ROF Sellafield was constructed by John Laing & Son in 1942 at Low Sellafield as a Second World War ROF . The nearby sister factory , ROF Drigg which produced TNT ( trinitrotoluene ) had been constructed in 1940 , 5 km to the south-east and adjacent to the village of Drigg . ROF Drigg and ROF Sellafield were built in these isolated and remote coastal sites because of the hazardous nature of the process and to minimise the risk of enemy air attack . They were both classed as Explosive ROFs specialising in high-explosive TNT ( ROF Drigg ) and propellant ( ROF Sellafield ) , and production ceased at both factories immediately following the defeat of Japan . <!-- --> After the war , the Sellafield site was briefly in the ownership of Courtaulds for development as a factory , but was reacquired by the Ministry of Supply to adapt the site for the production of materials for nuclear weapons , principally plutonium . Construction of the nuclear facilities commenced in September 1947 . The site was renamed Windscale Works ( after Windscale Nook , a bluff on the north bank of the River Calder ) to avoid confusion with the Springfields uranium processing factory near Preston . The building of the nuclear plants at Windscale Works was a huge construction project , requiring a peak of 5,000 workers . The two air-cooled and open-circuit , graphite-moderated Windscale reactors ( the " Windscale Piles " ) constituted the first British weapons grade plutonium-239 production facility , built for the British nuclear weapons programme of the late 1940s and the 1950s . Windscale Pile No. 1 was operational in October 1950 ( just over three years from the start of construction at Sellafield ) , and Pile No. 2 in June 1951 . Pile 1 was the site in 1957 of the Windscale fire , the worst nuclear accident in UK history , ranked in severity at level 5 out of a possible 7 on the International Nuclear Event Scale
Windscale fire
Windscale fire
Sellafield