CANDU reactor Totally Explained

Everything about The Candu totally explained

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 uranium fuel (originally, natural uranium). All current power reactors in Canada are of the CANDU type. Canada markets this power reactor abroad.

Design features

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 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're 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.
   At the time of its design, Canada lacked the heavy industry to cast and machine the large, heavy steel pressure vessel used in most light water reactors. Instead, the pressure is contained in much smaller tubes, 10 cm diameter, that contain the fuel bundles. These smaller tubes are easier to fabricate than a large pressure vessel. In order to allow the neutrons to flow freely between the bundles, the tubes are made of zircaloy, which is highly transparent to neutrons. The zircaloy tubes are surrounded by a much larger low-pressure tank known as a "calandria", which contains the majority of the moderator.
   Canada also lacked access to uranium enrichment facilities, which were then extremely expensive to construct and operate. The CANDU was therefore designed to use natural uranium as its fuel, like the ZEEP reactor, the first Canadian reactor. 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've good "spectral purity". At the same time, they're 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 can't 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 "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 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.

Purpose of using heavy water

ť 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'll 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 can't 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's 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 natural uranium, again precluding criticality. 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. Operators could reduce these issues by purchasing 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. 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. The advantage is that it already has the extra neutron that light water would normally tend to absorb, reducing the absorption rate. 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 isn't only able to "burn" natural uranium and other fuels, but tends to do so more effectively as well.

Fuel cycles

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's a worldwide surplus of plutonium due to the various United States 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's 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 isn't 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 U-235 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 U-235 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 doesn't 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 doesn't create plutonium at the same time as it's being consumed.
   CANDU reactors can also breed fuel from natural thorium, if uranium is unavailable.

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.
   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.
   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 wasn't 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 4,120 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 wouldn't 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.

Economics

The central functionality behind the CANDU design is heavy water moderation and on-line refuelling, which permits a range of fuel types to be used (including natural uranium, enriched uranium, thorium, and used fuel from Light Water Reactors). Significant fuel cost savings can be realized if the uranium doesn't have to be enriched, but simply formed into ceramic natural uranium-dioxide fuel. This saves not only on the construction of an enrichment plant, but also 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 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 with 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 with 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. Due to the lower fuelling costs compared to light water reactor designs, the levelized lifetime cost on a "per-kWh" basis tends to be comparable to these other designs.
   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. The latest-generation CANDU 6 reactors have demonstrated an 88-90% capacity factor, but overall fleet performance is dominated by the older Canadian units which generally report capacity factors on the order of 80%.
   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 Nuclear Generating Station near Toronto, Ontario. Technical problems and redesigns added about another billion to the resulting $14.4 billion price. 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.

Nuclear nonproliferation

In terms of safeguards against nuclear proliferation, CANDU reactors meet a similar level of international certification as other reactor designs. However, there's 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's based on the NRX design, a Canadian research reactor design. In addition to its two CANDU reactors, India has some unsafeguarded pressurised heavy water reactors (PHWRs) 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. 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.
   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 than 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's 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.
   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. 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's several orders of magnitude more efficient than the extraction of tritium from irradiated heavy water.

Active CANDU reactors

Today there are 30 CANDU reactors in use around the world, and a further 13 "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, +5 decommissioned)
South Korea - 4
China - 2
India - 2 (+13 CANDU-derivatives in use, +3 CANDU-derivatives under construction)
Argentina - 1
Romania - 2 (+3 under construction, currently dormant)
Pakistan - 1

New plants

Interest continues to be expressed in new CANDU construction around the world, and CANDU technology is typically involved in open bidding processes alongside LWR technology.
CANDU reactors have been proposed as the main vehicle for planned supply replacement and growth in Ontario, Canada, a province that currently generates over 50% of its electricity from CANDU reactors. Interest has also been expressed in Western Canada, where CANDU reactors are being considered as heat and electricity sources for the energy-intensive oil sands extraction process, which currently uses natural gas. Energy Alberta Corporation, headquartered in Calgary, announced August 27 2007 that they'd filed application for a license to build a new nuclear plant at Lac Cardinal (30 km west of the town of Peace River (Alberta). The application would see an initial twin AECL ACR-1000 plant go online in 2017, producing 2.2 gigawatt (electric). Romania is in discussions for the completion of its multi-unit nuclear plant at Cernavoda, now consisting of two operating CANDU reactors. Three more partially-completed CANDU reactors exist on the same site, part of a project discontinued at the close of the Nicolae Ceauşescu regime. Turkey has repeatedly shown interest in the CANDU reactor, but so far has chosen not to pursue nuclear energy. In the summer of 2006, Turks protested against plans for building nuclear reactors.

Enhanced CANDU 6

The Enhanced CANDU 6 is an evolutionary upgrade of the standard CANDU 6 design rated to deliver a gross output of 740 MWe per unit.
   The units are designed with a planned operating life of over fifty years, which will be achieved with a mid-life program to replace some of the key components, such as the fuel channels. The plants have a projected average annual capacity factor of more than nintety per cent.
   Enhancement of the CANDU 6 design to achieve higher plant output include: the installation of an Ultrasonic Flow Meter (UFM) to improve the accuracy of feedwater flow measurements, improvements in turbine design itself and change in condenser vacuum system design for operation at lower condenser pressures.
   AECL continues to develop other features to further improve the plant’s performance while maintaining the basic features of the CANDU 6 design, which over time have proven to be extremely reliable with an excellent production record since the early 1980s. The additional enhancements include:

Increased plant margins, both operational and safety

Enhanced environmental protection

Improved Severe Accident Response

Improved Fire Protection System

Improved Plant Security

Modern Computers and Control Systems

Improved Plant Operability and Maintainability

Optimized Plant Maintenance Outages

Reduced Overall Project Schedule

Advanced MACSTOR Design for Spent Fuel Storage

Advanced CANDU Reactor (ACR-1000)

The ACR-1000 represents the continuing evolution of CANDU design to match changing market conditions. ACR-1000 is the next-generation (officially, "Generation III+") CANDU technology from Atomic Energy of Canada Ltd. (AECL), which maintains proven elements of existing CANDU design, while making some significant modifications:

compact fuel-channel design, generating over 50% more power than a conventional CANDU-6 reactor, with approximately the same overall core diameter;

improved thermal efficiency through higher-pressure steam turbines (13 MPa primary pressure; 7 MPa steam outlet pressure, vs. approximately 10 MPa and 5 MPa, respectively, in current designs);

pressurized light-water coolant;

negative coolant void reactivity;

reduction in used fuel production by over 30%;

greater thermal efficiency due to higher operating temperatures and pressures;

reduced use of heavy water (more than half, for the same power output), thus reducing cost and eliminating many material handling concerns;

use of slightly enriched uranium (about 2%) to extend fuel life to three times that of existing *natural uranium fuel (reducing fuel waste volume by two-thirds);

average channel power increased from roughly 6 MW (CANDU 6) to roughly 7 MW;

flatter neutron flux shape, allowing 14% lower peak fuel element ratings;

longer plant operational lifetime (60 years);

longer operating cycles between maintenance outages (3 years);

90% design capacity factor;

pre-stressed concrete containment (1.8 m thick) with steel liner; and

further additions to CANDU's inherent passive safety. At the same time the basic and defining design features of CANDU are all maintained:

modular, horizontal fuel channel core;

heavy water moderation;

simple, economical fuel bundle;

separate, cool, low-pressure moderator with back-up heat sink capability;

two independent, fast-acting shutdown systems;

ability to perform long-term flux-shaping and failed fuel management through on-line refuelling. It is expected that the capital cost of constructing these plants will be reduced by up to 40% compared to current CANDU 6 plants.

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