Overflowing petrol blamed for Buncefield depot blast

Some of the unleaded petrol evaporated to form a shimmering white cloud, which investigators said was clearly visible from CCTV footage.

The cloud was then ignited by an unknown source, perhaps a spark from a generator at a nearby car park, pumphouse, or even a passing car, triggering the largest explosion in Britain since the Second World War.

The report from the Buncefield, Hertfordshire, investigation board stopped short of considering blame to avoid prejudicing any criminal prosecution or civil legal action that might result from the incident. But it did confirm that a filling gauge on a fuel tank at the Buncefield terminal got stuck, allowing an estimated 300 tonnes of petrol to spill out.

Hi-tech systems that should have automatically stopped fuel being pumped into the tank also failed, the report added.

Separate reports from the Health and Safety Executive (HSE), the Health Protection Agency and the Department for Environment, Food and Rural Affairs, also released yesterday, found that despite the scale of the explosion, there was no long-lasting damage to air quality or the water supply in the Hemel Hempstead area.

Fears that the water table would be polluted by a potentially lethal chemical used to douse the oil tank flames have proved unfounded. The HSE said 128 similar depots were on a "higher state of vigilance" following revelations of the faulty Buncefield tank.

"One contributory factor appears to have been a failure in the safety mechanisms designed to prevent this type of incident," said an HSE spokesman.

Taf Powell, who is heading the investigation into the explosion, said all his findings would be made available to the prosecuting authorities.

"The investigation has identified that it was tank 912 from which the fuel escaped," he said.

"This tank started receiving unleaded petrol from the pipeline at around 7pm on 10 December, pumping at around 550 cubic metres per hour.

"From about 3am on 11 December, the level gauge for tank 912 recorded an unchanged reading, as if no longer filling, but filling continued. Calculations show that the tank would have been full by about 5:20am, overflowing thereafter."

Mr Powell estimated that about 300 tonnes of unleaded petrol leaked out. He said a large vapour cloud would have formed. "Closed circuit television footage show a white mist emerging at 5:38am and spreading out," he said.

"Seismological evidence shows that the main explosion occurred at 06.01:32. Eyewitness accounts also describe the main explosion being followed by a number of smaller explosions."

Lord Newton, the inquiry board chairman, said all the evidence would be passed on to the HSE, which will bring any criminal proceedings. The report did not say whether staff at Buncefield made mistakes in failing to spot the over-filling tank.

Mr Powell said an alarm should have been triggered when fuel began to spill, but he would not say whether the alarm was triggered or not.

Sources said the alarm should have sounded and caused a warning light to flash in the control room - which was about 200 yards from tank 912 and manned by two people on 11 December.

Hertfordshire Oil Storage, (HOSL), the owner of most of the Buncefield site, said it was working with the investigation to prevent a repeat of the explosion.

The company - a joint venture between Total UK and Texaco said: "The industry now needs to work together to understand why this incident occurred and what steps we can take to prevent it from happening again.

"HOSL continues to work closely with the HSE to ensure it can conduct its investigation as quickly as possible. It is also working with the Environment Agency and other bodies to protect the environment during the clean-up of the site."