Shell was found culpable for failings in its basic maintenance regime, its safety management and its environmental control systems.
Experts from the Health and Safety Executive (HSE) said it was only good fortune that no one was killed or seriously injured in the blast.
The explosion blew the concrete roof off a buffering tank within the plant, hurling concrete and metal debris over a large area and sucking a nearby drain out of the ground. After investigating the incident, the HSE and Environment Agency (EA) jointly prosecuted the firm over safety, environmental control and pollution-prevention failures at the plant leading to the explosion.
Ipswich Crown Court heard how the blast happened shortly before 6pm on 28 February 2008 in the plant's water treatment plant. It was fortunate that at the time of the explosion daytime plant personnel were returning to offices to prepare for shift handovers
Ten appliances from Norfolk Fire Service attended the scene.
Investigators traced the cause of the explosion to a leak of highly flammable hydrocarbon liquid into a part of the plant responsible for treating waste water before discharging it into the sea.
The leak was caused by the failure of a corroded metal separator vessel, which allowed water contaminated with the highly flammable condensate to enter a concrete storage tank where it was heated by an electric heater. The heater's elements were exposed within the tank, raising the surface temperature significantly causing the explosion and fire.
Bacton is operated by several energy companies and houses gas processing plants along with the Interconnector system feeding gas between Britain and Europe.
Due to the large quantities of hazardous substances present on the site, Shell Bacton is classified as a top tier site under the Control of Major Accident Hazard (COMAH) regulations.
The court heard that during the incident there was an unauthorised release into the North Sea of 850t of fire water and fire fighting foam which ought to have been prevented.
Shell UK had failed to close the sea gate until about an hour after the fire started. It also failed to notify the Environment Agency, as required, meaning that valuable advice on environmental protection during the incident or its aftermath was not available to either Shell or the fire service - an emergency response priority first identified in 2004. The delay in notification also meant an assessment of environmental harm was not possible.
At an earlier hearing Shell pleaded guilty to seven charges covering safety, environmental control and pollution-prevention failures at the plant which led to the blast.
Shell UK was fined a total of £1milllion and ordered to pay £242,000 costs.
After sentencing, HSE inspector Steve Johnson, said: "The fact no-one was seriously hurt in this incident was solely down to good fortune as the company's internal report acknowledges. Shell UK neglected basic maintenance leading up to the explosion.
"Our investigation found key components had been failing for some years and the company knew this, yet there had been no appreciation of the potential for an incident such as this.
"In particular there had been no attempt to assess the risk that arose from condensate entering the water treatment plant despite the fact that the plant was not designed to handle highly flammable liquids like condensate.
"The investigation revealed significant failings in the safety management system operating on the plant and hopefully other operators will take note of the outcome of this incident and maybe review their own procedures."
Environment Agency environment manager for Norfolk and Suffolk Marcus Sibley said: "We are disappointed that a company such as Shell with its experience in the fuel industry should have operated in this fashion. This is a high risk industry and that is why we expect high standards.
"The explosion could have led to a major environmental disaster as other highly flammable materials were stored nearby."
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