Schoolchildren playing near the scene were rushed indoors and people living nearby were told to secure all windows and not leave their homes until told they could do so. At least one of the seriously injured had to be airlifted to the regional burns unit 10 miles away.
The explosion happened at the chemical plant operated by Hickson and Welsh, which supplies chemicals to the pharmaceutical industry. During a routine maintenance operation a toxic substance, nitrotolulene, which is highly flammable, was being moved in a tank. Nitrotoluene is used to make dyes, pharmaceuticals and fertilisers. The blast happened at lunchtime when most of the company's employees were not in the plant. The fireball ripped through a car park and destroyed a prefabricated building.
Dr James Fyfe, the managing director of the factory, said that a full inquiry would be held into the incident but as yet there was no explanation for what had happened. He said: 'At this stage we have no idea what went wrong. It was a routine maintenance operation that has been carried out countless times in the past.'
Witnesses said that they heard a roaring sound shortly before 1.15pm and then saw a huge fireball engulf a building within the vast plant. The plant is almost in the centre of Castleford and next to the town's rugby league club, local schools, shops and homes.
One witness, Sheila Malanszak, whose house overlooks the plant, said she heard a deafening explosion and thought there had been an aircraft crash. 'There was a huge bang. I raced outside and saw debris being hurled into the fields near the plant. The whole plant was just covered in a massive pall of smoke and flames. The smell was horrible. It just made you feel sick and it was difficult to breath.'
Jim Manuel, the chief fire officer for West Yorkshire, said that the fierce blaze which followed the explosion had destroyed several cars in a nearby car park. He said: 'The initial explosion was very, very severe. The explosion created a fireball which travelled 150 yards across the roadway and into a four-storey block. It also destroyed a prefabricated building at the plant.'
It is the second chemical blast in West Yorkshire in two months. In July a chemical fire broke out at the Allied Colloids factory in Bradford. In that incident smoke drifted for more than 20 miles from the plant although there were no serious injuries.
Yesterday's explosion was far more serious and again raised questions about the safety of such plants, which are situated near people's homes. Hicksons and Welsh is said locally to be extremely safety conscious and is well thought of in Castleford. It sponsors the rugby league team and is a significant employer. Nevertheless, people are well aware that there is a potential risk with the plant being located so close to homes.
The incident brought into play a full emergency plan with 17 fire engines and more than 100 firefighters being used to fight the blaze. Fire crews had to wear full protective gear, with breathing apparatus, to search for people missing and to deal with the injured. The survivors, said to be suffering from burns and blast injuries, were taken to Pontefract General Infirmary and the regional burns unit at Pinderfields Hospital in Wakefield.
Some three hours after the blast West Yorkshire police gave the all clear to people living in the area. Inspector Neville Oughtibridge said that people had been told to stay indoors initially as a precaution but it was not thought that the smoke contained noxious fumes.
The last serious incident at the plant was in the 1930s. The firm, - which with 800 workers is the largest employer in the town - has been operating in Castleford, on the banks of the River Calder, since 1915. The Control of Industrial Major Accident Hazards Regulations of 1984 are aimed at preventing chemical industrial accidents and to limit the consequences to people living nearby.
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