It started as a Royal Ordnance Factory in 1942, built far from cities because the explosives work was dangerous and the wartime sky belonged partly to the Luftwaffe. It had rail access, water from Wastwater, and a workforce drawn from the Cumbrian coastal villages. After Japan's surrender the factory closed, and Courtaulds briefly held the site for rayon manufacture. Then the Ministry of Supply took it back in 1947 for a different war - the post-war race to build a British atomic bomb. Construction of the Windscale Piles began in September of that year. The peak workforce reached five thousand. Pile No. 1 went critical in October 1950; Pile No. 2 in June 1951. The first British plutonium reached the weapons division at Aldermaston by August 1952. Operation Hurricane detonated Britain's first nuclear device in the Monte Bello Islands the following October.

Across the Calder River, four Magnox reactors rose during 1953-1956 - codenamed PIPPA, Pressurised Pile Producing Power and Plutonium, an honest acronym for a dual-use plant. Queen Elizabeth II officially opened Calder Hall on 17 October 1956. It was the first nuclear power station anywhere to export electricity on a commercial scale to a public grid. For much of its early life, electricity was the secondary purpose - the reactors made weapons-grade plutonium two fuel loads a year - but from 1964 the site shifted towards civilian production. The Magnox Reprocessing Plant followed in 1964, processing fifty-five thousand tonnes of fuel before its final batch on 17 July 2022. The Thermal Oxide Reprocessing Plant - THORP - opened in 1994 and operated until November 2018. In 1981, BNFL renamed Windscale and Calder Works as Sellafield. The change reflected a consolidation under one head, and a desire to put a difficult chapter under a different name.

The bulk of the work at Sellafield today is undoing what was built. The Pile Fuel Storage Pond, the Pile Fuel Cladding Silo, the First Generation Magnox Storage Pond, the Magnox Swarf Storage Silo - these are the priority legacy hazards, and they are extraordinary engineering challenges. The First Generation Magnox Storage Pond, originally called B30 and nicknamed Dirty 30, holds about 1,200 cubic metres of radioactive sludge of unknown characteristics and 14,000 cubic metres of contaminated water. The Magnox Swarf Storage Silo, where stripped fuel-rod cladding had been deposited since 1964, holds 11,000 cubic metres of historic waste. In June 2022, Sellafield Ltd announced the start of waste retrievals from the silo - a programme expected to take about twenty years. In August 2023, work began to retrieve waste from the Pile Fuel Cladding Silo, described as one of the most complex decommissioning challenges in the world. These are not glamorous jobs. They are the careful, decades-long labour of generations of Cumbrian workers who will spend their entire careers undoing what their predecessors built.

The relationship between Sellafield and the communities around it is woven from competing threads. The wages are decent. The work is long-term in an area where good jobs are scarce. Sellafield families - parents, grown children, sometimes grandparents - have worked the same site through Windscale, BNFL, Sellafield Ltd, the Nuclear Decommissioning Authority. They include welders, electricians, divers, health physicists, riggers, engineers and the laboratory chemists who design new ways to encapsulate sludge. They also include the people who lived through the Windscale fire of 1957 and the decades when the children of Seascale developed leukemia at elevated rates that science still cannot fully explain. The Republic of Ireland, the Isle of Man, and Norway have all formally protested the site's continued operation, citing risks of marine radioactive contamination. In 2003, the Committee on Medical Aspects of Radiation in the Environment found the local childhood leukemia cluster "unlikely to be due to chance" without identifying a convincing cause. The site continues to be the largest employer in west Cumbria. The site continues to be the source of profound ambivalence in the same region.