Candu units are pressurised heavy water reactors designed to operate for 30 years before refurbishment. Refurbishment is a major undertaking: it includes the replacement of key reactor components such as steam generators, pressure tubes, calandria tubes and feeder tubes, and involves removing all the reactor’s fuel and heavy water and isolating it from the rest of the power station before it is dismantled. Thousands of components are inspected before the plant is rebuilt and is ready for another 30 years of operation.
Ontario Power Generation’s (OPG’s) Darlington units were refurbished in a ten-year, CAD12.8 billion (USD9.4 billion) project that will extend the plant’s operating life to at least 2055. Unit 2 was the first to be refurbished and returned to service in June 2020; unit 3 returned to service in July 2023; and unit 1 returned to service in November 2024.
Stephen Lecce, Ontario Minister of Energy and Mines, on Monday announced the completion of construction on the refurbishment of the Darlington unit 4 nuclear reactor. Originally expected to return to power in August this year, the unit has now received approval from the Canadian Nuclear Safety Commission (CNSC) to enter the initial power testing, which is expected to begin on 12 February, and will be at full power shortly after. In September last year, the CNSC announced it was authorising the Darlington plant to continue operating for a further 20 years, until November 2045.
“With all four units refurbished and ready to be brought back online, this marks the completion of the world’s largest refurbishment project to date, with the Darlington Nuclear Generating Station ready to deliver more than 3,500 MW of clean, reliable, emission-free electricity – enough to power 3.5 million homes – for at least 30 more years,” the Ministry of Energy and Mines said. It noted that the Darlington refurbishment project – carried out by more than 6,000 Ontario workers – was completed CAD150 million under budget. During the construction phase, the project contributed an estimated CAD14.9 billion to Ontario’s GDP, with 96% of project spending retained in the province.
“Ontario is proving to the world that we can deliver major nuclear energy projects on-time and on-budget,” Lecce said. “Last year we returned another refurbished unit at Darlington four months ahead of schedule, and today we’ve done it again. Completing the full Darlington refurbishment on-time and under-budget is a made-in-Canada win for our economy, generating CAD90 billion in economic benefits and supporting 14,200 good-paying jobs. In a world of uncertainty, our government is doubling down on Canadian nuclear technology and workers, with 94% of investment benefiting Ontario’s supply chain. It is a tremendous industrial advantage that Canada is among only six nations that own civilian nuclear technology.”
“Bringing the Darlington refurbishment in ahead of schedule and under budget will clearly demonstrate our team’s ability to deliver large and complex nuclear projects,” said OPG President and CEO Nicolle Butcher. “The skills we developed and lessons we learned during this project are invaluable as we proceed with refurbishing Pickering Nuclear Generating Station, continue construction on the G7’s first small modular reactor at the Darlington New Nuclear Project, and study the potential for up to 10,000 MW of new nuclear at Wesleyville.”
According to a report by the Conference Board of Canada, the Darlington refurbishment project and the subsequent 30 years of station operation are expected to generate a total of CAD90 billion in economic benefits for Ontario.
In November 2025, the Ontario government approved OPG’s plan to refurbish four Candu nuclear reactors at its Pickering plant, clearing the way for a start to the execution phase of the project, subject to final regulatory approvals.
George Christidis, president and CEO of the Canadian Nuclear Association, added: “This milestone is the result of years of project management excellence and sustained collaboration across Ontario’s nuclear sector. Completing the Darlington refurbishment ahead of schedule and under budget demonstrates the strength of Canada’s nuclear industry, while helping position Canada as a global leader in nuclear refurbishment and life-extension projects. It also underscores the industry’s ability to deliver clean, reliable power that supports long-term economic growth.”
“Darlington’s ahead of schedule and under budget refurbishment stands as a remarkable achievement for Ontario and a powerful demonstration of what Canada’s nuclear sector can achieve,” said World Nuclear Association Director General Sama Bilbao y León. “This accomplishment is a testament to the excellence of the entire team, including Ontario Power Generation, Atkins Réalis, the Canadian Nuclear Safety Commission, and the many suppliers and technical partners from throughout the nuclear value chain, as well as to the support and the visionary leadership of the government of Ontario. This success is more than a milestone; it is a foundation. The industrial capabilities, the supply chain strength, and the project discipline proven at Darlington will be essential as Canada and Ontario move forward with new nuclear deployment, from next generation SMRs to large scale reactors. The world is watching, and Darlington shows that Canada is ready to lead the next era of clean energy with nuclear as its anchor.”
