One of Britain's oldest nuclear power stations shuts down on New Year's Eve after operating for over 40 years.
Returning the site to brown or green filed status could take 100 years
The government gave the go-ahead to decommission the Sizewell A nuclear reactors in Suffolk earlier this year.
Magnox Electric - which runs the facility - will then dismantle it once it stops generating electricity.
Another of the UK's first generation of nuclear power stations, Dungeness A in Kent, is also being closed down on the same day.
The buildings, plant and equipment from the sites will all be removed but it could take up to a 100 years before the areas are declared clean for new uses.
A spokesman for Sizewell told BBC News that the current projection for the time to completely clear the site and take it back to greenfield or brownfield status is 80 to a 100 years.
"However, there are possibilities for this process to be accelerated to as low as 20 to 30 years, but this would of course incur larger costs," he said.
The spent nuclear fuel will be sent to Sellafield in Cumbria over the next two years.
The Health and Safety Executive said that conditions were attached concerning mitigation measures to prevent, reduce and, where possible, offset adverse environmental effects.
Another 10 years
Environmental benefits would far outweigh any problems and that there will be no significant effects on other countries, it said.
Sizewell A is owned by the NDA (Nuclear Decommissioning Authority), which is a government body.
The site is licensed by Magnox, and is managed by British Nuclear Group.
The nuclear power station Sizewell B is set to continue operating for at least another 10 years.