Three of four reactors at Ontario’s refurbished Darlington nuclear power plant out of commission

Three of four reactors at Ontario’s Darlington Nuclear Generating Station are experiencing prolonged outages, data from the Independent Electricity System Operator show, even as the province faces a further large drop in nuclear power generation in just a few weeks’ time.

Ontario Power Generation completed a $12.8-billion, decade-long refurbishment of its station in Clarington, Ont., in March. Although provincial officials have portrayed that overhaul as an unqualified success, Darlington’s units have meandered in and out of service ever since.

The most recently refurbished unit, Unit 4, suddenly stopped generating power on Aug. 1 – its third outage since returning to service. Two days later, Unit 2 abruptly stopped supplying power to the grid. Unit 3 had already been out of service for several months because of long-planned work, leaving Unit 1 as the lone operational unit.

Officials have disclosed little about the causes and implications of the outages at Units 2 and 4, nor when they might return to service. In response to questions from The Globe seeking details of the outages, OPG spokesperson Neal Kelly wrote in a statement that the two units were taken offline “to complete required maintenance.” He added that Unit 2 is expected to restart in September, and Unit 4 will follow in October.

Braeson Holland, a spokesperson for the Canadian Nuclear Safety Commission, wrote in a statement that OPG reported the forced outages of Units 2 and 4 to its staff.

“CNSC inspectors maintain regulatory oversight of the outage work to verify that the appropriate actions are being taken. CNSC staff are satisfied that these events do not present a risk to the health and safety of persons or to the environment,” he wrote.

Representing nearly 900 megawatts of generating capacity each, Darlington’s units are the largest in Ontario. The station boasts a total generating capacity of roughly 3,500 megawatts, making it the province’s second largest, eclipsed only by the eight-reactor Bruce Nuclear Generating Station in Tiverton, Ont., operated by Bruce Power.

Ontario’s third nuclear power plant, OPG’s Pickering Nuclear Generating Station, is just weeks away from shutting down. Its four operational reactors, representing 2,100 megawatts of generating capacity, are scheduled to come offline next month for their own $26.8-billion refurbishment, expected to wrap up in the mid-2030s.

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Michael Dodsworth, a spokesperson for the Independent Electricity System Operator, said his organization had no concerns that the outages would affect Ontario’s ability to meet electricity demand in the coming months.

“Ontario’s electricity system has remained reliable this summer thanks to our diverse supply mix, bolstered by recent investments in new generation and energy storage,” he wrote.

In a July filing to the Ontario Energy Board, OPG forecast that there would be 286 outage days at Darlington in 2026 – the most taken in any year since at least 2008. Nonetheless, the forced outages of Units 2 and 4 seem likely to push the actual total higher still.

OPG forecast planned outages totaling 512 days in 2027, including a major outage for work on Unit 2’s steam generators. The entire Darlington station will be out of service next year for nearly 46 days to allow work on its vacuum building, which is designed to suck out and condense steam from a reactor during a major accident.

Since outages of a single reactor can cost a utility $1-million or more a day in lost revenue – and because large, unexpected fluctuations in power generation can stress power grids, utilities invest considerable manpower and resources to avoiding unplanned outages, according to David Novog, a professor of engineering at McMaster University who specializes in nuclear safety and maintenance.

“Of course there’ll be unplanned outages,” he said. “But because of the value that the reactor represents on the grid and the amount of revenue it provides, it is probably one of the most looked-after assets that the government has.”

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However, several of this year’s outages at Darlington have been entirely unexpected. Unit 4 was officially returned to service on March 13 following a refurbishment lasting nearly three years. Since then, it has been taken offline three times with little explanation; OPG has disclosed only that the first outage was taken “to assess an operational matter unrelated to the refurbishment” and that it had completed repairs.

“When you stop and start and stop and start, that’s atypical of how we would typically choose to run the reactors,” Prof. Novog observed.

The Bruce and Darlington plants suffered many unplanned outages in their early years of operation, he added. During the years leading up to refurbishment, however, Darlington’s units were “exceptionally well-run.”

Prof. Novog said OPG should pro-actively publish more information about outages.

“When you’re not upfront with the information, it can lead to speculation and a lack of confidence,” he said.

“Without knowing the details, it’s hard to even tell whether it’s teething pain [at Darlington] or whether it is something else.”

The provincial government has said that Ontario’s nuclear projects were completed ahead of schedule and under budget, which Energy Minister Stephen Lecce repeated Tuesday at a press conference in St. Clair Township.

As the output of Ontario’s nuclear fleet has fallen because of refurbishments and outages, the province has increasingly relied on natural gas-fired plants to supply electricity. On Thursday afternoon, the 11 operational reactors provided 38.5 per cent of the province’s electricity, as compared to 31.5 per cent from gas plants, according to the Gridwatch monitoring web site. For 2024 and 2025, the nuclear plants provided roughly half of Ontario’s power; gas plants provided less than 20 per cent.