The chemical plant, owned by Nypro (UK) (a joint venture between Dutch State Mines and the British National Coal Board), and in operation since 1967, produced caprolactam, a precursor chemical used in the manufacture of nylon. Residents of the village of Flixborough were not keen to have such a large industrial development so close to their homes and had expressed concern when the plant was first proposed.
The process involved oxidation of cyclohexane with air in a series of six reactors to produce a mixture of cyclohexanol and cyclohexanone. Two months prior to the explosion, a crack was discovered in the number 5 reactor. It was decided to install a temporary 50 cm (20 inch) diameter pipe to bypass the leaking reactor to allow continued operation of the plant while repairs were made.
At 16:53 on Saturday 1 June 1974, the temporary bypass pipe (containing cyclohexane at 150°C (302°F) and 1 MPa) ruptured, possibly as a result of a fire on a nearby 8 inch (20 cm) pipe which had been burning for nearly an hour. Within a minute, about 40 tonnes of the plant's 400 tonne store of cyclohexane leaked from the pipe and formed a vapour cloud 100–200 metres (320-650 feet) in diameter. The cloud, on coming in contact with an ignition source (probably a furnace at a nearby hydrogen production plant) exploded, completely destroying the plant. Around 1,800 buildings within a mile radius of the site were damaged.
The fuel-air explosion was estimated to be equivalent to 15 tonnes of TNT (60 gigajoules) and it killed all 18 employees in the nearby control room. Nine other site workers were killed, and a delivery driver died of a heart attack in his cab.
Observers have said that had the explosion occurred on a weekday, more than 500 plant employees would likely have been killed. Resulting fires raged in the area for over 10 days. It was Britain's biggest ever peacetime explosion until the Buncefield Depot explosion in 2005.
Substantial destruction of property was recorded in Flixborough itself, as well as in the neighbouring villages of Burton-upon-Stather and Amcotts. Significant structural damage affected Scunthorpe (eight miles away) and the blast was heard (and felt) twenty-five miles away in Grimsby.
Although the area was quite remote, graphic images of the disaster were soon shown on television due to BBC and Yorkshire Television crews who had been covering the Appleby-Frodingham Gala in Scunthorpe that afternoon.
The official inquiry into the accident determined that the bypass pipe had failed due to unforeseen lateral stresses in the pipe during a pressure surge. The bypass had been designed by engineers who were not experienced in high-pressure pipework, no plans or calculations had been produced, the pipe was not pressure-tested, and was mounted on temporary scaffolding poles that allowed the pipe to twist under pressure. These shortcomings led to a widespread public outcry over industrial plant safety, and significant tightening of the UK government's regulations covering hazardous industrial processes. See COMAH Regulations.
Despite protests from the local community the plant was re-built but due to a subsequent collapse in the price of nylon, it closed down a few years later. The site was demolished in 1981 although the administration block still remains. The site today is home to the Flixborough Industrial Estate, occupied by various businesses and Glanford Power Station.
The foundations of properties severely damaged by the blast and subsequently demolished can be found on land between the estate and the village, on the route known as Stather Road. A memorial to those who died was erected in front of offices at the rebuilt site in 1977. Cast in bronze, it showed a number of mallards in flight landing on water: When the plant was closed the statue was moved to the pond at the parish church in Flixborough. During the night on New Years' Day 1984 the sculpture was stolen. It has never been recovered and those responsible for the crime have never been found.
The plinth it stood on, featuring a plaque listing all those who died that day, can still be found outside the church.
Previously, in April 2000, allegations of cover-up had been unveiled by a whistle-blowing scientist sacked from the original inquiry, Ralph King. He said the original inquiry was wrong to blame a simple mechanical failure: "I realised that what we were really there for was to come up with a cause which would not embarrass the company". His criticism led to the HSE ordering laboratory experiments. The test results released in November 2000 seemed to back up Mr King's theory that the presence of water inside the reactors and the simultaneous shutting down of crucial equipment, generated a massive build-up of pressure that blew the valve apart.