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| Fig. 1: Bruce Nuclear Generating Station in Ontario (Source: Wikimedia Commons) |
As of 2023, Canada is the 6th highest energy producer in the world, accounting for roughly 4% of global energy production. Canada is a key producer and exporter of uranium-based energy, ranking second in the world for both uranium production and export. [1]
Of the 24726 petajoule (2.47 × 1019 joules) of energy produced in Canada, 11% comes from uranium, third only to crude oil (44%) and natural gas (28%). [1] There is substantial province-to-province variation in the fraction of energy produced from uranium. For example, almost 2/3 of the energy produced in Saskatchewan comes from uranium, while some provinces, such as Newfoundland and the maritime provinces, do not produce any uranium-derived energy. [1]
A large share of the uranium-derived energy that Canada produces is exported rather than used domestically. Only 8% of Canada's total energy supply (production + imports - exports) comes from nuclear energy (Table 1). [1] All nuclear reactors in Canada use uranium fuel and follow the CANDU (Canada deuterium-uranium) design. Ontario has the most number of CANDU reactor plants at 18 (Fig. 1).
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| Table 1: Canada total energy supply breakdown. [1] |
In the same year as Enrico Fermi's first proof-of-concept fission reactor (December 2nd 1942), the Western Allies decided to move much of the British nuclear effort to Canada. [2] At the time, it was known that fission of a uranium-235 atom by a neutron is more likely if the neutron is traveling at a lower velocity than the speed at which it is initially released from U-235. Therefore, a self-sustaining fission chain reaction was understood to require a moderator that could slow down neutrons.
Three practical moderators were being considered: graphite, beryllium, and deuterium. Since the United States focused on graphite, and beryllium was scarce and toxic, a tripartite agreement between the United Kingdom, Canada, and the United States assigned the Canadian team to work on deuterium as the moderator. [2] These efforts led to the NRX (National Research Experimental) reactor, which achieved criticality in 1945.
This line of heavy-water, natural-uranium reactors ultimately evolved into the CANDU (Canadian deuterium-uranium) reactor design, which supplies 8% of Canadian energy today.
In a typical fission reaction, a neutron strikes the nucleus of a uranium atom, producing a lighter element, energy, and more neutrons, which then continue the chain reaction. Neutrons produced in fission have energies of around 2 MeV. However, the probability of fission when a neutron strikes a U-235 nucleus is much higher when the neutron energy has been reduced to around 0.024 eV. A moderating medium is therefore required to slow neutrons and reduce their kinetic energy. [3]
When light water is used as the moderator, there is a non-negligible probability that hydrogen nuclei will absorb neutrons. To compensate for these neutron losses, uranium must be enriched to about 23% U-235 from its naturally occurring 0.7%. [3]
The uniqueness of the CANDU reactor lies in its use of deuterium (heavy water) as the moderator. Because the hydrogen atoms in deuterium already have an extra neutron, they are less likely to absorb additional neutrons. As a result, a self-sustaining fission reaction is possible with unenriched (natural) uranium. [3]
Because the CANDU reactor design does not require uranium enrichment, it can bypass the enrichment bottleneck. CANDU reactors can also generate plutonium-239. This occurs when a neutron produced in the fission of uranium-235 is captured by the more abundant uranium-238. Since CANDU fuel is not enriched and therefore contains a higher fraction of uranium-238, it is particularly conducive to producing plutonium-239.
India's first nuclear test, Smiling Buddha, in May 1974 used plutonium produced in a Canadian-supplied, heavy-water, natural-uranium research reactor related to the CANDU design. [4]
© Kent Kotaka. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Energy Fact Book 2023-2024," Natural Resources Canada, 2024.
[2] G. L. Brooks, "A Short History of the CANDU Nuclear Power System," Atomic Energy of Canada Ltd., AECL-10788, April 1993.
[3] C. M. Morad, G. L. de Stefani, and T. A. dos Santos, "CANDU: Study and Review," 2017 Intl. Nuclear Atlantic Conf. INAC 2017, Belo Horizonte, Brazil, 22 Oct 17.
[4] M. Donohue, "Pokhran-I: India's First Nuclear Bomb," Phyiscs 241, Stanford University, Winter 2014.
