Qinshan Phase III Units 1 & 2, located in Zhejiang China: Two CANDU 6 reactors, designed by Atomic Energy of Canada Limited (AECL), owned and operated by the Third Qinshan Nuclear Power Company Limited. Credit: AECLThe CANDU reactor is a Canadian-invented, pressurized heavy water reactor developed initially in the late 1950s and 1960s by a partnership between Atomic Energy of Canada Limited (AECL), the Hydro-Electric Power Commission of Ontario (now known as Ontario Power Generation), Canadian General Electric (now known as GE Canada), as well as several private industry participants. The acronym "CANDU", a registered trademark of Atomic Energy of Canada Limited, stands for "CANada Deuterium Uranium". This is a reference to its deuterium-oxide (heavy water) moderator and its use of uranium fuel (originally, natural uranium). All current power reactors in Canada are of the CANDU type. Canada markets this power reactor abroad.

Reactor design

The CANDU reactor is conceptually similar to most light water reactors, although it differs in the details.

Fission reactions in the nuclear reactor core heat a fluid, in this case heavy water (see below). This coolant is kept under high pressure to raise its boiling point and avoid significant steam formation in the core. The hot heavy water generated in this primary cooling loop is passed into a heat exchanger heating light water in the less-pressurized secondary cooling loop. This water turns to steam and powers a conventional turbine with a electrical generator attached to it. Any excess heat energy in the steam after flowing through the turbine is rejected into the environment in a variety of ways, most typically into a large body of cool water, such as a lake, river or ocean. Heat can also be disposed of using a cooling tower, but they are avoided whenever possible because they reduce the plant's efficiency. More recently-built CANDU plants, such as the Darlington Nuclear Generating Station near Toronto, Ontario, use a discharge-diffuser system that limits the thermal effects in the environment to within natural variations.

Schematic diagram of a CANDU reactor Source: US Nuclear Regulatory Commission. Source: CANDU Owners GroupThe CANDU was designed to use natural uranium as its fuel. Traditional designs using light water as a moderator will absorb too many neutrons to allow a chain reaction to occur in natural uranium due to the low density of active nuclei. Heavy water absorbs fewer neutrons than light water, allowing a high neutron economy that can sustain a chain reaction even in unenriched fuel. Also, the low temperature of the moderator (below the boiling point of water) reduces changes in the neutrons' speeds from collisions with the moving particles of the moderator ("neutron scattering"). The neutrons therefore are easier to keep near the optimum speed to cause fissioning; they have good spectral purity. At the same time, they are still somewhat scattered, giving an efficient range of neutron energies.

The large thermal mass of the moderator provides a significant heat sink that acts as an additional safety feature. If a fuel assembly were to overheat and deform within its fuel channel, the resulting change of geometry permits high heat transfer to the cool moderator, thus preventing the breach of the fuel channel, and the possibility of a meltdown. Furthermore, because of the use of natural uranium as fuel, this reactor cannot sustain a chain reaction if its original fuel channel geometry is altered in any significant manner.

In a traditional light water reactor (LWR) design, the entire reactor core is a single large pressure vessel containing the light water, which acts as moderator and coolant, and the fuel arranged in a series of long bundles running the length of the core. To refuel such a reactor, it must be shut down, the pressure dropped, the lid removed, and a significant fraction of the core inventory, such as one-third, replaced in a batch procedure. The CANDU's calandria-based design allows individual fuel bundles to be removed without taking the reactor off-line, improving overall duty cycle or capacity factor. A pair of remotely-controlled fueling machines visit each end of an individual fuel string. One machine inserts new fuel while the other receives discharged fuel.

A lower ²³⁵U density also generally implies that less of the fuel will be consumed before the fission rate drops too low to sustain criticality (due primarily to the relative depletion of ²³⁵U compared with the build-up of parasitic fission products). However, through increased efficiency which, among other benefits, avoids the need for enriched uranium, CANDU reactors use about 30-40% less mined uranium than light-water reactors per unit of electrical energy produced.

Two CANDU fuel bundles: Each about 50 cm in length and 10 cm in diameter, and generating about 1 GWh of electricity during its lifetime. Credit: Atomic Energy of Canada Limited.A CANDU fuel assembly consists of a number of zircaloy tubes containing ceramic pellets of fuel arranged into a cylinder that fits within the fuel channel in the reactor. In older designs the assembly had 28 or 37 half-meter long fuel tubes with 12 such assemblies lying end to end in a fuel channel. The relatively new CANFLEX bundle has 43 tubes, with two pellet sizes. It is about 10 cm (four inches) in diameter, 0.5 m (20 inches) long and weighs about 20 kg (44 lb) and replaces the 37-tube bundle. It has been designed specifically to increase fuel performance by utilizing two different pellet diameters.

A number of distributed light-water compartments called liquid zone controllers help control the rate of fission. The liquid zone controllers absorb excess neutrons and slow the fission reaction in their regions of the reactor core.

CANDU reactors employ two independent, fast-acting safety shutdown systems. Shutoff rods penetrate the calandria vertically and lower into the core in the case of a safety-system trip. A secondary shutdown system involves injecting high-pressure gadolinium nitrate solution directly into the low-pressure moderator.

Use of heavy water

The key to maintaining a nuclear reaction within a nuclear reactor is to use the neutrons being released during fission to stimulate fission in other nuclei. With careful control over the geometry and reaction rates, this can lead to a self-sustaining chain reaction, a state known as "criticality".

Natural uranium consists of a mixture of various isotopes, primarily ²³⁸U and a much smaller amount (about 0.72% by weight) of ²³⁵U. ²³⁸U can only be fissioned by neutrons that are fairly energetic, about 1 MeV or above. No amount of ²³⁸U can be made "critical", however, since it will tend to absorb more neutrons than it releases by the fission process. ^235U, on the other hand, can support a self-sustained chain reaction, but due to the low natural abundance of ²³⁵U, natural uranium cannot achieve criticality by itself.

The key to making a working reactor is to slow some of the neutrons to the point where their probability of causing nuclear fission in ²³⁵U increases to a level that permits a sustained chain reaction in the uranium as a whole. This requires the use of a neutron moderator, which absorbs some of the neutrons' kinetic energy, slowing them down to an energy comparable to the thermal energy of the moderator nuclei themselves (leading to the terminology of "thermal neutrons" and "thermal reactors"). During this slowing-down process it is beneficial to physically separate the neutrons from the uranium, since ²³⁸U nuclei have an enormous  affinity for neutrons in this intermediate energy range (a reaction known as "resonance" absorption). This is a fundamental reason for designing reactors with discrete solid fuel separated by moderator, rather than employing a more homogeneous mixture of the two materials.

Water makes an excellent moderator; the hydrogen atoms in the water molecules are very close in mass to a single neutron, and thus have a potential for high energy transfer, similar conceptually to the collision of two billiard balls. However, in addition to being a good moderator, water is also fairly effective at absorbing neutrons. Using water as a moderator will absorb enough neutrons that there will be too few left over to react with the small amount of ²³⁵U in the fuel, again precluding criticality in natural uranium. Instead, light water reactors first enhance the amount of ²³⁵U in the uranium, producing enriched uranium, which generally contains between 3% and 5% ²³⁵U by weight (the waste from this process is known as depleted uranium, consisting primarily of ²³⁸U). In this enriched form there is enough ²³⁵U to react with the water-moderated neutrons to maintain criticality.

One drawback of this approach is the requirement to build an uranium enrichment facility, which are generally expensive to build and operate. They also present a nuclear proliferation concern; the same systems used to enrich the ²³⁵U can also be used to produce much more "pure" weapons-grade material (90% or more ²³⁵U), suitable for producing a nuclear bomb. This is not a trivial exercise, by any means, but feasible enough that enrichment facilities present a nuclear proliferation risk.

An alternative solution to the problem is to use a moderator that does not absorb neutrons as readily as water. In this case potentially all of the neutrons being released can be moderated and used in reactions with the ²³⁵U, in which case there is enough ²³⁵U in natural uranium to sustain criticality. One such moderator is heavy water, or deuterium-oxide (D_²O). Although it reacts dynamically with the neutrons in a similar fashion to light water (albeit with less energy transfer on average, given that heavy hydrogen, or deuterium, is about twice the mass of hydrogen), it already has the extra neutron that light water would normally tend to absorb.

Fuel cycles

Range of possible CANDU fuel cycles: CANDU reactors can accept a variety of fuel types, including the used fuel from light-water reactors. Credit: Atomic Energy of Canada Limited.Compared with light water reactors, a heavy water design is "neutron rich". This makes the CANDU design suitable for "burning" a number of alternative nuclear fuels. To date, the fuel to gain the most attention is mixed oxide fuel (MOX). MOX is a mixture of natural uranium and plutonium, such as that extracted from former nuclear weapons. Currently, there is a worldwide surplus of plutonium due to the various agreements between the United States and the former Soviet Union to dismantle many of their warheads. However, the security of these supplies is a cause for concern. One way to address this security issue is by converting the warhead into fuel and burning the plutonium in a CANDU reactor.

Plutonium can also be extracted from spent nuclear fuel reprocessing. While this consists usually of a mixture of isotopes that is not attractive for use in weapons, it can be used in a MOX formulation reducing the net amount of nuclear waste that has to be disposed of.

Plutonium isn't the only fissile material in spent nuclear fuel that CANDU reactors can utilize. Because the CANDU reactor was designed to work with natural uranium, CANDU fuel can be manufactured from the used (depleted) uranium found in light water reactor (LWR) spent fuel. Typically this "Recovered Uranium" (RU) has a ²³⁵U enrichment of around 0.9%, which makes it unusable to an LWR, but a rich source of fuel to a CANDU (natural uranium has a ²³⁵U abundance of roughly 0.7%). It is estimated that a CANDU reactor can extract a further 30-40% energy from LWR fuel by recycling it in a CANDU reactor.

Recycling of LWR fuel does not necessarily need to involve a reprocessing step. Fuel cycle tests have also included the DUPIC fuel cycle, or direct use of spent PWR fuel in CANDU, where used fuel from a pressurized water reactor is packaged into a CANDU fuel bundle with only physical reprocessing (cut into pieces) but no chemical reprocessing. Again, where light-water reactors require the reactivity associated with enriched fuel, the DUPIC fuel cycle is possible in a CANDU reactor due to the neutron economy which allows for the low reactivity of natural uranium and used enriched fuel.

Several Inert-Matrix Fuels have been proposed for the CANDU design, which have the ability to "burn" plutonium and other actinides from spent nuclear fuel, much more efficiently than in MOX fuel. This is due to the "inert" nature of the fuel, so-called because it lacks uranium and thus does not create plutonium at the same time as it is being consumed.

CANDU reactors can also breed fuel from natural thorium, if uranium is unavailable.

Tritium release

Tritium is a radioactive form of hydrogen that occurs naturally in the environment as a result of cosmic ray interactions in the atmosphere. There is also residual tritium in the environment from the testing of nuclear weapons in the atmosphere in the 1950s and 1960s. Tritium is produced in nuclear reactors by the fissioning of nuclear fuel and by the neutron irradiation of heavy water used in nuclear research and power reactors. Emissions from these nuclear facilities, and from some industrial facilities that use tritium, add to the naturally- and weapons-produced tritium in the environment.

Tritium is considered to be a weak radionuclide because of the low energy of its radioactive emissions (beta particle energy 0 -19 keV). The beta particles do not travel very far in air and no do not penetrate skin; therefore the main biological hazard of tritium is due to its intake into the body (inhalation, ingestion, or absorption).

Tritium is generated in all nuclear power designs; however, CANDU reactors generate more tritium in their coolant and moderator than light-water designs, due to neutron capture in heavy hydrogen. Some of this tritium escapes into containment and is generally recovered; however a small percentage (about 1%) escapes containment and constitutes a routine radioactive emission from CANDU plants (also higher than from an LWR of comparable size). Operation of a CANDU plant therefore includes monitoring of this effluent in the surrounding biota (and publishing the results), in order to ensure that emissions are maintained below regulatory limits.

In some CANDU reactors the tritium concentration in the moderator is periodically reduced by an extraction process, in order to further reduce this risk. Typical reported tritium emissions from CANDU plants in Canada are less than 1% of the national regulatory limit, which is based upon the guidelines of the International Commission on Radiological Protection (ICRP). Some environmental and health NGOs have questioned the government's data, contending the public health risk is greater than those data suggest.

Sources

• CANDU Owners Group, Accessed 31 October 2008.
• Hore-Lacy, Ian (Lead Author); World Nuclear Association (Content Partner); Cutler J. Cleveland (Topic Editor). 2008. Nuclear power reactor. In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth August 22, 2006; Last revised March 3, 2008; Retrieved October 30, 2008].
• Wikipedia Contributors, CANDU reactor, Wikipedia The free Encyclopedia, Accessed 31 October 2008.