The Sellafield site is built on land that was formerly part of the Windscale nuclear site (named after another nearby village). Windscale was owned by the United Kingdom Atomic Energy Authority, but when part of it was transferred to the ownership of British Nuclear Fuels Limited, the transferred part was renamed Sellafield. The remainder remains in the hands of the UKAEA and is still called Windscale.
In 1957, there was a fire at the Windscale reactor there, which was considered the world's worst nuclear accident until Chernobyl. Produce, especially milk, from the surrounding farming areas had to be destroyed. An estimated 20,000 Curies of radioactive material were released. (For comparison 7 million Curies were released by Chernobyl, and 15 Curies by Three Mile Island).
The Windscale reactor was the first British weapons grade plutonium 239 production facility built for the British nuclear weapons program in the late 40s and the 50s. The facility preceeded efforts for peaceful use of nuclear energy. In the hasty effort building the 'British Bomb', radioactive waste was simply pipelined out into the Irish Sea, which is still some of the most heavily contaminated water in the world. It has been estimated that about 250kg ['The worst accident in the world. Chernobyl: The end of the nuclear dream', Observer] of plutonium have been deposited in the marine sediments surrounding the site during its lifetime. Some studies have shown a leukaemia pocket in the surrounding villages (though their findings are disputed) and it has been shown that cattle and fish are contaminated with plutonium 239 and Caesium 137, originating from the contaminated sediments.
Windscale was also the site of the prototype British Advanced Gas-cooled Reactor. This reactor was shut down in 1981.