Three of the four reactors at Ontario’s Darlington Nuclear Generating Station have experienced extended outages in recent months, raising questions about the long-term reliability of the facility despite a major refurbishment completed earlier this year. The station, located in Clarington, Ontario, underwent a $12.8-billion, decade-long overhaul that was officially finished in March 2026.

Since returning to service, Darlington’s Unit 4 reactor has been taken offline three times, the most recent outage beginning on August 1. Unit 2 also abruptly stopped generating power on August 3, while Unit 3 remains offline due to planned maintenance. This leaves only Unit 1 currently operating.

Ontario Power Generation (OPG), which operates the facility, has provided limited details on the incidents. An OPG spokesperson stated the outages for Units 2 and 4 were necessary to complete required maintenance, with Unit 2 expected to restart in September and Unit 4 in October. The Canadian Nuclear Safety Commission (CNSC) confirmed it is overseeing the outages, emphasizing that the events do not pose any health or environmental risks.

Darlington’s reactors each have roughly 900 megawatts of capacity, totaling about 3,500 megawatts, making the station Ontario’s second-largest nuclear power producer after the Bruce Nuclear Generating Station. However, the reliability of Darlington’s reactors has come under scrutiny this year, with OPG forecasting a significant increase in outage days. In a recent filing to the Ontario Energy Board, OPG projected 286 outage days in 2026—the highest annual total since at least 2008—with 512 outage days expected in 2027 due to planned refurbishments.

Experts note that while some unplanned outages are inevitable due to the complexity and value of nuclear reactors, the frequency and timing of the recent disruptions are unusual. David Novog, a nuclear safety professor at McMaster University, highlighted that frequent stop-and-start cycles like those at Unit 4 are atypical, and more transparency from OPG would help clarify whether these issues represent normal “teething pains” or something more concerning.

Despite these challenges, the Independent Electricity System Operator (IESO) maintains that Ontario’s electricity grid will remain stable. IESO spokesperson Michael Dodsworth stated that the province’s diverse electricity supply mix, including investments in new generation and energy storage, has ensured reliability during recent outages. The IESO also noted that the entire Darlington station is scheduled to be offline for nearly 46 days in 2027 for critical upgrades, including work on the vacuum building that enhances safety in the event of an accident.

These developments come as the province’s Pickering Nuclear Generating Station approaches permanent closure, further affecting Ontario’s nuclear generating capacity. In response to the reduced output from nuclear plants, natural gas-fired power stations have seen increased use. As of August, nuclear plants accounted for 38.5 percent of the province’s electricity supply, while gas plants provided 31.5 percent, a notable shift from previous years when nuclear generation made up about half of Ontario’s power mix. The provincial government continues to defend the refurbishment projects, citing their completion ahead of schedule and under budget.