The CANDU (Canada deuterium uranium) is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide (heavy water) moderator and its use of (originally, natural) uranium fuel. CANDU reactors were first developed in the late 1950s and 1960s by a partnership between Atomic Energy of Canada Limited (AECL), the Hydro-Electric Power Commission of Ontario, Canadian General Electric, and other companies. It was later exported, and in 2026 there were 26 operational CANDU reactors worldwide: 17 in Canada, three in South Korea, two each in Romania and China, and one each in India and Argentina.
There have been two major types of CANDU reactors, the original design of around 500 MWe that was intended to be used in multi-reactor installations in large plants, and the optimized CANDU 6 in the 600 MWe class that is designed to be used in single stand-alone units or in small multi-unit plants. CANDU 6 units were built in Quebec and New Brunswick, as well as Pakistan, Argentina, South Korea, Romania, and China. A single example of a non-CANDU 6 design was sold to India. The multi-unit design was used only in Ontario, Canada, and grew in size and power as more units were installed in the province, reaching ~880 MWe in the units installed at the Darlington Nuclear Generating Station. An effort to optimize the larger units in a fashion similar to CANDU 6 led to the CANDU 9.
By the early 2000s, sales prospects for the original CANDU designs were dwindling due to the introduction of newer designs from other companies. AECL responded by cancelling CANDU 9 development and moving to the Advanced CANDU reactor (ACR) design. ACR failed to find any buyers; its last potential sale was for an expansion at Darlington, but this was cancelled in 2009. In October 2011, the Canadian Federal Government licensed the CANDU design to Candu Energy (a wholly owned subsidiary of SNC-Lavalin, now the AtkinsRéalis Group Inc.), which also acquired the former reactor development and marketing division of AECL at that time.
In 2017, a consultation with industry led Natural Resources Canada to establish a "SMR Roadmap" targeting the development of small modular reactors (SMRs). In response, SNC-Lavalin developed a 300 MWe SMR version of the CANDU, the CANDU SMR, which it began to highlight on its website. In 2020, the CANDU SMR was not selected for further design work for a Canadian demonstration project. SNC-Lavalin is still looking at marketing a 300 MW SMR in part due to projected demand due to climate change mitigation.
Design and operation
The basic operation of the CANDU design is similar to other nuclear reactors. Fission reactions in the reactor core heat pressurized water in a primary cooling loop. A heat exchanger, also known as a steam generator, transfers the heat to a secondary cooling loop, which powers a steam turbine with an electric generator attached to it (for a typical Rankine thermodynamic cycle). The exhaust steam from the turbines is then cooled, condensed and returned as feedwater to the steam generator. The final cooling often uses cooling water from a nearby source, such as a lake, river, or ocean. Newer CANDU plants, such as the Darlington Nuclear Generating Station near Toronto, Ontario, use a diffuser to spread the warm outlet water over a larger volume and limit the effects on the environment. Although all CANDU plants to date have used open-cycle cooling, modern CANDU designs can use cooling towers instead.
Where the CANDU design differs from most other designs is in the details of the fissile core and the primary cooling loop. Natural uranium consists of a mix of mostly uranium-238 with small amounts of uranium-235 and trace amounts of other isotopes. Fission in these elements releases high-energy neutrons, which can cause other 235U atoms in the fuel to undergo fission as well. This process is more effective when the neutron energies are lower than what the reactions release naturally. Most reactors use some form of neutron moderator to lower the energy of the neutrons, or "thermalize" them, which makes the reaction more efficient. The energy lost by the neutrons during this moderation process heats the moderator, and this heat is extracted for power.
Most commercial reactor designs use normal water as the moderator. Water absorbs some of the neutrons, enough that it is not possible to keep the reaction going in natural uranium. CANDU replaces this "light" water with heavy water. Heavy water's extra neutron decreases its ability to absorb excess neutrons, resulting in a better neutron economy. This allows CANDU to run on unenriched natural uranium, or uranium mixed with a wide variety of other materials such as plutonium and thorium. This was a major goal of the CANDU design; by operating on natural uranium the cost of enrichment is removed. This also presents an advantage in nuclear proliferation terms, as there is no need for enrichment facilities, which might also be used for weapons.
Calandria and fuel design
In conventional light-water reactor (LWR) designs, the entire fissile core is placed in a large pressure vessel. The amount of heat that can be removed by a unit of a coolant is a function of the temperature; by pressurizing the core, the water can be heated to much greater temperatures before boiling, thereby removing more heat and allowing the core to be smaller and more efficient.
Building a pressure vessel of the required size is a significant challenge, and at the time of the CANDU's design, Canada's heavy industry lacked the requisite experience and capability to cast and machine reactor pressure vessels of the required size. This problem is amplified by natural uranium fuel's lower fissile density, which requires a larger reactor core. This issue was so major that even the relatively small pressure vessel originally intended for use in the NPD prior to its mid-construction redesign could not be fabricated domestically and had to be manufactured in Scotland instead. Domestic development of the technology required to produce pressure vessels of the size required for commercial-scale heavy water moderated power reactors was thought to be very unlikely.
In CANDU the fuel bundles of about 10 cm diameter are composed of many smaller metal tubes. The bundles are contained in pressure tubes within a larger vessel containing additional heavy water acting as a moderator. This larger vessel, known as a calandria, is not pressurized and remains at lower temperatures, making it easier to fabricate. In order to prevent the heat from the pressure tubes from leaking into the surrounding moderator, each pressure tube is enclosed in a calandria tube. Carbon dioxide gas in the gap between the two tubes acts as an insulator. The moderator tank also acts as a large heat sink that provides an additional safety feature.
In a conventional pressurized water reactor, refuelling the system requires shutting down the core and opening the pressure vessel. In CANDU reactors, the tube being refuelled remains pressurized. This allows the CANDU system to be continually refuelled without shutting down, another major design goal. In modern systems, two robotic machines attach to the reactor faces and open the end caps of a pressure tube. One machine pushes in the new fuel, whereby the depleted fuel is pushed out and collected at the other end. A significant operational advantage of online refuelling is that a failed or leaking fuel bundle can be removed from the core once it has been located, thus reducing the radiation levels in the primary cooling loop.
Each fuel bundle is a cylinder assembled from thin tubes filled with ceramic pellets of uranium oxide fuel (fuel elements). In older designs, the bundle had 28 or 37 half-meter-long fuel elements with 12–13 such assemblies lying end-to-end in a pressure tube. The newer CANFLEX bundle has 43 fuel elements, with two element sizes (so the power rating can be increased without melting the hottest fuel elements). It is about in diameter, long, weighs about , and is intended to eventually replace the 37-element bundle. To allow the neutrons to flow freely between the bundles, the tubes and bundles are made of neutron-transparent zircaloy (zirconium + 2.5% wt niobium).
Purpose of using heavy water
Natural uranium is a mix of isotopes: approximately 99.28% uranium-238 and 0.72% uranium-235 by atom fraction. Nuclear power reactors are usually operated at constant power for long periods of time, which requires a constant rate of fission over time. In order to keep the fission rate constant, the neutrons released by fission must produce an equal number of fissions in other fuel atoms. This balance is referred to as criticality. Neutrons released by nuclear fission are fairly energetic and are not readily absorbed (or "captured") by the surrounding fissile material. In order to improve the capture rate, the neutron energy must be reduced, or moderated, to be as low as possible. In practice, the lower energy limit is the energy where the neutrons are in thermal equilibrium with the moderator. When neutrons approach this lower energy limit, they are referred to as thermal neutrons.
During moderation it helps to separate the neutrons and uranium, since 238U has a large affinity for intermediate-energy neutrons ("resonance" absorption), but is only easily fissioned by the few energetic neutrons above ≈1.5–2 MeV. Since most of the fuel material is usually 238U, most reactor designs are based on thin fuel rods separated by moderator, allowing the neutrons to travel in the moderator before entering the fuel again. More neutrons are released than the minimum needed to maintain the chain reaction; when uranium-238 absorbs neutrons, plutonium is created, which helps to make up for the depletion of uranium-235. Eventually the build-up of fission products that are more neutron-absorbing than 238U slows the reaction and calls for refuelling.
Light water makes an excellent moderator: the light hydrogen atoms are very close in mass to a neutron and can absorb a lot of energy in a single collision (like a collision of two billiard balls). However, light hydrogen can absorb neutrons, reducing the number available to react with the small amount of 235U in natural uranium, preventing criticality. In order to allow criticality, the fuel must be enriched, increasing the amount of 235U to a usable level. In light-water reactors, the fuel is typically enriched to between 2% and 5% 235U (the leftover fraction with less 235U is called depleted uranium). Enrichment facilities are expensive to build and operate. They may also pose a proliferation concern, as they can be used to enrich the 235U much further, up to weapons-grade material (90% or more 235U). This can be remedied if the fuel is supplied and reprocessed by an internationally approved supplier.