U.S. nuclear power provider Constellation Energy on Friday agreed to buy privately-held natural gas and geothermal company Calpine Corp for $16.4 billion, marking one of the biggest acquisitions in the history of the U.

The net purchase price of the deal is roughly $26.6 billion, which includes the assumption of debt for the privately held Calpine. Constellation Energy rose 9.5% to $266.89 in pre

Constellation Energy — the electricity provider whose stock has doubled over the last year on hopes it’ll help meet the voracious energy demand from artificial intelligence — reached a $16 billion deal to buy Calpine.