CANDU nuclear reactors: history and characteristics

The CANDU reactor, a Canadian technology developed in the 1950s-1960s, uses heavy water as moderator and natural uranium as fuel to generate electricity. CANDU models have evolved, with units ranging from 500 to 880 MWe, followed by the CANDU 6 and CANDU 9. Faced with competition, AECL developed the Advanced CANDU Reactor (ACR), but without commercial success. Today, SNC-Lavalin subsidiary Candu Energy supports existing projects and is developing a 300 MWe small module version (SMR) of the CANDU reactor.

History of CANDU reactors

The development of CANDU reactors began in the 1950s, when Canadian scientists began to explore the possibilities offered by nuclear energy for power generation [1]. The first CANDU reactor, the NPD (Nuclear Power Demonstration) nuclear power plant, was commissioned in 1962 in Ontario, Canada [2]. Since then, several CANDU nuclear power plants have been built in Canada and other countries, including Argentina, China, South Korea, India and Romania [3].

Characteristics of CANDU reactors

CANDU reactors, like most nuclear reactors, use fission reactions to heat pressurized water in a primary cooling circuit. The heat is transferred via a heat exchanger to a secondary circuit, powering a steam turbine and an electrical generator. The exhaust steam is cooled, condensed and returned to the steam generator.

CANDU reactors use heavy water as moderator and coolant. Heavy water, containing a high proportion of deuterium, is richer in neutrons than light water (“normal” H20) and therefore less limiting to the chain reaction. This makes it possible to use natural uranium as fuel. This is the big difference with most reactors, which use enriched uranium. This eliminates the need for uranium enrichment, a costly operation requiring a technology that is not very widespread: there are fewer than 10 installations worldwide.

The main features of CANDU reactors include :

  1. Pressure tubes: CANDU reactors feature horizontal pressure tubes, which contain the fuel bundles and enable reactor cooling [5].
  2. The ability to refuel during operation: Unlike many other types of nuclear reactor, CANDU reactors can be refueled during operation, enabling continuous power generation and reducing maintenance downtime [7].

CANDU reactor fuel

CANDU reactors use fuel bundles around 10 cm in diameter, made up of several small metal tubes. These bundles are placed in pressure tubes inside a calandria, a large vessel containing heavy water that acts solely as a moderator. The calandria, unpressurized and at lower temperatures, is easier to manufacture. To prevent heat leakage from the pressure tubes to the moderator, each pressure tube is enclosed in a calandria tube, with insulating CO2 gas between the two tubes.

Unlike conventional pressurized water reactors, the CANDU system enables refuelling without shutting down the reactor, which was a major design objective. Two robotic machines perform continuous refuelling.

Each fuel bundle is a cylinder made up of thin tubes filled with uranium oxide fuel pellets. Older designs had 28 or 37 fuel elements, while the new CANFLEX bundle has 43, with two element sizes, enabling power to be increased without overheating the elements. The tubes and bundles, made of zircaloy (zirconium 2.5% niobium by weight), are transparent to neutrons, allowing them to circulate between the bundles.

CANDU reactor applications and exports

CANDU reactors account for a significant share of electricity generation in Canada, particularly in Ontario, where they supply around 60% of the province’s electricity [8]. In addition to their use in Canada, CANDU reactors have been exported to several countries, where they have been adapted to local energy needs.

India, for example, has developed its own version of the CANDU reactor, called PHWR (Pressurized Heavy Water Reactor). These reactors are widely used in India’s nuclear power program, and account for a significant proportion of the country’s electricity production [9].

In Argentina, the Embalse nuclear power plant, using a CANDU-6 reactor, was commissioned in 1984 and continues to supply electricity to the national grid [10]. South Korea, China and Romania have also incorporated CANDU technology into their nuclear programs [11].

Conclusion

CANDU reactors offer an attractive alternative to enriched uranium nuclear reactors, and are particularly suited to countries with natural uranium resources. Their ability to use natural uranium, their modular design and their ability to refuel during operation make them a viable option for large-scale power generation. As global demand for clean energy continues to grow, CANDU reactor technology remains a key element of the global energy landscape.

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