The Wylfa nuclear power station will shut down after 44 years of electricity production.
Reactor One is due to be switched off today, five years after the plant’s original planned closure date.
Located at Anglesey, it has been generating electricity since 1971, enough to power more than a million homes every year.
Secretary of State for Wales Stephen Crabb said: Wylfa has been a terrific success story for Anglesey and for the UK nuclear industry. As attention turns to the decommissioning and clean-up of the site, it will continue to remain a vital source of employment in the local area.
“But the future of UK nuclear is about more than just one reactor. UK nuclear is also about developing and exporting our world-leading decommissioning expertise and boosting the domestic supply-chain, creating new skilled jobs across the country.”
The second reactor was shut down in 2012.
Earlier this year the UK Government said it would provide £2 billion of initial support for a new nuclear power station at Hinkley Point in Somerset.