Two of the four nuclear reactors at Ontario’s Darlington Nuclear Generating Station remain offline following a recent cycle of refurbishments that provincial officials have described as successful. Despite the completion of the decade-long refurbishment project earlier this year—delivered ahead of schedule and under budget—ongoing outages have raised questions about the station’s current operational status.

The refurbishment of Darlington’s reactors began in October 2016 and concluded in early March 2026 when the final unit returned to service. The project, managed by Ontario Power Generation (OPG), was completed four months ahead of its original timeline and approximately $150 million under the $12.8 billion budget. Throughout the process, both OPG and the Ontario government emphasized the refurbishment as a technical and financial success.

However, operational data from the Independent Electricity System Operator (IESO) shows that Unit 4 ceased electricity production on April 11, about a month after returning to service. The Canadian Nuclear Safety Commission (CNSC), the federal regulatory agency, linked this outage to signs of a hydrogen leak in the generator stator cooling water system, which is separate from the nuclear reactor itself.

OPG stated in May that repairs on Unit 4 had been completed and forecast its return to service by early June. The unit briefly returned on May 26 but went offline again on June 10 and has not generated power since. OPG told the CNSC that the current outage involves electrical repairs unrelated to the refurbishment work and expected the unit to resume operations shortly. The CNSC confirmed that the repairs were finished and the unit was being returned to service.

Meanwhile, Unit 3 has been offline since March 30 for a planned 230-day outage to replace deteriorating steam generators. Originally inspected during refurbishments and deemed fit for an additional 30 years of operation, components known as primary moisture separators inside the steam generators were later found to be degraded and in need of replacement. Moisture separators for Units 1 and 4 were replaced during refurbishment, and replacements for Unit 3 are underway, with the OPG board approving over $250 million to complete the work.

Further costly maintenance is anticipated through 2029 and beyond, including a $2 billion project to replace turbine rotors on the first three units, with additional major work planned on the Tritium Removal Facility, turbine control systems, and generator stators at Darlington’s units.

The Ontario Energy Minister’s office stated that outages are standard maintenance procedures necessary to meet regulatory requirements while underscoring the government’s focus on ensuring reliable electricity supply to support economic growth amid rising demand. The IESO’s latest seasonal assessment projects sufficient capacity this summer to meet peak electricity needs, even during extreme conditions.

In regulatory filings, OPG has requested significant hikes in payments for power generated by Darlington and its other nuclear station, Pickering. If approved, these increases could lead to higher electricity rates for consumers.

Despite recent operational challenges, analysis of Darlington’s post-refurbishment performance shows generally strong capacity factors, notably outperforming other refurbished Canadian Candu stations such as New Brunswick’s Point Lepreau, which faced significant operational difficulties after its uplift.

Looking ahead, all four reactors at Pickering are scheduled for shutdown starting in September. The Ontario government ordered their refurbishment last year with an estimated $26.8 billion budget, signaling continued investment in the province’s nuclear infrastructure.