North American construction is back—smaller and faster—at OPG’s Darlington

Darlington, nearly 45 miles east of Toronto on the northern shore of Lake Ontario, has four 878-MWe pressurized heavy water reactors. The four GE Vernova Hitachi Nuclear Energy (GVH) BWRX-300 modules OPG plans to add are Canada’s first new nuclear build in more than 30 years, and the nation’s first power reactors that are not PHWRs.

Sinnathamby has been working on nuclear operations and projects at OPG for 25 years and is now chief projects officer. Prior to taking on her current role, she led work on an extensive refurbishment of Darlington’s Canadian deuterium-uranium (CANDU) reactors, extending their operating lives by 30 years. That project came in under budget and ahead of schedule.

Now, she is working on another major project, known in Ontario as the Darlington New Nuclear Project (DNNP), with OPG, GVH, and industry partners. The one 300-MWe small modular reactor under construction in Ontario now—and the three that could follow—is an important test case in Canada and the United States.

As executive vice president for advanced nuclear at GVH, Sean Sexstone is working closely with OPG and vendors to deliver the BWRX-300 at Darlington and other proposed sites. “We haven’t built new nuclear in a long time. Outside of maybe Vogtle, there has not been a significant build. And now, everyone wants to build all the time,” he said.

Sexstone was witness to work at Vogtle. He started his career with GE and later worked for CB&I when AP1000 construction was underway at Vogtle and Summer, and in China. He moved to the construction of combined-cycle gas generation before taking a role leading Aecon’s nuclear business when Aecon was participating in the Darlington refurbishment. Sexstone returned to GE in 2022. It was a “coming home,” he said, “seeing that there was a real need for new nuclear, and then from there it has just skyrocketed.”

Refurbishment sets the stage

Ahead of the Darlington refurbishment, operators were trained on a full-scale CANDU reactor mock-up in a nonradiological environment. Fuel load training is shown here. (Photo: OPG)

All four units of the C$12.8 billion Darlington refurbishment were completed within a decade in what Sinnathamby said was “a tremendous experience for us… . I think we can claim to be probably the only organization in recent history that has had a nuclear project come in ahead of schedule and under budget.”

Sinnathamby identified key ingredients to that success: process and tools, partnering with vendors in a “one-team” culture that put the project first, building a training mock-up to ensure that workforce training was not on critical path, and upfront procurement of major components to refurbish all four units.

“The full-size mock-up paid dividends within the first unit in terms of the trades,” she said. “Not just training the trades, but also our ability to test our tooling that we use, the automated tooling, the manual tooling, full dress rehearsals like they’re working in the reactor vault.”

Getting past first-of-a-kind challenges to “rinse and repeat” for later refurbishments is a goal for the new reactor project, as well. And given that both Sexstone and Sinnathamby cited it in separate conversations, “rinse and repeat” is something of a mantra for the project team.

Building a supply chain

“We’re not new to this,” said Sexstone, noting that the “X” in BWRX-300 stands for the 10th generation and that GE has built 67 boiling water reactors.

Hitachi, GE Vernova’s partner in the nuclear business, “has built a number of ABWRs, large reactors in Japan,” Sexstone said. “They are a key partner and an owner of the GVH business. They’re very involved up front in our design, how we modularize, how we engineer, how we integrate with construction.”

Key vendors involved in the DNNP include Aecon as constructor, working with Kiewit as Aecon Kiewit Nuclear Partners; AtkinsRéalis as integration engineer; BWXT Canada as reactor pressure vessel manufacturer; and Velan Valves. In all, over 100 companies in Ontario are part of the supply chain.

“In Ontario, we’re quite fortunate that we have a very mature nuclear supply chain,” Sinnathamby said. Despite Ontario’s supply chain being focused on CANDU technology, “the quality that’s required to manufacture for nuclear was already established. What I would say is that we now have that first-mover advantage and the ability to be able to support global deployment as other countries decide to build the BWRX-300.”

While about 80 percent of the supply chain for the Darlington BWRX-300s is located in Ontario, “there are some components that we just don’t manufacture in Canada,” Sinnathamby said, naming turbines as an example. “It’s not a component that’s manufactured in Canada or in Ontario, regardless of technology.”

Canada has developed and then exported its CANDU technology and has exported its uranium as well. Now, the nation is importing light water reactor technology and the enriched fuel it requires. Canada’s operating fleet of 17 reactors uses fuel pellets manufactured in Canada from unenriched uranium.

While the fuel for Darlington’s BWRX-300s will be fabricated by GVH’s Global Nuclear Fuel, the uranium originates in the mines of Canada’s Cameco. Enrichment will be performed in the United States or in Europe. “We’ve built some diversity in our supply chain that we’re quite confident around the fuel supply risk,” Sinnathamby said.

Finding a way to integrate work between the United States and Canada is “a big thing,” Sexstone said, made easier by collaboration between the Canadian Nuclear Safety Commission (CNSC) and the Nuclear Regulatory Commission.

“Canada has done a fantastic job, and we’ve put a lot of focus on the Canadian supply chain,” Sexstone said, “but it’s also a very diverse supply chain. So we have suppliers that are in Japan or in Europe or in the U.S., and so it’s not all focused on one country… . We’re very confident that we can supply multiple projects across the globe.”

GVH is planning to establish a BWRX-300 engineering service center in Ontario. “That’s a big thing for us,” Sexstone said. “It’s going to be in the Durham region near the Darlington site. That’s a roughly C$70 million investment, just to provide engineering and technical services to support a fleet of X-300s. So we’re very invested in the Canadian market right now and supporting our partners.”

Workforce ready

The one-team approach that OPG built for the Darlington refurbishment and is repeating for the DNNP is something that GVH plans to carry over as an integrated project delivery approach to other BWRX-300 builds.

“OPG is a world-class operator. And what is really interesting to note is how they run their projects,” Sexstone said. As the Darlington refurbishments got underway, “one of the best strategies OPG had was seeing that the refurbishments were going to roll and then asking, ‘Where do we utilize these people?’ We utilize those people in this new unit build.”

Sinnathamby agreed. “We’re very fortunate that we have a very skilled workforce here in Ontario with the nuclear refurbishments that we’ve been doing,” she said. “With the Darlington refurbishment coming to completion, we have been able to transition some of that workforce over to the SMR project and some have been transitioned over to the Pickering refurbishment project.”

At the Canadian Nuclear Association’s 2026 conference, held in late April, Ontario Energy Minister Stephen Lecce cited 19,000 direct and indirect jobs as part of the economic impact to the province. On-site, Sinnathamby said that at peak, the DNNP’s workforce will be “probably around 2,000 as we go through the various phases of the project.”

Driving down cost and schedule

A view in April of the Darlington site, roughly 45 miles east of Toronto on the northern shore of Lake Ontario. The DNNP site is in the foreground, and Darlington’s four operating CANDU reactors are visible in the background. (Photo: OPG)

A depiction of the BWRX-300 now under construction at Darlington. (Image: OPG)

The first Darlington CANDU went off line for refurbishment in October 2016. The last of the four units returned to service in March 2026. Over nearly a decade of work, OPG improved on cost and schedule delivery.

“Our philosophy on how we executed refurb was do the first unit standalone. We knew it was going to be tough, and it was tough,” Sinnathamby said. “It was a ‘night fight’ right to the last day. But the key part of that was we had a very robust lessons-learned process. Between our first unit and the last unit, we were able to shave a year off in construction schedule and had significant cost savings.”

OPG is working with a C$20.9 billion project envelope and a similar strategy for the DNNP. “We have released $7.7 billion for what we call the common systems, common to all four units, and for the first unit,” Sinnathamby said. That includes about C$1.6 billion for common systems, including the cooling water intake system, administration building, and other common site infrastructure.

“The first unit cost is C$6.1 billion as a standalone,” she added, with the remaining units ranging from about C$5 billion down to about C$4 billion for the final unit.

“When we started construction, [the design] was at the maturity level that we needed to start construction. The beauty of doing this first unit standalone is you get the construction experience, you get the first-of-a-kind learnings behind you, the engineering is complete, all the analysis to support it is complete,” Sinnathamby said. She added that the first unit is proving to be “just as hard as the first unit on Darlington refurb was, and we’re not surprised by how hard it is. The team will figure it out. But the key is that we get the last three units done most effectively within the envelope that we have.”

Lessons learned will come into play as OPG works toward receiving approval to progress with Units 2–4. That approval milestone is expected in 2028.

“I think OPG is doing it in a very intelligent way,” Sexstone said. Instead of building one unit and then deciding whether to proceed with subsequent builds, “they’re already looking way ahead of how do we really integrate a team into this process now so that Units 2, 3, and 4 go that much faster. You’re going to have a civil team, and that civil team will do their work, excavate the hole, and they might go right next to it and do number 3 and then number 4.”

Small, but “a beauty”

The last BWRs built in the United States were under construction in the 1980s. Limerick-2 was the last to enter commercial operation, in 1990. Asked how the BWRX-300 construction site at Darlington would compare to a large BWR construction site in the United States, Sexstone replied that it’s “much smaller. You can walk the site in two minutes. So it’s a different feel than large builds, and I’ve been on a number of them.”

A typical large LWR build might have up to 6,000 people on site at peak, Sexstone said, and a project with four or five times the workforce is “not just four or five times harder … . It’s exponentially harder.”

“I think the other interesting thing about our technology versus a Limerick, for instance, is the amount of equipment is far less,” he added. “We’ve leveraged our partnership with Hitachi, who does a lot of our reactor internals. But outside of the reactor building itself, it’s pretty standard equipment.”

Safety-related equipment typically makes nuclear construction a lot harder than similar nonnuclear projects, Sexstone said. At the Darlington site, “it’s still a nuclear project, and we still have all the nuclear pedigree, safety, and quality, but you can build at least your secondary island—your turbine island—in more of an industrial fashion.”

Sinnathamby agreed that it’s “a very tight site. It’s a very busy site, lots of work going on. You can see it from afar, the number of cranes when the sky is clear. It is a beauty.”

To date, teams on-site have been working on three shafts: the launch shaft, which will launch a tunnel boring machine to bring in cooling water from deep in the lake; the forebay shaft, which is where cooling water will come in from the lake; and the reactor building shaft. That’s in addition to a services building, turbine building, administration building, assembly building, and warehouses.