Trawsfynydd, which had two 195-MW gas-cooled Magnox reactors, is on a 15-hectare site, on an inland lake in Snowdonia National Park, North Wales.
It started service in 1965 and generated 69 TWh of electricity over the 26 years until its closure in 1991.
The twin reactors at the site will now become the very first in the UK to be fully decommissioned.
In 2016 a committee of MPs said the Trawsfynydd site should be designated as a site for a first-of-its kind small modular reactor station in the UK.
According to the International Atomic Energy Agency’s PRIS reactor database, there are 11 former commercial Magnox nuclear power stations in the UK, which have all now stopped generating electricity, the last being Wylfa on Anglesey in 2015.
The Magnox stations are Berkeley (two units); Bradwell (2); Chapelcross (4); Dungeness A (2); Hinkley Point A (2); Hunterston A (2); Oldbury (2); Sizewell A (2); Trawsfynydd (2); Winfrith (1); and Wylfa (2).