Canada's Big Modular Reactor

by Jeremy Whitlock

February 2024

Canada is a big country.

It has big energy needs, big natural resources, big environmental challenges – and big ideas.

One of the biggest came as the country emerged from WWII, punching well above its weight on several fronts (manufacturing, science and engineering among them) – and looking increasingly outward to the needs of global peace and sustainable development.

At the time Canada boasted the world's second-largest nuclear program – birthed in fire with the Manhattan Project as midwife, now unleashed as an unprecedented force for global good.

The big idea was to make electricity from a nuclear reactor that Canada could design and build, and to do this cheaper (and of course cleaner) than fossil fuels.

It had to run on natural uranium, since Canada has a lot of that, and no enrichment capability.

It had to use pressure tubes instead of pressure vessels, partly since Canada lacked a large steel forging capability.

The result, by the early 1960s, was CANDU: a reactor that any country could build and fuel if it had the medium-scale industrial capacity of Canada circa 1950s, and access to natural uranium (conveniently one of earth's abundant elements).

The name recalls the spirit of Canadian engineers at the time: not only was this 'hewer of wood, drawer of water' nation now a splitter of atoms (and one of the first), but it was doing so on its own terms, keeping pace with the giant to the south despite that nation's head start through its nuclear navy program.

Canada's was the nuclear road less travelled: it's actually very hard to make a reactor work with natural uranium, and maintain commercial viability – the machine has to be comparatively huge, filled with exotic heavy water, and refuelled every day without shutting down.

The heavy water can't leak, the fuel must be extra robust, and the control and safety systems demand numerous in situ detectors and devices – all while minimizing extra materials in the core.

But the genius of it all is the pressure-tube design, and the robotic fuelling system that feeds it: hundreds of subcritical microreactors linked to each other by a bath of low-temperature, low-pressure heavy water.

The fuelling system must latch to these pressure tubes – daily, at 300 degrees and 100 atmospheres, keeping the coolant flowing while swapping fuel bundles.

More power? More pressure tubes: CANDU designs range from 20 to over 1000 megawatts of electrical power, mainly by adding more pressurized fuel channels.

In some ways, it was the world's first commercial modular reactor.

The many side benefits include passive safety: that large heavy-water inventory makes any change to the fission process naturally sluggish, and absorbs excess heat when needed. Both features translate to more time, and time is your friend when managing enormous flows of energy.

The design also enables refurbishment – an option now exercised throughout Canada's nuclear fleet with the recent announcement of an overhaul at the Pickering station near Toronto. Like swapping out the engine of a classic car, new life can be bestowed upon aging but still elegant bodies, rather than starting from scratch.

Perhaps the most interesting benefit is CANDU's ability to operate with different fuels, including thorium – more abundant than uranium, and able to ‘breed' uranium fuel while in the reactor.

These things – passive safety, fuelling flexibility, extendable core life – are the stuff of advanced reactors today, and CANDU does this all without needing enrichment of its uranium fuel (presenting both fuel-efficiency and nonproliferation benefits).

Small modular reactors are the rage and have their place – small grids, load following, integration with renewables, remote and off-grid locations, industrial applications, newcomer countries. But the big need for big reactors in a big country with big energy needs has not gone away – and Canadians spent billions over 80 years perfecting a big answer to this need.

With energy flexibility and sustainable development as its calling cards, nuclear's time has finally come: whether big or small, we'll need them all.


Discussion welcome.

©2024 Jeremy Whitlock

About the Author

Dr. Jeremy Whitlock has three decades' experience as a scientist and manager in the Canadian and international nuclear community. Since January 2017 he has worked in the Department of Safeguards at the International Atomic Energy Agency (IAEA) in Vienna, helping to ensure that countries meet their obligations under the Nuclear Non-proliferation Treaty (NPT). Prior to that he worked at Chalk River Laboratories in nuclear nonproliferation and reactor physics. Dr. Whitlock has a B.Sc. in Physics from the University of Waterloo (1988), and an M.Eng. and PhD in Engineering Physics (reactor physics) from McMaster University (1995). Dr. Whitlock is a Past President and Fellow of the Canadian Nuclear Society, and a public speaker and author on nuclear issues – including The Canadian Nuclear FAQ (, a personal website of frequently-asked questions (FAQs) on Canadian nuclear technology. Dr. Whitlock feels that humans and nature can coexist in harmony when technology is used sustainably, and strongly suspects that canoes are the closest humans have come to inventing a perfect machine.

Go to CNF homepage
The Canadian Nuclear FAQ  

by Dr. Jeremy Whitlock