Darlington Nuclear Refurbishment Finishes Early, Under Budget at CAD12.8 Billion

Ontario Power Generation completed what it and industry observers are calling the world's largest nuclear refurbishment project when Darlington Unit 4 reconnected to the Ontario grid at full power on March 16, 2026, closing out a ten-year, CAD12.8 billion overhaul of all four CANDU reactors at the Clarington, Ontario station.

The final tally landed CAD150 million under budget and four months ahead of schedule. "A milestone 10 years in the making," OPG wrote in its announcement. "With this, our four-unit Darlington Refurbishment project, which began in 2016, officially comes to a close — $150 million under budget and four months ahead of schedule."

The sequencing stretched across nearly a decade. Unit 2 went offline in October 2016 as the first reactor into the program and returned to service in June 2020, a month ahead of its revised schedule. Unit 3 followed, beginning refurbishment in September 2020; the Canadian Nuclear Safety Commission cleared fuel loading in December 2022 and gave restart authorization in May 2023, with Unit 3 back at full capacity in July 2023. Unit 1 returned to service in November 2024. Unit 4, which originally entered commercial operation in 1993 as an 878-MWe PHWR, was the last to go through. The CNSC authorized it to enter initial power testing, which began February 12; by March 16, OPG confirmed reconnection at 100%.

The technical scope of each refurbishment was substantial. For every reactor, crews removed all fuel and heavy water, isolated and dismantled the unit, and replaced 480 fuel channels and 960 feeder tubes alongside thousands of calandria tubes, end fittings, and other components that are inaccessible when a reactor is assembled. Across all four units the project replaced 1,920 fuel channels and 3,840 feeder pipes in total. A full-scale reactor mock-up at the Darlington site was used to train the more than 6,000 Ontario workers who carried out the work.

The economic footprint was substantial even by the standards of a CAD12.8 billion project. During the construction phase the refurbishment contributed an estimated CAD14.9 billion to Ontario's GDP, with 96% of project spending retained within the province. OPG projects the refurbished station will support 14,200 workers over the next 30 years.

Ontario's Ministry of Energy and Mines framed the completion in grid terms: "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." Nuclear power currently supplies roughly 50% of Ontario's electricity, and Darlington's 3,500-plus MW represents a significant share of that baseline.

The question of exactly how long the refurbished station will operate carries some complexity. Multiple sources report the refurbishment extends Darlington's operating life to at least 2055, while the CNSC separately announced in September 2025 that it was authorizing the plant to continue operating for a further 20 years, until November 2045. OPG and the CNSC have not publicly reconciled those two dates; the 2045 figure likely reflects the current licence term while the 2055 projection may represent OPG's planning horizon for the refurbished units.

OPG indicated it intends to apply lessons from Darlington directly to its next major undertakings. The Ontario government and OPG have already approved a plan to build the first of four BWRX-300 small modular reactors at the Darlington site, and Ontario has given OPG the formal go-ahead to begin refurbishment of four units at the Pickering nuclear station. Delivering a ten-year, multi-billion-dollar nuclear project under budget and ahead of schedule gives OPG a credible template to point to as both of those programs move into execution.