Unit 4 at Ontario Power Generation’s (OPG’s) Darlington Nuclear Generating Station has been reconnected to the grid at 100% power, generating power for the first time since July 2023, when the unit was taken offline for refurbishment.
This marks completion of the four-unit Darlington Refurbishment project, which began in October 2016, which has been achieved CAD150m ($110m) under budget and four months ahead of schedule. As a result of the CAD12.8bn project, the station will now continue to provide roughly 20% of Ontario’s electricity until at least 2055.
The four original Candu pressurised heavy water reactors (PHWRs) at Darlington Nuclear Power Plant (NPP) were constructed in the 1980s and entered commercial service in the early 1990s. Each of these units has a net electrical capacity of 878 MWe (roughly 934 MWe nameplate). 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 in November 2024.
Refurbishment of unit 4 was completed in 968 days – the first Darlington unit to be completed in under 1,000 days. OPG said the unit’s return to service is a culmination of meticulous planning and preparation, millions of hours worked, and the dedication of thousands of people.
With approximately 8,000 lessons learned from the Darlington Refurbishment, OPG will leverage this experience to execute future projects, including the Pickering Nuclear refurbishment, the deployment of four small modular reactors (SMRs) at Darlington, and the proposed large nuclear development in Port Hope.
“We give our thanks to all who made the Darlington Refurbishment a resounding success – from our employees to our project partners to the province’s robust nuclear supply chain. They have helped set the standard and now we’re ready to keep delivering for Ontario’s future,” said Nicolle Butcher, OPG’s President & CEO. “Our experience on this refurbishment, and the thousands of lessons we have learned, will serve as our foundation as we advance the future of nuclear. Darlington’s refurbishment has given us the confidence, the tools, and the skills to forge ahead.”
Candu units are designed to operate for 30 years before refurbishment – 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. During the refurbishment, a total of 1,920 fuel channels and 3,840 feeder pipes were replaced, and millions of hours worked.
A major refurbishment project for Pickering units 5-8 (Pickering B) was approved by the provincial government in November 2025 and the four units will be removed from service at the end of the third quarter this year. Following defueling, the refurbishment project will begin in January 2027 with unit 5 expected to return to service in 2031, and all four back by 2034. The approved budget to refurbish the four units is CAD26.8bn “including interest, cost escalation, and contingency”.
Pickering B began operations in the mid-1980s and was scheduled to end electricity production in 2025. However, in 2022, the provincial government directed OPG to keep them in operation until 2026 and to reassess the feasibility of their refurbishment. OPG received approval for the initiation phase of the refurbishment project in January 2024, and for the start of the project definition phase in 2025. The project will enable the plant to continue in operation for up to 38 more years.
Butcher noted: “We have the talent and experience, honed during the Darlington Refurbishment and ready to transition to this project, to deliver Pickering Refurbishment within budget and make it as successful as Darlington’s.”
