Sellafield's decommissioning marks the beginning of the end for the controversial nuclear energy site.
When closed, Calder Hall was the oldest nuclear reactor in the world.
But the story is far from over - the process could take up to 100 years, according to the Nuclear Decommissioning Authority (NDA).
Sellafield, originally known as Windscale, was built in the late 1940s, and first generated electricity with its Calder Hall reactor in 1956.
However, it soon sparked controversy, when a fire broke out in a chimney the following year, spreading radioactivity across the Cumbrian countryside.
At the time, it was the world's worst nuclear accident and is still regarded as Britain's most severe.
Because of the amount of radioactivity released, workers were only recently allowed access to the chimney to demolish it.
The NDA says decommissioning Sellafield could take 100 years
In the 1960s, the distinctive second phase of the Calder Hall reactor - nicknamed The Golf Ball - came into operation. It remains one of the most well-recognised parts of Sellafield's skyline, though it is no longer a working part of the plant.
BNFL renamed the plant Sellafield in 1981, but many environmentalists refused to accept the re-branding, and still refer to the site as Windscale.
The Sellafield mixed oxide (Mox) plant was built in 1997. It processes waste from other countries' nuclear power plants, making the material reusable.
Unfortunately it also produces vast quantities of waste water and high-level nuclear waste which has to be kept far away from humans for 250,000 years - 50 times longer than the history of the written word.
The construction of the Mox plant was to mark a turning point for Sellafield.
In 2003, the plant ceased its job of generating electricity. Calder Hall was closed down after almost 50 years, by which point it was the world's oldest nuclear reactor.
It had been cutting-edge technology in the 1950s, but by 21st century standards its 196 megawatt capacity was considered small.
Nuclear reprocessing produces vast quantities of waste water
BNFL blamed the closure on the depressed price for electricity, along with the relatively high overheads of such a small plant.
So, despite concerns from environmentalists about the safety of transporting radioactive material to and from the site, nuclear re-processing became the primary function of Sellafield.
However, in June 2003 the government published a draft bill to create the NDA. It would be charged with clearing up the "radioactive legacy" left by 50 years of nuclear power and weapons' development.
Its creation was a trigger for decommissioning nuclear facilities across the county.
Sellafield's slow demise will no doubt be welcomed by environmentalists who have spent decades campaigning for its closure.
Long-term critics include the Irish government and a number of Scandinavian countries, who are worried about the contamination of seawater.
But the closure is a delicate matter, especially when the site employs 12,000 people in the region.
According to trade union Amicus, around 8,000 jobs are likely to be cut on the site in the next 10 years, which is extremely bad news for an area where very few other large employers exist.