The small modular reactor has been the nuclear industry’s favorite slide for about a decade. Utilities pitch it, governments model it, conference panels argue about it, and for years that was as far as it got in the Western world: a promising design nobody had actually built at the scale of a power grid. Then this spring, a crane in Ontario lowered a 953-tonne slab of steel and concrete into a shaft 35 meters deep and quietly ended the talking phase.<\/p>\n
That slab is the basemat, the foundation of the first GE Vernova Hitachi BWRX-300, going up at the Darlington site east of Toronto. Ontario Power Generation<\/a>, the provincial utility building it, and GE Vernova<\/a> both call it the Western world’s first grid-scale small modular reactor. It is designed to make 300 megawatts, enough for roughly 300,000 homes, and it is the first of four planned for the same site.<\/p>\n None of them will generate a watt until the end of 2030. But the concrete is in the ground now, which is further than anyone in the West has gotten with this technology before.<\/p>\n The basemat went into place on May 1, and it is a genuinely strange piece of engineering. It weighs close to 953 metric tons (about 2.1 million pounds, or more than three fully loaded Airbus A380s), measures 37 meters across, and was welded together in one piece before one of the world’s largest crawler cranes set it 35 meters below grade in the excavated reactor shaft. According to World Nuclear News<\/a>, it is the first time a reactor-building foundation in Canada has been assembled modularly, which OPG described as “putting the ‘M’ in SMR.”<\/p>\n The reason a slab of concrete counts as news comes down to nuclear bookkeeping. For a conventional plant, pouring the first concrete for the reactor basemat is the moment a project officially becomes a nuclear unit under construction. The Canadian Nuclear Safety Commission issued the construction license in April 2025, the province gave its final go-ahead a few weeks later, and the basemat is the physical proof that the build is real. It is also the first brand-new reactor Ontario has started since the early 1990s, when the last of the existing Darlington units came online. The province spent the years since refurbishing what it already had rather than building anything new.<\/p>\n The BWRX-300 is exactly what the name implies: a 300-megawatt boiling water reactor, about a third the output of the large units that anchor most nuclear stations. GE Vernova calls it a tenth-generation design, which is a polite way of saying it is the latest in a very long line of boiling water reactors and borrows its safety case from the ESBWR, a bigger design the U.S. Nuclear Regulatory Commission already certified. It cools itself by natural circulation instead of relying on electric pumps, sits in a containment buried below grade, and according to GE Vernova the entire power block fits within two international soccer pitches, roughly 430 by 200 feet. The build time, the developer says, drops to 24 to 36 months for later, repeat units, once the first one has worked out the kinks.<\/p>\n There is one wrinkle, and it has to do with fuel. The BWRX-300 runs on standard low-enriched uranium, the same kind feeding most of the world’s reactors, not the high-assay HALEU that more exotic designs need. That part is routine. The catch is local: Canada’s existing CANDU fleet runs on natural, unenriched uranium, so the country does not enrich uranium domestically, and the new reactors will need a fuel supply it currently has to source from elsewhere. It is a solvable problem. It is not solved yet.<\/p>\n The press releases lean hard on the word “first,” so it helps to pin down which kind of first you are looking at. Both OPG and GE Vernova are careful to say Western world’s first, or G7’s first, grid-scale SMR, and that scoping matters. Russia and China already operate small modular reactors, and Argentina has a pilot under construction, so this is not the first SMR on Earth. It is the first one a G7 country has built and wired to a major grid.<\/p>\n It is also a different machine from the other advanced-reactor projects breaking ground right now. Bill Gates’ TerraPower just started building a sodium-cooled reactor in Wyoming<\/a>, and a company in Italy is testing a full-scale lead-cooled rig with electric heaters standing in for the fuel<\/a>. Those are fast reactors chasing exotic coolants. The BWRX-300 is the opposite bet: a deliberately conventional, water-cooled reactor scaled down and built in a factory-style sequence, designed to be boring and repeatable rather than novel. And none of this is making electricity yet. OPG is targeting the end of 2030 for grid connection, with construction itself wrapping up around 2029.<\/p>\n This is not a cheap way to find out. OPG’s release-quality estimate puts the first reactor at CAD 6.1 billion, plus another CAD 1.6 billion for roads, tunnels, cooling-water lines and other infrastructure shared across all four units, for CAD 7.7 billion to get the first one standing. The full four-unit program is budgeted at CAD 20.9 billion (about USD 15 billion) in 2024 dollars, with interest and contingencies baked in, and OPG expects each later unit to cost less as the supply chain matures, dropping to roughly CAD 4.1 billion for the fourth.<\/p>\n For that money, the four reactors are meant to deliver 1,200 megawatts, enough for about 1.2 million homes. The Conference Board of Canada estimates the program will add CAD 38.5 billion to the national economy over 65 years and sustain 18,000 jobs a year during the five-year construction phase, with more than 80 percent of the spending going to Canadian companies and about 5 percent to U.S. firms, mostly GE Vernova for the design.<\/p>\n The Canada Growth Fund and the Building Ontario Fund are each taking a minority stake, putting up CAD 3 billion in equity between them. Ontario’s grid operator estimated the power at about 14.9 cents per kilowatt-hour and judged a comparable build-out of wind, solar and storage to be both pricier and riskier. Whether that holds depends entirely on the first reactor coming in on budget, which is the part nobody can promise.<\/p>\n The reason this particular construction site matters beyond Ontario is that Darlington is the reference unit for a much bigger plan. In Tennessee, the Tennessee Valley Authority became the first U.S. utility to file a construction permit application for a BWRX-300, for a single reactor at its Clinch River site near Oak Ridge. The U.S. Nuclear Regulatory Commission<\/a> docketed that application in July 2025 and expects to finish its review by the end of 2026; TVA has said preliminary site work could start as early as this year, and the Department of Energy put a USD 400 million grant behind it. Poland’s Orlen Synthos Green Energy plans a fleet of about 24 of the reactors, with its first unit targeted near Wloclawek by 2032, and utilities in Sweden, Estonia, Hungary and the Canadian province of Saskatchewan are all somewhere on the same path.<\/p>\n The whole economic argument for SMRs rests on standardization, on building the identical machine over and over until the price comes down, the way the Darlington refurbishment shaved 250 days off its second reactor compared with its first. Darlington is where that theory either holds up or it doesn’t. Get the first one right and the next two dozen get easier and cheaper to finance. Blow the budget and every utility watching quietly revises its plans.<\/p>\n The basemat is set, the cranes are lifting the reactor building up out of the shaft, and OPG is leaning hard on the roughly 7,000 lessons it logged refurbishing the existing Darlington reactors, a project that finished ahead of schedule and about CAD 150 million under budget. That track record is the strongest card in the deck. But a foundation is still just a foundation. The reactor has to be built on top of it, fueled, licensed to operate and actually switched on, and all of that lands in 2030. The West has spent the better part of two decades talking about small modular reactors. Canada is the first to find out whether the pitch survives contact with poured concrete.<\/p>\n","protected":false},"excerpt":{"rendered":" The small modular reactor has been the nuclear industry’s favorite slide for about a decade. Utilities pitch it, governments model … <\/p>\nWhat OPG actually put in the ground<\/h2>\n
The reactor is small on purpose<\/h2>\n
The “first” comes with asterisks<\/h2>\n
Twenty-one billion dollars, and what it buys<\/h2>\n
Why the rest of the world is watching a hole in the ground<\/h2>\n