AtkinsRéalis Group Inc.’s ATRL-T Monark nuclear power reactor, which had aimed to produce substantially more electricity than previous Candu reactors, will actually produce slightly less, according to the Canadian Nuclear Safety Commission.
The Monark has been marketed by its designer, AtkinsRéalis Group Inc., as boasting a electricity generation capacity of 1,000 megawatts. However, the CNSC recently updated its website to reflect a much lower capacity as the regulator begins a preliminary review of the Monark.
“As the design has progressed, AtkinsRéalis has further defined the power of the reactor,” wrote CNSC spokesperson Braeson Holland in a statement confirming the new output, which she said is around 850 megawatts.
AtkinsRéalis disputed that, asserting that its submission to the CNSC was for a 925-megawatt reactor. In either case, the Monark’s lower-than-promised output could represent a problem as it competes head-to-head with much larger reactors.
Since the last Candus were built decades ago, the reactor market has continued a longstanding trend toward ever-larger units. According to data from Mycle Schneider Consulting, construction began on 73 reactors worldwide since 2016. Their average output approached 1,100 megawatts; only four, all in India, fell in the range between 800 and 999 megawatts.
When AtkinsRéalis announced the Monark in November, 2023, it claimed the reactor would boast “the highest output of any CANDU technology at 1,000 megawatts.” It assembled a team with hundreds of members to design it.
Taxpayer support followed. In March, 2025, the federal government announced it had entered into an agreement with AtkinsRéalis with the intention of loaning the company more than $300-million over four years to finance half of the Monark’s development. (Natural Resources Canada spokesperson Miriam Galipeau said that agreement was subject to further negotiations and due diligence, and that no loan has been granted yet.)
That same month, Patrick Reed, the project’s technical director, said during a presentation that one-third of the Monark’s higher output would be achieved through “improvements on the secondary side,” a reference to the portion of the plant where water is turned into steam to drive turbines. Also, the Monark’s steam generators would be much larger than those of previous Candus, increasing the amount of heat that could be extracted from the reactor core.
It’s unclear how the Monark’s diminished output might affect its prospects as AtkinsRéalis pursues orders from Ontario’s two largest power utilities. Ontario Power Generation Inc. proposes to build a new nuclear plant dubbed Wesleyville with a capacity of up to 10,000 megawatts; Bruce Power LP is planning a new station at its existing Bruce facility, featuring up to 4,800 megawatts.
OPG reported in a January regulatory filing that it had considered the Monark, Westinghouse Electric Co.’s AP1000 and Electricité de France’s European Pressurized Reactor (EPR) for Wesleyville. (It also contemplated the BWRX-300, a much smaller reactor it is already building at its Darlington station.) Bruce Power has not revealed which reactors made its short list. Both utilities denied interview requests for this story.
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The Monark’s competitors are substantially larger. The AP1000, which Westinghouse is aggressively marketing in Canada, puts out 1,100 megawatts. France’s EPR is even larger, with a gross output of around 1,600 megawatts. Korea Electric Power Corp.’s APR-1400 reactor, sometimes raised as a dark horse for deployment in Canada, puts out 1,400 megawatts.
Reactor selection for Ontario’s plants, which is being co-ordinated by the province’s government, could strongly influence other Canadian utilities that are considering building nuclear plants. On Thursday, Saskatchewan’s government announced that its Crown utility, SaskPower, had entered into an agreement with Bruce Power to access expertise as it evaluates large reactors. Separately, Energy Alberta, which seeks to build large reactors in Alberta, announced it had entered into its own agreement to access Bruce Power’s expertise.
Output is just one of many considerations when buying a reactor; as a Canadian design, the Monark might enjoy a measure of home-court advantage. OPG and Bruce Power already possess extensive experience with Candus. And Ontario Energy Minister Stephen Lecce has also placed strong emphasis on local supply chains and domestic economic benefits; any foreign-designed reactor would likely offer less on that front.
Chris Gadomski, a nuclear energy analyst with BloombergNEF, said the Monark’s size will be of secondary importance to its cost. If its capital cost per megawatt is competitive, then its smaller size “shouldn’t make much difference,” he said. It might even be more attractive to certain utilities, he added, because disconnecting one from the grid for maintenance or other reasons would be less disruptive.
AtkinsRéalis spokesperson Daniela Pizzuto said the Monark will be “a close replication” of the Candu 850 reactors at Darlington, which were built in the 1980s and early 1990s. She added that the Monark’s lower-than-promised output reflects the company’s intention to duplicate the Darlington reactor’s design as closely as possible while avoiding risks associated with a first-of-a-kind reactor.
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Ibrahim Attieh, a reactor physicist who worked on Candu designs for many years, had warned The Globe and Mail more than two years ago that the Monark was unlikely to meet the 1,000-megawatt target, owing to technical decisions made in the early stages of its design.
Atomic Energy of Canada Ltd. (which AtkinsRéalis purchased from the federal government in 2011) previously attempted to update the Candu 850 in the 1990s with a 935-megawatt version called the Candu-9. Mr. Attieh said the Monark used many of the same strategies employed unsuccessfully to boost the Candu-9’s output.
He added that when Ontario Hydro built the province’s original nuclear stations, it employed a massive contingent of experienced engineers, as did AECL. The work force available today is tiny by comparison, and far less experienced.
“Canada has failed to successfully design a nuclear reactor for the last 40 years,” he wrote.
The source of the disagreement between AtkinsRéalis and the CNSC regarding the Monark’s output is unclear. Although electricity is the most important product from most nuclear plants, strictly speaking, reactors generate heat; thus, they are rated in megawatts thermal. The thermal output is generally three times higher than the plant’s capacity to generate electricity, which is expressed in megawatts electric. That capacity is often further subdivided into “gross” and “net” figures, the latter of which takes into account the substantial amount of electricity consumed by the plant itself. The plethora of different ways of measuring reactor output sometimes conjures confusion.
Ms. Pizzuto clarified that the Monark will have a net output of 925 megawatts.
With a file from Emma Graney