Light water reactors work by loading enriched uranium, sealing the reactor core and then running it for 18-24 months before shutting down, reopening the core and replacing fuel. This makes it relatively easy to keep track of how much fuel is going in, and what is happening to the spent fuel that comes out. It’s impossible to secretly swap fuel rods while the reactor is generating power.
The Canadian heavy water reactor doesn’t work that way.
All uranium-fuelled reactors produce plutonium. The issue with the CANDU is that it has online refuelling," explained Ed Lyman, director of nuclear safety for the Union of Concerned Scientists in Washington, D.C. "You're refuelling continuously, which makes it harder for the IAEA [International Atomic Energy Agency]."
Canada, which produces all of its own uranium, would be in a strong position to divert fissionable material undetected if so inclined, say the experts.
The plutonium produced in a heavy water reactor is also much easier to turn into a nuclear weapon, said Fetter.
"Of the countries that have made nuclear weapons with plutonium, many of them have used a heavy water reactor," he said.
That’s why India used a Canadian heavy water reactor to produce its first nuclear weapon in 1974, an all-plutonium implosion-type bomb. The "Smiling Buddha" test was the first to break the nuclear monopoly of the "Big Five" UN Security Council powers.
Fetter said the U.S. had heavy water reactors only for the purpose of producing plutonium and tritium. (Tritium, also produced in abundance by the CANDU reactor, is another critical component of a modern two-stage thermonuclear weapon that Canada has in abundance.)
Canada is also branching out into a new kind of technology that will require enriched uranium: the small modular reactor (SMR). Some of the proposed SMRs would use uranium enriched to a level as high as 20 per cent, known as high-assay low-enriched uranium (HALEU).
HALEU is already a proliferation concern in itself, said Lyman.
Anything below 20 per cent is considered low-enriched uranium and has a much lower level of oversight. The IAEA considers it indirect-use material you can't directly use in a bomb,” he explained. "But there's evidence, both historical and technical, that that's not true, and that the actual cutoff for weapon-usable uranium is below 20 per cent. Could be 15 per cent, could be 10 per cent.
"And so I worry about this push for advanced reactors, SMRs that use HALEU up near 20 per cent, like the
ARC 100 in Canada, that the material itself is in the dangerous band. And if Canada were to pursue a whole fleet of SMRs using HALEU, and especially if they were to build their own enrichment plants to support that, that might give them a threshold capability for bombs, even if they never make highly enriched [over 20 per cent] uranium."
ARC Canada points out that its proposed reactor in New Brunswick, which would become the only commercial fast-breeder reactor in the world outside Russia, mitigates against proliferation risks by having a long fuel cycle with a core that stays sealed for 20 years.
But there’s another factor: The arrival of SMRs that consume enriched uranium has got the government of Canada thinking about enrichment.
Any country which has a complete fuel cycle is a latent nuclear weapons country, in the sense that it is not far from making a nuclear weapon," said former IAEA director and Nobel Peace Prize laureate Mohamed ElBaradei.
