The delays and resulting cost overruns have made Darlington a primary case study for the anti-nuclear movement in Canada, and was one of the main reasons Ontario Hydro was broken up in 1999 and its debts paid off by special billings.

Room for a second four-reactor unit had been in place since the original site selection, with a large area to the east of the current plant set aside for what was known as Darlington B.

Darlington was part of a large nuclear buildout planned by Ontario Hydro based on their predictions of almost linear growth in power demand at 7% per year essentially forever.

Based on this predicted growth, the company stated that nuclear would account for 60 to 70% of the province's supply by 1990, and for that to occur, a large number of new reactors would have to be built.

[6] Land at the Darlington Provincial Park was identified as a potential site in the late 1960s, and Hydro purchased the plot in 1971 as an "energy centre".

The first official plans to develop the site for nuclear were approved in 1973, apparently at the personal direction of Conservative Ontario premier Bill Davis without discussion by cabinet.

Given the public scrutiny and generally negative reports, in 1976 Energy Minister Darcy McKeough told Hydro to slow down its demands and spread out the budget or the province would not guarantee the company's bonds.

[8] A more detailed budget, this time accounting for inflation during the expected construction period through 1988 put the final figure at $7.4 billion, equivalent to $30 in 2023.

The engineering staff on Darlington were assigned to other projects, including the expansions at Pickering and Bruce that were now about to come online.

Additionally, during the pause a number of issues in the design were found, this being the first plant of the ~900 MW size to be built, introducing further delay in getting construction started again.

[18] It was during this period that the labour force at the site reached its peak of 7,000,[17] making it the largest construction program in North America at that time.

Almost immediately on entering service, the power shafts on Unit 2 connecting the steam turbines to the alternators were found to suffer from cracking, which led to lengthy shutdowns in 1990 and 1991.

As a result of Farlinger's suggestions, Strong began the process of breaking up the company into five divisions, each with a separate area of responsibility.

[25] By this time, Hydro had $34 billion in debt, almost half of that due to Darlington and a significant portion of the rest from construction and expansions on Pickering and Bruce.

The Darlington reactors have been among the best performing in OPG's CANDU fleet, including a top year in 2008 in which the plant achieved a combined 94.5% capacity factor.

[29] In March 2017, Ontario Power Generation (OPG) and its venture arm, Canadian Nuclear Partners, announced plans to produce Plutonium-238 as a second source for NASA.

Rods containing Np-237[30] would be fabricated by Pacific Northwest National Laboratory (PNNL) in Washington State and shipped to OPG's Darlington Nuclear Generating Station where they would be irradiated with neutrons inside the reactor's core to produce Pu-238.

[33] A year-long period of public hearings and study by an Ontario government all-party committee finished in 1986 with the decision to proceed with the project, which had then risen to $7 billion in actual and committed costs.

In 1989, Hydro filed its latest 25-year Demand Supply Plan, Providing the Balance of Power, calling for another 10 reactors and 32 fossil plants.

In 1993, this plan was withdrawn, after Darlington entered service and the province now had a surplus of generation[36] and was forced to sell at very low and sometimes negative prices.

The project was adversely affected by declining electricity demand forecasts, mounting debt of Ontario Hydro, and the Chernobyl disaster, which necessitated safety reviews in mid-construction.

[38] Discussion of who is to blame for the costs and subsequent debts associated with Darlington often arise during provincial election campaigns, and are often mentioned in anti-nuclear literature.

[40] On October 14, 2016, OPG began Canada’s largest clean infrastructure project – the refurbishment of all four of Darlington’s reactors.

A request for proposals (RFP) process for design and construction resulted in bids from Areva NP, Westinghouse, and Atomic Energy of Canada Limited (AECL).

In June 2009, the Government of Ontario put the RFP process on hold, citing unexpectedly high bids, and the uncertainty surrounding the future of the only compliant bidder (AECL).

[46][47][48] In August 2011, the three-member Joint Review Panel (mandated by the Ontario Ministry of the Environment and the Canadian Nuclear Safety Commission) released a report finding that the Darlington new build project would not result in any significant adverse environmental impacts (after taking into account mitigation measures).

OPG has proposed the construction and operation of a deep geologic repository for the long-term storage of this low and intermediate level waste on lands adjacent to WWMF.

On May 6, 2015 the Joint Review Panel issued the Environmental Assessment (EA) Report recommending the approval of the Deep Geologic Repository for Ontario’s low and intermediate level waste to the federal government.