The CANDU reactor and heavy water reactors, explained
Most of the world's nuclear electricity comes from light water reactors that need enriched uranium. The CANDU reactor took a different path. By using heavy water to slow neutrons, it runs on natural uranium straight from the mine, after conversion and fuel fabrication but without enrichment.
What is a heavy water reactor?
A heavy water reactor uses deuterium oxide (D2O), water in which the hydrogen atoms are the heavier isotope deuterium, as its moderator. The moderator is the material that slows the neutrons released by fission so they cause more fission. The World Nuclear Association (WNA) explains that if graphite or heavy water is used as moderator, it is possible to run a power reactor on natural instead of enriched uranium.
The reason is neutron economy. Ordinary (light) water absorbs neutrons as well as slowing them, so WNA describes it as less efficient as a moderator than heavy water or graphite. Light water reactors make up for that by enriching the fuel, commonly to 3.5 to 5.0% U-235. Natural uranium has the same composition as when it was mined: 0.7% U-235 and over 99.2% U-238.
How a CANDU reactor works
WNA notes that the PHWR has been developed since the 1950s in Canada as the CANDU, and from the 1980s also in India. The main parts are:
- Calandria. The heavy water moderator sits in a large tank called a calandria, penetrated by several hundred horizontal pressure tubes.
- Pressure tubes. Each tube forms a channel for fuel and carries heavy water coolant under high pressure, about 100 times atmospheric pressure, typically reaching 290°C.
- Fuel bundles. A CANDU fuel bundle is 37 half-metre fuel rods, ceramic uranium oxide pellets in zircaloy tubes, with 12 bundles lying end to end in each fuel channel.
- Steam cycle. As in a pressurized water reactor, the primary coolant makes steam in a secondary circuit that drives the turbines.
Advantages of the CANDU design
No enrichment. Natural uranium fuel removes the enrichment step from the fuel cycle. That matters for countries and customers who want a fuel supply that does not depend on a small number of enrichment suppliers. For the contrast with advanced designs that need high-assay fuel, see HALEU vs natural uranium.
On-power refuelling. WNA notes that the reactor can be refuelled progressively without shutting down, by isolating individual pressure tubes from the cooling circuit. Most other reactors must shut down and open the vessel to refuel.
No single large pressure vessel. The pressure boundary is spread across many tubes instead of one large steel vessel.
The trade-offs
- More used fuel. PHWRs produce a much larger amount of used fuel per unit output.
- Pressure tube life. The tubes have not proved as durable as a pressure vessel. WNA reports that in CANDU reactors, pressure tube replacement has been undertaken on some plants after about 30 years of operation.
- Heavy water. D2O is a specialised material that has to be produced, kept pure and managed through the plant's life.
Heavy water and small modular reactors
Most SMR designs in development use either light water with low-enriched fuel, or advanced coolants with HALEU. The U.S. EIA notes that several designs use high-assay low-enriched uranium (HALEU) fuel, enriched between 5% and under 20%. A heavy-water-moderated SMR takes the opposite route and keeps the natural-uranium fuel cycle. For a wider view of the field, see small modular reactors, SMR vs microreactor, or pebble bed reactor.
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Frequently asked questions
What is a CANDU reactor?
CANDU is the Canadian pressurized heavy water reactor (PHWR). It uses natural uranium oxide fuel, heavy water as the moderator and coolant, and a calandria tank pierced by horizontal pressure tubes that hold the fuel.
Why does a CANDU reactor not need enriched uranium?
Heavy water absorbs far fewer neutrons than ordinary water, so it is a more efficient moderator. That efficiency lets the reactor sustain a chain reaction with natural uranium, which contains about 0.7% U-235.
Can a CANDU reactor be refuelled while running?
Yes. Because the fuel sits in individual pressure tubes rather than one large vessel, CANDU reactors can be refuelled progressively without shutting down, by isolating individual pressure tubes from the cooling circuit.
What is the difference between heavy water and light water reactors?
Light water reactors use ordinary water as moderator and coolant and need enriched fuel, commonly 3.5 to 5% U-235. Heavy water reactors use deuterium oxide as the moderator and can run on natural uranium.
What are the disadvantages of heavy water reactors?
WNA notes that PHWRs produce a much larger amount of used fuel per unit of output, and that pressure tubes have not proved as durable as a large pressure vessel; some plants have replaced them after about 30 years. Heavy water itself is also a specialised material that must be produced and managed.