators who had gathered to watch the blaze -- drawn by the spectacular flames visible from the motorway. The Feyzin disaster was, at the time, the worst accident to occur in a petroleum or petrochemical facility anywhere in Western Europe. It would hold that grim distinction until the Flixborough disaster in England in 1974. The human toll was made worse by the proximity of everything: the spheres were spaced between 11.3 and 17.2 meters apart, and the nearest houses in the village of Feyzin stood only 300 meters away.

Feyzin forced a fundamental rethinking of how the petrochemical industry handles pressurized liquefied gas storage. The disaster demonstrated that BLEVEs produce intense but relatively localized thermal radiation compared with unconfined vapor cloud explosions, meaning that evacuation of 500 meters is generally sufficient to protect lives -- if people actually evacuate rather than watch. Storage sphere design changed dramatically: fire protection for support legs became standard, water deluge systems were improved, and spacing requirements between vessels were reconsidered. Perhaps the most significant change was philosophical. So many firefighters had been killed attempting to control burning pressurized vessels that the industry's prevailing wisdom shifted toward a strategy of evacuation and sheltering, allowing the material to burn itself out rather than risking lives to extinguish fires that could not be safely fought. A single missing valve spanner had exposed the lethal consequences of inadequate procedures, insufficient training, and industrial facilities built too close to public roads.