How Buncefield fire unfolded

The fire raged at the depot for several days

The Buncefield fuel depot fire in December 2005 was the UK's biggest peacetime blaze. Below are key graphics, maps and satellite images showing what caused the explosion, how the blaze was tackled and its impact.

The Hertfordshire Oil Storage Terminal, or HOSL as it is also known, handled around 2.37 million metric tonnes of oil products a year - mainly petrol, diesel and aviation fuel - delivered by tankers and pipeline.

The depot, opened in 1968, is outside the town of Hemel Hempstead, 40km (25 miles) northwest of London and just off the busy M1 motorway, which was closed twice because of the fire. Some residents had to be evacuated from nearby housing, while offices and warehouses around the site suffered major damage.

Disaster struck early in the morning of Sunday 11 December as unleaded motor fuel was being pumped into storage tank 912, in the north west corner of the site. Safeguards on the tank failed and none of the staff on duty realised its capacity had been reached. By 0520 GMT, investigators believe, the tank was overflowing:

HOW TANK 912 OVERFLOWED

Under normal circumstances, gauges monitor the level of the fuel in the tank as it fills from a pipeline.

An automatic high level safety switch should trigger an alarm if the tank reaches its maximum capacity. This should result in shutdown.

But on this occasion, automatic shutdown did not happen and when fuel continued to be pumped in, it overflowed through roof vents.

The overflow from the tank led to the rapid formation of a rich fuel and air vapour. It thickened to about 2m (6.6ft) and started spreading in all directions:

HOW THE VAPOUR SPREAD

1) Fuel cascaded down the tank and formed a rich fuel/air mix, which collected in bund A (the area surrounding the tank bounded by a low wall designed to prevent leaked liquid spreading).

2) CCTV footage showed vapour flowing out of bund A from 0538. The cloud was initially about 1m deep, but thickened to 2m.

3) By 0550 vapour started flowing off the site, near the junction of Cherry Tree Lane and Buncefield Lane.

4) Between 0550 and 0600 the rate at which fuel was being pumped into tank 912 gradually increased from 550 cubic metres (1,805 cubic feet) an hour to around 890 cubic metres (2,920 cubic feet) an hour

5) At 0601, with the vapour cloud cloaked over a large area and reaching buildings next to the site, the first explosion occurred.

Further explosions followed and a large fire took hold, eventually engulfing 20 large storage tanks. Emergency services declared a major emergency at 0608 and a huge firefighting effort began, peaking with 25 fire engines, 20 support vehicles and 180 firefighters on site before the blaze was finally extinguished on 15 December:

HOW THE FIRE WAS FOUGHT

1) Artificial pool with water pumped from nearby lake used to create foam and water mix. 2) Fire engines, foam cannon and fixed fire-fighting units already at depot used to pump mixture onto blaze. 3) Huge clouds of smoke produced, at one point engulfing large parts of south eastern England (see satellite images below).

4) Fire brigade water curtain protected intact tanks on eastern part of site. 5) The bunds around each tank fail to contain all the contaminated liquid run-off - a mix of firefighting water and escaped fuel. 6) Investigations find "large amounts" of run-off escaped from the site, contaminating ground and surface water.

The plant - co-owned by Total and Texaco, with sections operated by other firms including BP and the British Pipeline Agency - suffered extensive damage, although the water curtain helped save large areas:

At one point black smoke covered much of south-eastern England, as satellite images show:

UK SATELLITE IMAGE 11 DECEMBER 2005, 1336 GMT

See how satellites tracked the blaze from space