Sellafield is a large multi-function nuclear site close to Seascale on the coast of Cumbria, England. As of August 2020, activities at the site include nuclear fuel reprocessing, nuclear waste storage and nuclear decommissioning, and it is a former nuclear power generating site. The licensed site covers an area of 265 hectares (650 acres), and comprises more than 200 nuclear facilities and more than 1,000 buildings. It is Europe's largest nuclear site and has the most diverse range of nuclear facilities in the world situated on a single site.Sellafield site incorporates the UK's first generation nuclear reactors and associated fuel re-processing facilities at Windscale, and the world's first nuclear power station to export electricity on a commercial scale to a public grid at Calder Hall. The UK's National Nuclear Laboratory has its Central Laboratory and headquarters on the site.

Sellafield was the site in 1957 of one of the world's worst nuclear incidents. This was the Windscale fire which occurred when uranium metal fuel ignited inside Windscale Pile no.1. Radioactive contamination was released into the environment, which it is now estimated caused around 240 cancers in the long term, with 100 to 240 of these being fatal. The incident was rated 5 out of a possible 7 on the International Nuclear Event Scale.Originally built as a Royal Ordnance Factory in 1942, the site briefly passed into the ownership of Courtaulds for rayon manufacture following WW2, but was re-acquired by the Ministry of Supply in 1947 for the production of plutonium for nuclear weapons, and was given the name "Windscale Works". Subsequent key developments include the building of Calder Hall nuclear power station, the Magnox fuel reprocessing plant, the prototype Advanced Gas-cooled Reactor (AGR) and the Thermal Oxide Reprocessing Plant (THORP).

Activities at the Sellafield site are primarily decommissioning of historic plants, and reprocessing of spent fuel from UK and international nuclear reactors, which will completely cease when the Magnox fuel reprocessing plant closes in 2021. Decommissioning projects include the Windscale Piles, Calder Hall nuclear power station, and a number of historic reprocessing facilities and waste stores.

The site currently directly employs about 10,000 people, and is owned by the Nuclear Decommissioning Authority (NDA) which is a non-departmental public body of the UK government. Following a period 2008–2016 of management by a private consortium, the site has been returned to direct government control by making the Site Management Company, Sellafield Ltd, a subsidiary of the NDA. The site is due to be fully decommissioned by 2120 at a cost of £121 billion.