Brussels in March announced it was delaying its long-planned exit from nuclear energy for 10 years as Russia's war on Ukraine sent power prices soaring. Belgium had planned to be nuclear-free by 2025.
The accord with Engie comes after months of fierce negotiation with the Belgian authorities over key questions such as how to set up the joint venture running the reactors and capping costs for the disposal of nuclear waste.
Engie said the "agreement in principle constitutes an important step, and paves the way for the conclusion of full agreements in the upcoming months.
"It also provides for the immediate start of environmental and technical studies prior to obtaining the authorisations related to this extension," it said.
Belgian Prime Minister Alexander De Croo told a press conference that "work can start tomorrow for the extension of the two most recent reactors".
Wedged between nuclear-powered France and gas-and-coal-dependent Germany, Belgium has relied on an ageing stable of seven nuclear reactors operated by Engie for about half of its electricity needs.
The promise of a gradual phase-out of nuclear power has been enshrined in Belgian law since 2003 and the decision to delay the moratorium was fiercely resisted by the Greens party.
The plan outlined last year saw Belgium's fragile coalition government agree to extend the operating lives of the Doel 4 reactor near the port city of Antwerp and Tihange 3 near Liege until 2035.
The move came after years of squabbling over the wisdom of the country's nuclear exit.
Belgium's Greens had made an exit from nuclear power in 2025 a condition when they joined a seven-party coalition that was painfully cobbled together in 2020, more than a year after inconclusive elections.
But after Russia's invasion of Ukraine on 24 February and the surge in energy prices, the party agreed to consider an alternative scenario.
Nonetheless, the country has stuck to a plan to shut down two older reactors.