The UK's next nuclear power plant could be built on the island of Anglesey by 2020.
Wylfa, the existing nuclear plant, will be decommissioned in December.
Plaid Cymru leader and Anglesey AM Ieuan Wyn Jones said it was an economic boost, and Labour MP Albert Owen called it a vote of confidence in the area.
County council leader Clive McGregor said the move would have a "positive impact" on the community and deliver "high quality jobs".
He said: "We have suffered tremendous job losses over the last few years.
Alan Smith, Horizon's Wylfa project manager, said the plant "would make a vital contribution to meeting the country's energy needs and tackling the challenge of climate change"."
So, one assumes, the "Celtic Alliance" could be in for a bit of falling-out over, for example, whether to have any future minimum carbon price benefiting nuclear as well as renewables.