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The CANDU reactor is a 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 natural uranium fuel. All current power reactors in Canada are of the CANDU type. Canada markets this power reactor abroad.

Contents 1 Design features 2 Purpose of using heavy water 3 Fuel cycles 4 Chronology 5 Economics 6 Nuclear Nonproliferation 7 Active CANDU reactors 8 New plants 9 References 10 See also 11 External links

[edit] Design features

A CANDU reactor is similar to most "classic" nuclear power plants in design. Fission reactions in the reactor core heat a fluid, in this case heavy water (see below), which 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 (ordinary) water in the less-pressurized secondary cooling loop. This water turns to steam and powers a conventional turbine with a 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 (lake, river, or ocean). More recently-built CANDU plants (such as the Darlington station near Toronto, Ontario) use a discharge-diffuser system that limits the thermal effects in the environment to within natural variations.

The CANDU concept incorporates a number of novel design features. In a traditional light-water reactor (LWR) design, the entire reactor core is a single large pressure vessel containing the moderator/coolant (as one fluid) 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 replaced in a batch procedure. In contrast, the CANDU design encloses each string of fuel bundles in its own cylindrical pressure vessel, or "fuel channel", made of a special zirconium alloy (zircaloy) that is relatively transparent to neutrons. The individual fuel channels are in turn suspended in a low-pressure "calandria" which does not need to be as thick walled as a pressure vessel. Most of the heat is removed by the high-pressure heavy water in the fuel channels and so the additional heavy water moderator in the calandria remains at low pressure and temperature. Because of its size and relatively low temperature, this bulk moderator provides a more "thermalized", and therefore more reactive, neutron spectrum (neutrons will tend to slow down to the thermal energy of the moderating medium). In addition, 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.

The calandria-based design allows individual fuel bundles to be removed without taking the reactor off-line, improving overall duty cycle or "capacity factor". To make this task easier, the calandria is mounted horizontally, allowing a pair of remotely-controlled fuelling machines to visit each end of an individual fuel string, one machine inserting new fuel while the other receives discharged fuel. A lower ²³⁵U density also generally implies that less of the fuel will be "burned" 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, by avoiding the uranium enrichment process, overall utilization of mined uranium in CANDU reactors is significantly less than in light-water reactors (about 30-40% less, using current designs).

A CANDU fuel assembly consists of a bundle of 28 or 37 half-meter long fuel rods (depending on the design): Ceramic fuel pellets in zircaloy tubes plus a support structure, with 12 bundles lying end to end in a fuel channel. The relatively new CANFLEX bundle has 43 fuel elements, with two element 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-pin standard bundle. It has been designed specifically to increase fuel performance by utilizing two different pin diameters.

A number of distributed light-water compartments (Liquid Zone Controllers) are used to control the rate of the nuclear fission chain reaction. The Liquid Zone Controllers work by absorbing excess neutrons and slowing the fission reaction within each of their regions in the reactor core.

CANDU reactors employ two independent, fast-acting safety shutdown systems. Control 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.<ref>Canadian Nuclear FAQ. The Canadian Nuclear FAQ by Dr. Jeremy Whitlock. Retrieved on March 5, 2005. A. CANDU Nuclear Power Technology A.12 How are CANDU reactors controlled? Shutdown System 2 (SDS 2), in most CANDU designs, works by high-pressure injection of a liquid poison (gadolinium nitrate) into the low-pressure moderator.</ref>

For more information on CANDU technical details, see the CANTEACH web site

[edit] Purpose of using heavy water

See nuclear reactor physics and nuclear fission and heavy water for complete details.

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 parasitically 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 "trick" 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 parasitic 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. So, light water reactors require fuel with an enhanced amount of ²³⁵U in the uranium, that is, 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 complication of this approach is the requirement to build uranium enrichment facilities which are generally expensive to build and operate. They also present a nuclear proliferation concern since 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 making a nuclear bomb. This is not a trivial exercise, by any means, but simple enough that enrichment facilities present a significant nuclear proliferation risk. Of course, an operator could opt to buy ready made fuel assemblies from the reactor supplier and have the latter reprocess the spent fuel.

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. 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.

The use of heavy water moderator is the key to the CANDU system, enabling the use of natural uranium as fuel (in the form of ceramic UO₂), which means that it can be operated without expensive uranium enrichment facilities. Additionally, the mechanical arrangement of the CANDU, which places most of the moderator at lower temperatures, is particularly efficient because the resulting thermal neutrons are "more thermal" than in traditional designs, where the moderator normally runs hot. This means that the CANDU is not only able to "burn" natural uranium and other fuels, but tends to do so more effectively as well.

[edit] Fuel cycles

Compared to 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, or 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 US and Soviet agreements to dismantle many of their warheads, and the security of these supplies is a cause for concern. By burning this plutonium in a CANDU it is removed from use, turning it into energy. Plutonium can also be extracted from spent nuclear fuel reprocessing. While this consists usually of a mixture of isotopes that is not usable for weapons, it can be used in a MOX formulation reducing the net amount of nuclear waste that has to be disposed of.

Fuel cycle tests also have 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. Where light-water reactors require the reactivity associated with enriched fuel, the DUPIC fuel cycle is possible in a CANDU due to the neutron economy which allows for the low reactivity of natural uranium and used enriched fuel.

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

[edit] Chronology

The first CANDU-type reactor was Nuclear Power Demonstration (NPD), in Rolphton, Ontario. It was intended as a proof-of-concept design, and was rated for only 22 MWe, a very low power for a commercial power reactor. It produced the first nuclear-generated electricity in Canada, and ran successfully from 1962 to 1987.[1], [2]

The second CANDU was the Douglas Point reactor, a more powerful version rated at roughly 200 MWe and located near Kincardine, Ontario. Douglas Point went into service in 1968, and ran until 1984. Uniquely among CANDU stations, Douglas Point incorporated an oil-filled window which offered a view of the east reactor face, even when the reactor was operating. The Douglas Point type was exported to India, and was the basis for India's fleet of domestically-designed and built 'CANDU-derivatives'. Douglas Point was originally planned to be a two-unit station, but the second unit was cancelled because of the success of the larger 515 MWe units at Pickering.[3], [4]

In parallel to the development of the classic CANDU heavy-water design, experimental CANDU variants were developed. WR-1, located at the AECL's Whiteshell Laboratories in Pinawa, Manitoba, used vertical pressure tubes and organic oil as the primary coolant. The oil used has a higher boiling point than water, allowing the reactor to operate at higher temperatures and lower pressures than a conventional reactor. This reactor operated successfully for many years, and promised a significantly higher thermal efficiency than water-cooled versions. Gentilly-1, near Trois-Rivières, Québec, was also an experimental version of CANDU, using a boiling light-water coolant and vertical pressure tubes, but was not considered successful and was closed after 7 years of fitful operation.

The successes at NPD and Douglas Point led to the decision to construct the first multi-unit station in Pickering, Ontario. Pickering A, consisting of units 1 to 4, went into service in 1971. Pickering B, consisting of units 5 to 8, went into service in 1983, giving a full-station capacity of 4120 MWe. The station is placed very close to the city of Toronto, in order to reduce transmission costs.

Pickering A was placed into voluntary lay-up in 1997, as a part of Ontario Hydro's Nuclear Improvement plan. Units 1 and 4 have since been returned to service, although not without considerable controversy regarding significant cost-overruns, especially on Unit 4. (The refurbishment of Unit 1 was essentially on-time and on-budget, accounting for delays in project startup imposed by the Ontario provincial government.)

In 2005, Ontario Power Generation announced that refurbishment of Units 2 and 3 at Pickering A would not be pursued, contrary to expectations. The reason for this change in plan was economic: the material condition of these units was much poorer than had existed for Units 1 and 4, particularly the condition of the steam generators, and thus the refurbishment costs would be much higher. This rendered a return-to-service of Units 2 and 3 uneconomical. A project to decommission these units is currently in the early stages of planning.

[edit] Economics

The "whole idea" of the CANDU design is that the uranium does not have to be enriched, but simply formed into ceramic uranium-dioxide rods. This saves on the construction of an enrichment plant, and on the costs of processing the fuel.

However, some of this potential savings is offset by the initial, one time cost of the heavy water. The heavy water required must be more than 99.75% pure<ref>Canadian Nuclear FAQ. The Canadian Nuclear FAQ by Dr. Jeremy Whitlock. Retrieved on March 5, 2005. A. CANDU Nuclear Power Technology A.3 What is "heavy water"? "reactor-grade" heavy water, nominally 99.75 wt% deuterium content.</ref> and tonnes of this are required to fill the calandria and the heat transfer system. The next generation reactor (the Advanced CANDU Reactor, also called the "ACR") mitigates this disadvantage by having a smaller moderator size and by using light water as a coolant.

Since heavy water is less efficient at transferring energy from neutrons, the moderator volume (relative to fuel volume) is larger in CANDU reactors compared to light-water designs, making a CANDU reactor core generally larger than a light water reactor of the same power output. In turn, this implies higher building costs for standard features like the containment building. This is offset to some degree by the calandria-based construction, but even considering this, the CANDU tends to have higher capital costs compared to other designs. In fact, CANDU plant costs are dominated by construction costs, the price of fuel representing perhaps 10% of the cost of the power it delivers. This is true in general of nuclear plants, where the plant cost and cost of operations represent about 65% of overall lifetime cost.

When first being offered, CANDUs offered much better "running" time statistics, the capacity factor, than light-water reactors of a similar generation. At the time, light-water (LWR) designs spent, on average, about half of their time in maintenance or refueling outages. However, since the 1980s dramatic improvements in LWR outage management have narrowed the gap between LWR and CANDU, with several LWR units achieving capacity factors in the 90% and higher range, with an overall fleet performance of 89.5% in 2005<ref>US Fleet Performance</ref>. The latest-generation CANDU 6 reactors have demonstrated an 88% capacity factor, but overall fleet performance is dominated by the older Canadian units which generally report capacity factors on the order of 80%<ref>CANDU Lifetime Performance to November 30, 2001</ref>.

Some CANDU plants suffered from cost overruns during construction, primarily due to external factors. For instance, a number of imposed construction delays led to roughly a doubling of the projected cost of the Darlington station near Toronto, Ontario. Technical problems and redesigns added about another billion to the resulting $14.4 billion price<ref>CANDU, Debunking Darlington</ref>. In contrast, the two CANDU 6 reactors more recently installed in China at the Qinshan site were completed on-schedule and on-budget, an achievement attributed to tight control over scope and schedule.

[edit] Nuclear Nonproliferation

In terms of safeguards against nuclear proliferation, CANDU reactors meet a similar level of international certification as other reactor designs.

However, there is a common misconception that the plutonium for India's Operation Smiling Buddha nuclear test was produced in a CANDU design. In fact, the plutonium was produced in the unsafeguarded CIRUS reactor that is based on the NRX design, a Canadian research reactor design.

In addition to its two CANDU reactors, India has some unsafeguarded Pressurised Heavy Water Reactor (PHWR) reactors based on the CANDU design, and two safeguarded light-water reactors supplied by the US. Plutonium has been extracted from the spent fuel from all of these sources in the PREFRE reprocessing facility^[5]. While all of these reactors could in principle be used for plutonium production, India has a locally-designed military plutonium production reactor called Dhruva which is a scaled-up version of the CIRUS designed for plutonium production. It is this reactor which is thought to have produced the plutonium for India's more recent Operation Shakti nuclear tests.<ref>Albright, David, and Mark Hibbs (September 1992). "India's Silent Bomb". Bulletin of the Atomic Scientist 48 (7): pp. 27-31.</ref>

Another concern is tritium production. Although heavy water is relatively immune to neutron capture, a small amount of the deuterium turns into tritium via this process. Tritium, when mixed with deuterium, undergoes nuclear fusion more easily that any other elemental mixture. Small amounts of tritium can be used in both the "trigger" of an A-bomb and the "fusion boost" of a boosted fission weapon. Tritium can also be used in the main fusion process of an H-bomb, but in this application it is typically generated in situ by neutron irradiation of lithium-6.

Tritium is extracted from the CANDU plants in operation in Canada, primarily to improve safety in case of heavy-water leakage. The gas is stockpiled and used in a variety of commercial products, notably "powerless" lighting systems and medical devices. In 1985 what was then Ontario Hydro sparked controversy in Ontario due to its plans to sell tritium to the US. The plan, by law, involved sales to non-military applications only, but some speculated that even this minor penetration of the market would aid the U.S. nuclear weapon program. Demands for this supply in the future appear to outstrip production; in particular the needs of future generations of experimental fusion reactors like ITER will use up a significant amount of any potential stockpile. Currently between 1.5 and 2.1 kg of tritium are recovered yearly at the Darlington separation facility, of which a minor fraction is sold.^[6].

The 1998 Operation Shakti test series in India included one bomb of about 45 kT yield that India has publicly claimed was a hydrogen bomb. An offhand comment in the BARC publication Heavy Water - Properties, Production and Analysis appears to suggest that the tritium was extracted from the heavy water in the CANDU and PHWR reactors in commercial operation. Janes Intelligence Review quotes the Chairman of the Indian Atomic Energy Commission as admitting to the tritium extraction plant, but refusing to comment on its use ^[7]. It is known, however, that India possesses the indigenous technology to create tritium from the neutron-irradiation of lithium-6 in reactors, a process that is several orders of magnitude more efficient than the extraction of tritium from irradiated heavy water.

[edit] Active CANDU reactors

Today there are 29 CANDU reactors in use around the world, and a further 11 "CANDU-Derivatives" in use in India (these reactors were developed from the CANDU design after India detonated a nuclear bomb and Canada stopped nuclear dealings with India). The countries the reactors are located in are:

• Canada - 18 (+2 refurbishing, +6 decommissioned)
• South Korea - 4
• China - 2
• India - 2
• Argentina - 1
• Romania - 1
• Pakistan - 1

[edit] New plants

Turkey has repeatedly shown interest in the CANDU reactor, but so far has chosen not to pursue nuclear energy. In Summer 2006, Turks protested against plans for building nuclear reactors, erroneously believing that all designs are as dangerous as the RBMK when in fact all designs, but that, have fail safe features.

[edit] References

[edit] See also

• Nuclear power in Canada
• List of nuclear reactors

[edit] External links

• The Evolution of CANDU Fuel Cycles and Their Potential Contribution to World Peace
• CANDU Owner's Group
• A history of the CANDU reactor
• CanTeach - Educational and Reference Library on Candu Technology
• Ontario Power Generation
• Bruce Power
• New Brunswick Power
• Hydro Quebec
• Atomic Energy of Canada Limited
• Canadian Nuclear Safety Commission
• Canadian Nuclear Society
• Canadian Nuclear Association
• Canadian Nuclear FAQ
• CBC Digital Archives - Candu: The Canadian Nuclear Reactorde:CANDU-Reaktor

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