The problem
Ontario gets more than half its electricity from nuclear plants, and demand is set to grow with electric vehicles, heat pumps, industry and data centres. Several older reactors will retire or need refurbishment. The province wants new firm, clean power, but a new gigawatt-scale reactor is a huge, slow and risky project.
Utilities around the world face the same choice. SMRs promise smaller up-front costs, shorter construction and the option to add units one at a time. But they need a design that regulators can approve, a first plant that proves the costs, and a supply chain.
The product
The BWRX-300 is a 300 MWe water-cooled boiling water reactor. Its tall reactor pressure vessel works like a chimney: water boils in the core and rises naturally, then cools and falls, so no recirculation pumps are needed. Isolation valves fitted directly to the vessel can quickly isolate large pipe breaks, removing the need for many of the large safety systems of older reactors (GE Vernova Hitachi).
In an emergency, three isolation condenser trains passively condense steam from the reactor and dump the heat into pools above the reactor. With two trains working, decay heat removal continues for seven days without operator action, and the pools can be topped up with a fire truck. The design uses GE’s existing, licensed GNF2 fuel. Its natural-circulation lineage runs from early US plants through the Dodewaard reactor in the Netherlands to the ESBWR.
How it works
- Boiling in the core: enriched uranium fuel heats water directly in the reactor vessel, producing a steam and water mixture.
- Natural circulation: the hot mixture rises through the tall vessel; cooler water flows back down the sides, with no recirculation pumps.
- Steam to turbine: steam separated at the top of the vessel goes directly to the turbine generator.
- Condense and return: the steam condenses and feedwater is pumped back into the reactor.
- Passive safety: if needed, isolation condensers remove decay heat to water pools by natural circulation, without pumps or AC power.
- Compact build: the reactor building sits largely below ground in a deep shaft; one unit and its buildings take up about the area of a football field.
Timeline
| Date | Milestone |
|---|---|
| 2 Dec 2021 | OPG selects GE Hitachi’s BWRX-300 for Darlington |
| Jul 2023 | Ontario plans three more units, four in total |
| Nov 2023 | Fuel suppliers chosen: Cameco, Urenco USA, Orano and Global Nuclear Fuel |
| 4 Apr 2025 | Canadian Nuclear Safety Commission issues licence to construct one unit |
| 8 May 2025 | Ontario approves construction of the first unit |
| Oct 2025 | Federal and provincial equity commitments through the Canada Growth Fund and Building Ontario Fund |
| Mar 2026 | Regulator clears first hold point; OPG applies for a licence to operate |
| Jun 2026 | Williams Treaties First Nations announce a C$700 million investment |
| 30 Jun 2026 | Poland’s OSGE applies for a Contract for Difference for 14 BWRX-300s |
| End of decade | Target for completing construction of the first unit |
| End 2030 | Target for grid connection |
Impact and numbers
- Four-unit plan: four BWRX-300s at Darlington would supply 1,200 MW, enough for about 1.2 million homes, according to OPG (OPG).
- Costs falling unit by unit: OPG expects the fourth unit to cost about C$4.1 billion, about 33% less than the first. The C$20.9 billion total includes interest, escalation and contingency (WNN).
- Power price: OPG projects about 14.9 Canadian cents per kWh over the plant’s life, within the range of 13.5 to 18.4 cents estimated for a wind, solar and storage alternative.
- Construction progress: by mid 2026 a 953 tonne basemat had been installed about 35 metres below ground, the first outer shaft wall rings were in place and a tunnel boring machine was being readied to dig a 3.4 km cooling water tunnel.
- Climate: OPG says a single 300 MWe SMR can avoid between 0.3 and 2 million tonnes of CO2 a year, depending on what it displaces.
Honest caveats. The BWRX-300 has not yet operated anywhere, so its costs and performance are still forecasts. At about C$20,000 per kW for the first unit including shared infrastructure, it is expensive compared with gas, wind or solar, and the “nth of a kind” savings depend on building a fleet. First-of-a-kind nuclear projects often slip, and Darlington’s licence includes regulatory hold points that must be cleared before key steps, such as installing the reactor vessel. The plant depends on enriched uranium, and Western enrichment and fuel supply are tight after the move away from Russian supplies. Public funding is large: Canada and Ontario are equity investors, and costs will flow into regulated electricity prices. Spent fuel will be stored on site until Canada’s planned deep repository is ready.
What’s next
OPG’s next regulatory steps include clearing the hold point for installing the reactor pressure vessel and the public hearing on its licence to operate. In the US, the Tennessee Valley Authority is pursuing a BWRX-300 at Clinch River. In Europe, Orlen Synthos Green Energy (OSGE) has applied to Poland’s energy minister for a Contract for Difference covering 14 units at Włocławek, Stawy Monowskie and Stalowa Wola, as the first phase of a programme of up to 26 units, with the first unit planned at Włocławek in 2032 (WNN).
Why it matters for Europe / green buyers
Darlington is the reference plant for Europe’s BWRX-300 projects. OSGE says Poland’s programme follows it directly; in Estonia, Fermi Energia is choosing a site for two units, with a construction permit application around 2029 and first operation targeted for 2035 (NucNet). In Sweden, Vattenfall’s Videberg project has shortlisted the BWRX-300 and the Rolls-Royce SMR (Neutron Bytes). European suppliers already play a role: France’s Orano and the Urenco group, with Dutch, German and British shareholders, supply enriched uranium for the first Darlington unit. For green buyers, the Darlington cost and schedule record over the next four years will decide whether SMRs become a mainstream option for clean firm power in Central and Eastern Europe, where coal still dominates.
In India, where NPCIL is developing its own 220 MW Bharat Small Reactors and the SHANTI Act now allows private Indian operators, the Darlington experience on licensing and cost will also be a key reference.
Sources & image credits
- Ontario Power Generation, “Darlington SMR”: https://www.opg.com/projects-services/projects/nuclear/smr/darlington-smr/
- World Nuclear News, “Canada’s first SMR project: How is CAD20.9 billion cost calculated?”, 23 May 2025: https://world-nuclear-news.org/articles/what-is-the-budget-for-canadas-first-smr-project
- GE Vernova Hitachi Nuclear Energy, “BWRX-300 General Description”, Revision J: https://www.gevernova.com/content/dam/gevernova-nuclear/global/en_us/documents/carbon-free-power/005N9751-BWRX-300-General-Description.pdf
- Canadian Nuclear Safety Commission, “Decision by the Commission to authorize Ontario Power Generation Inc. to construct 1 BWRX-300 reactor at the Darlington New Nuclear Project site”, April 2025: https://www.canada.ca/en/nuclear-safety-commission/news/2025/04/decision-by-the-commission-to-authorize-ontario-power-generation-inc-to-construct-1-bwrx-300-reactor-at-the-darlington-new-nuclear-project-site.html
- Government of Ontario, “Ontario Leads the G7 by Building First Small Modular Reactor”, 8 May 2025: https://news.ontario.ca/en/release/1005889/ontario-leads-the-g7-by-building-first-small-modular-reactor
- World Nuclear News, “Polish developer applies for CfD for three SMR plants”, 30 June 2026: https://www.world-nuclear-news.org/articles/polish-developer-applies-for-state-funding-for-three-smr-plants
- NucNet, “Fermi Energia focused on site selection for Estonia SMR project, says CEO”, March 2026: https://www.nucnet.org/news/fermi-energia-focused-on-site-selection-for-estonia-smr-project-says-ceo-4-3-2026
- Neutron Bytes, “UK & Rolls Royce sign deal for three SMRs at Wylfa” (includes Sweden’s Videberg shortlist), 19 April 2026: https://neutronbytes.com/2026/04/19/uk-rolls-royce-sign-deal-for-three-smrs-at-wylfa/
Images:
- “Darlington Nuclear Generating Station panorama” by Óðinn, licensed under CC BY-SA 2.5 Canada (https://creativecommons.org/licenses/by-sa/2.5/ca/deed.en), via Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Darlington_Nuclear_Generating_Station_panorama.jpg
