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86-480: A single prototype of the design, the 100 MWe "Winfrith Reactor", was connected to the grid in 1967 and ran until 1990. A larger commercial design with a 650 MWe power rating was selected in 1974 as the basis for future reactor builds in the UK, but declining electricity use led to this decision being reversed in 1976 and no production models were ever built. SGHWR was among a number of similar designs, which include

172-548: A CANDU plant therefore includes monitoring tritium in the surrounding environment (and publishing the results). In some CANDU reactors the tritium is periodically extracted. Typical emissions from CANDU plants in Canada are less than 1% of the national regulatory limit, which is based on International Commission on Radiological Protection (ICRP) guidelines (for example, the maximal permitted drinking-water concentration for tritium in Canada, 7,000  Bq /L, corresponds to 1/10 of

258-513: A CANDU-6 reactor, began operating in 1983. Following statements from the in-coming Parti Québécois government in September 2012 that Gentilly would close, the operator, Hydro-Québec , decided to cancel a previously announced refurbishment of the plant and announced its shutdown at the end of 2012, citing economic reasons for the decision. The company has started a 50-year decommissioning process estimated to cost $ 1.8 billion. In parallel with

344-758: A fusion reactor and so dozens of kilograms being required for a fleet. Between 1.5 to 2.1 kilograms (3.3 to 4.6 lb) of tritium were recovered annually at the Darlington separation facility by 2003, of which a minor fraction was sold. Consequently, the Canadian Nuclear Laboratories in 2024 announced a decades-long program to refurbish existing CANDU plants and equip them with tritium breeding facilities. The 1998 Operation Shakti test series in India included one bomb of about 45 kilotons of TNT (190 TJ) yield that India has publicly claimed

430-425: A larger moderator-to-fuel ratio and a larger core for the same power output. Although a calandria-based core is cheaper to build, its size increases the cost for standard features like the containment building . Generally nuclear plant construction and operations are ≈65% of overall lifetime cost; for CANDU, costs are dominated by construction even more. Fueling CANDU is cheaper than other reactors, costing only ≈10% of

516-655: A lower concentration of fissile atoms than light-water reactors, allowing it to use some alternative fuels; for example, " recovered uranium " (RU) from used LWR fuel. CANDU was designed for natural uranium with only 0.7%  U, so reprocessed uranium with 0.9%  U is a comparatively rich fuel. This extracts a further 30–40% energy from the uranium. The Qinshan CANDU reactor in China has used recovered uranium. The DUPIC ( Direct Use of spent PWR fuel in CANDU ) process under development can recycle it even without reprocessing. The fuel

602-591: A number of imposed construction delays led to roughly a doubling of the 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, in 2002 two CANDU 6 reactors at Qinshan in China were completed on-schedule and on-budget, an achievement attributed to tight control over scope and schedule. In terms of safeguards against nuclear weapons proliferation , CANDUs meet

688-402: 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 10 centimetres (3.9 in) in diameter, 0.5 metres (20 in) long, weighs about 20 kilograms (44 lb), and is intended to eventually replace the 37-element bundle. To allow the neutrons to flow freely between

774-421: A response from the rest of the reactor, allowing various negative feedbacks to stabilize the reaction. On the other hand, the fission neutrons are thoroughly slowed down before they reach another fuel rod, meaning that it takes neutrons a longer time to get from one part of the reactor to the other. Thus if the chain reaction accelerates in one section of the reactor, the change will propagate itself only slowly to

860-546: A shrinking nuclear market. Given the limited number of new reactors expected in the future, modified versions of the AGR were selected over SGHWR as no further development effort was needed. The Winfrith Reactor reactor remained operational and was used for a wide variety of purposes until it ceased operation in October 1990 after 23 years of operations. As of 2019 it is in the process of being decommissioned by Magnox Ltd on behalf of

946-450: A similar generation. The light-water designs spent, on average, about half the time being refueled or maintained. Since the 1980s, dramatic improvements in LWR outage management have narrowed the gap, with several units achieving capacity factors ~90% and higher, with an overall US fleet performance of 92% in 2010. The latest-generation CANDU 6 reactors have an 88–90% CF, but overall performance

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1032-543: A similar level of international certification as other reactors. The plutonium for India's first nuclear detonation, Operation Smiling Buddha in 1974, was produced in a CIRUS reactor supplied by Canada and partially paid for by the Canadian government using heavy water supplied by the United States. In addition to its two PHWR reactors, India has some safeguarded pressurised heavy-water reactors (PHWRs) based on

1118-469: 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

1204-440: A very large pressure vessel would be needed. The low U density in natural uranium also implies that less of the fuel will be consumed before the fission rate drops too low to sustain criticality, because the ratio of U to fission products + U is lower. In CANDU most of the moderator is at lower temperatures than in other designs, reducing the spread of speeds and the overall speed of the moderator particles. This means that most of

1290-427: A wide range of fuels other than enriched uranium, e.g., natural uranium, reprocessed uranium, thorium , plutonium , and used LWR fuel. Given the expense of enrichment, this can make fuel much cheaper. There is an initial investment into the tonnes of 99.75% pure heavy water to fill the core and heat-transfer system. In the case of the Darlington plant, costs released as part of a freedom of information act request put

1376-430: Is 10–12 days. Tritium is generated in the fuel of all reactors; CANDU reactors generate tritium also in their coolant and moderator, due to neutron capture in heavy hydrogen. Some of this tritium escapes into containment and is generally recovered; a small percentage (about 1%) escapes containment and is considered a routine radioactive emission (also higher than from an LWR of comparable size). Responsible operation of

1462-613: Is also capable of creating tritium more efficiently by irradiation of lithium-6 in reactors. Tritium , H, is a radioactive isotope of hydrogen , with a half-life of 12.3 years. It is produced in small amounts in nature (about 4 kg per year globally) by cosmic ray interactions in the upper atmosphere. Tritium is considered a weak radionuclide because of its low-energy radioactive emissions ( beta particle energy up to 18.6 keV). The beta particles travel 6 mm in air and only penetrate skin up to 6 micrometers. The biological half-life of inhaled, ingested, or absorbed tritium

1548-579: Is completing formerly stalled installations in Romania and Argentina through a partnership with China National Nuclear Corporation . SNC Lavalin, the successor to AECL, is pursuing new CANDU 6 reactor sales in Argentina (Atucha 3), as well as China and Britain. Sales effort for the ACR reactor has ended. In 2017, a consultation with industry led Natural Resources Canada to establish a "SMR Roadmap" targeting

1634-473: Is dominated by the older Canadian units with CFs on the order of 80%. Refurbished units had historically demonstrated poor performance, on the order of 65%. This has since improved with the return of Bruce units A1 and A2 to operation, which have post-refurbishment (2013+) capacity factors of 90.78% and 90.38%, respectively. Some CANDU plants suffered from cost overruns during construction, often from external factors such as government action. For instance,

1720-400: Is much less expensive as well. A further unique feature of heavy-water moderation is the greater stability of the chain reaction . This is due to the relatively low binding energy of the deuterium nucleus (2.2 MeV), leading to some energetic neutrons and especially gamma rays breaking the deuterium nuclei apart to produce extra neutrons. Both gammas produced directly by fission and by

1806-406: Is normally kept relatively cool. Heat generated by fission products would initially be at about 7% of full reactor power, which requires significant cooling. The CANDU designs have several emergency cooling systems, as well as having limited self-pumping capability through thermal means (the steam generator is well above the reactor). Even in the event of a catastrophic accident and core meltdown ,

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1892-524: Is not attractive for weapons, but can be used as fuel (instead of being simply nuclear waste), while consuming weapons-grade plutonium eliminates a proliferation hazard. If the aim is explicitly to utilize plutonium or other actinides from spent fuel, then special inert-matrix fuels are proposed to do this more efficiently than MOX. Since they contain no uranium, these fuels do not breed any extra plutonium. The neutron economy of heavy-water moderation and precise control of on-line refueling allow CANDU to use

1978-552: Is one of the many reasons for the cooler mass of moderator in the calandria, as even a serious steam incident in the core would not have a major impact on the overall moderation cycle. Only if the moderator itself starts to boil would there be any significant effect, and the large thermal mass ensures that this will occur slowly. The deliberately "sluggish" response of the fission process in CANDU allows controllers more time to diagnose and deal with problems. The fuel channels can only maintain criticality if they are mechanically sound. If

2064-519: Is sintered in air (oxidized), then in hydrogen (reduced) to break it into a powder, which is then formed into CANDU fuel pellets. CANDU reactors can also breed fuel from the more abundant thorium . This is being investigated by India to take advantage of its natural thorium reserves. Even better than LWRs , CANDU can utilize a mix of uranium and plutonium oxides ( MOX fuel ), the plutonium either from dismantled nuclear weapons or reprocessed reactor fuel. The mix of isotopes in reprocessed plutonium

2150-613: 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 are capable of using cooling towers instead. Where

2236-495: Is thought to have produced the plutonium for India's more recent (1998) Operation Shakti nuclear tests. Although heavy water is relatively immune to neutron capture, a small amount of the deuterium turns into tritium in this way. This tritium is extracted from some CANDU plants in Canada, mainly 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

2322-711: 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. Candu Energy offers support services for existing sites and

2408-514: The American Nuclear Society . The design boils ordinary water like a boiling water reactor (BWR) but uses heavy water as a moderator as in a CANDU reactor. The electrical output was 165 MW and the thermal output was 557 MW. The plant is located on a site that covers 267,694 m (66 acres); buildings occupy 7,762 m (1.9 acres), and it has 46,488 m of floor space. It employed 256 workers. During dismantling operations it

2494-695: The CANDU -derived Gentilly Nuclear Generating Station in Quebec , the Fugen Advanced Test Reactor in Japan , and the never-commissioned CIRENE reactor in Italy . These designs differ with the baseline CANDU design, which uses heavy water as the coolant as well as the moderator. SGHWR was a departure from previous UK designs, which had used graphite as the moderator and carbon dioxide gas as

2580-630: The Nuclear Decommissioning Authority . Between 2022 and 2024, 1068 drums of radioactive waste were transported by train to the Low Level Waste Repository . The material was once intermediate-level waste but had decayed down to low-level waste while being stored at Winfrith. SGHWR is similar to the Canadian CANDU reactor designs in that it uses a low-pressure reactor vessel containing

2666-414: The overnight cost of the plant (four reactors totalling 3,512 MW e net capacity) at $ 5.117 billion CAD (about US$ 4.2 billion at early-1990s exchange rates). Total capital costs including interest were $ 14.319 billion CAD (about US$ 11.9 billion) with the heavy water accounting for $ 1.528 billion, or 11%, of this. Since heavy water is less efficient than light water at slowing neutrons, CANDU needs

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2752-467: The 600 MW e 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

2838-420: 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 U atoms in the fuel to undergo fission as well. This process is much more effective when

2924-409: 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

3010-554: 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 reactors; India mainly relies on an Indian designed and built military reactor called Dhruva . The design is believed to be derived from the CIRUS reactor, with the Dhruva being scaled-up for more efficient plutonium production. It is this reactor which

3096-737: The CANDU ;6 design, which first went into operation in the early 1980s. CANDU 6 was essentially a version of the Pickering power plant that was redesigned to be able to be built in single-reactor units. CANDU 6 was used in several installations outside Ontario, including the Gentilly-2 in Quebec, and Point Lepreau Nuclear Generating Station in New Brunswick. CANDU 6 forms the majority of foreign CANDU systems, including

3182-460: The ICRP's dose limit for members of the public). Tritium emissions from other CANDU plants are similarly low. In general, there is significant public controversy about radioactive emissions from nuclear power plants, and for CANDU plants one of the main concerns is tritium. In 2007 Greenpeace published a critique of tritium emissions from Canadian nuclear power plants by Ian Fairlie . This report

3268-693: The SGHWR was built at Winfrith in the 1960s and was connected to the grid in 1967. It is often known simply as the "Winfrith Reactor". The other designs produced similar sub-scale prototypes of the High Temperature Reactor also at Winfrith, the Magnox-derived AGR at Windscale , and the Prototype Fast Reactor at Dounreay . This contest ultimately selected the AGR design, and several AGRs began construction in

3354-417: The bundles, the tubes and bundles are made of neutron-transparent zircaloy ( zirconium + 2.5% wt niobium ). 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,

3440-505: The classic CANDU design, experimental variants were being 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. WR-1's outlet temperature

3526-503: The controllers to adjust reactivity across the fuel mass, as different portions would normally burn at different rates depending on their position. The adjuster rods can also be used to slow or stop criticality. Because these rods are inserted into the low-pressure calandria, not the high-pressure fuel tubes, they would not be "ejected" by steam, a design issue for many pressurized-water reactors. There are two independent, fast-acting safety shutdown systems as well. Shutoff rods are held above

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3612-448: The coolant. The original Magnox was designed to run on natural uranium but the subsequent Advanced Gas-cooled Reactor (AGR) abandoned this for a variety of reasons, using low-enriched uranium instead. Although Magnox was technically successful it was expensive. For future orders, several alternative reactor designs concepts were studied during the early 1960s. As part of this program, a 100  megawatt electrical (MWe) prototype of

3698-414: The decay of fission fragments have enough energy, and the half-lives of the fission fragments range from seconds to hours or even years. The slow response of these gamma-generated neutrons delays the response of the reactor and gives the operators extra time in case of an emergency. Since gamma rays travel for meters through water, an increased rate of chain reaction in one part of the reactor will produce

3784-410: 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 with units 5 to 8 came online in 1983, giving a full-station capacity of 4,120 MW e . The station is very close to the city of Toronto , in order to reduce transmission costs. A series of improvements to the basic Pickering design led to

3870-429: The designs exported to Argentina, Romania, China and South Korea. Only India operates a CANDU system that is not based on the CANDU 6 design. The economics of nuclear power plants generally scale well with size. This improvement at larger sizes is offset by the sudden appearance of large quantities of power on the grid, which leads to a lowering of electricity prices through supply and demand effects. Predictions in

3956-557: The development of small modular reactors (SMRs). In response, SNC-Lavalin developed a 300 MW e 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 . The basic operation of

4042-471: 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 MW e and located near Kincardine , Ontario. It went into service in 1968 and ran until 1984. Uniquely among CANDU stations, Douglas Point had an oil-filled window with a view of the east reactor face, even when

4128-429: The fuel is not critical in light water. This means that cooling the core with water from nearby sources will not add to the reactivity of the fuel mass. Normally the rate of fission is controlled by light-water compartments called liquid zone controllers, which absorb excess neutrons, and by adjuster rods, which can be raised or lowered in the core to control the neutron flux. These are used for normal operation, allowing

4214-430: The fuel is supplied and reprocessed by an internationally approved supplier. The main advantage of heavy water moderator over light water is the reduced absorption of the neutrons that sustain the chain reaction, allowing a lower concentration of fissile atoms (to the point of using unenriched natural uranium fuel). Deuterium ("heavy hydrogen") already has the extra neutron that light hydrogen would absorb, reducing

4300-475: The fuel material is usually U, 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 U slows

4386-454: The fuel must be enriched , increasing the amount of U to a usable level. In light-water reactors , the fuel is typically enriched to between 2% and 5% U (the leftover fraction with less U 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 U much further, up to weapons-grade material (90% or more U). This can be remedied if

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4472-417: 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 to shut down the core and to open

4558-445: The inter-fuel pellet fission reaction. This will not stop heat production from fission product decay, which would continue to supply a considerable heat output. If this process further weakens the fuel bundles, the pressure tube they are in will eventually bend far enough to touch the calandria tube, allowing heat to be efficiently transferred into the moderator tank. The moderator vessel has a considerable thermal capability on its own and

4644-490: 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. There have been two major types of CANDU reactors, the original design of around 500  MW e that was intended to be used in multi-reactor installations in large plants, and the rationalized CANDU 6 in

4730-461: The late 1960s suggested that growth in electricity demand would overwhelm these downward pricing pressures, leading most designers to introduce plants in the 1000 MW e range. Fugen Nuclear Power Plant Fugen ふげん ( Fugen ) was a prototype Japanese nuclear test reactor. Fugen was a domestic Japanese design for a demonstration Advanced Thermal Reactor . It was a heavy water moderated, boiling light water cooled reactor. The reactor

4816-406: The late 1960s. These quickly ran into problems, and by the early 1970s the design was considered a failure. In 1974, a larger version of the SGHWR with a design power of 650 MWe was selected for future power plant builds. In 1976 this decision was reversed due to the combination of a predicted sharp drop in electricity demand, higher than expected costs, and the lack of obvious export potential in

4902-451: 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 U 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

4988-497: The moderator and high-pressure piping for the coolant. This both reduces the total amount of expensive heavy water required, as well as reducing the complexity of the reactor vessel, which in turn reduces construction costs and complexity. It differs in that it uses ordinary "light" water as a coolant, whereas CANDU uses heavy water here as well. Light water reduces the neutron economy to the point where natural uranium can no longer be used as fuel. The ability to run on natural uranium

5074-510: The moderator and light water for the coolant was explored by a number of designs during this period. The Gentilly-1 Nuclear Generating Station in Quebec used the same solution, but this was not successful and shut down after a short lifetime. The Fugen Advanced Test Reactor in Japan suffered a similar fate. The Italian CIRENE design, hosted at Latina Nuclear Power Plant , was built but never commissioned. The last attempt to use this basic design

5160-454: 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

5246-407: The neutron energies are much 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

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5332-416: 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 don't readily get 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,

5418-411: The neutrons will end up at a lower energy and be more likely to cause fission, so CANDU not only "burns" natural uranium, but it does so more effectively as well. Overall, CANDU reactors use 30–40% less mined uranium than light-water reactors per unit of electricity produced. This is a major advantage of the heavy-water design; it not only requires less fuel, but as the fuel does not have to be enriched, it

5504-475: 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

5590-422: The post– World War II era to explore nuclear energy while lacking access to enrichment facilities. War-era enrichment systems were extremely expensive to build and operate, whereas the heavy water solution allowed the use of natural uranium in the experimental ZEEP reactor. A much less expensive enrichment system was developed, but the United States classified work on the cheaper gas centrifuge process. The CANDU

5676-404: The pressure vessel. In CANDU, only the single tube being refuelled needs to be depressurized. 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

5762-417: The province, reaching ~880 MW e in the units installed at the Darlington Nuclear Generating Station . An effort to rationalize 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

5848-427: 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 U in natural uranium, preventing criticality. In order to allow criticality,

5934-412: The reactor by electromagnets and drop under gravity into the core to quickly end criticality. This system works even in the event of a complete power failure, as the electromagnets only hold the rods out of the reactor when power is available. A secondary system injects a high-pressure gadolinium nitrate neutron absorber solution into the calandria. A heavy-water design can sustain a chain reaction with

6020-534: The reactor was operating. 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 MW e units at Pickering . Gentilly-1 , in Bécancour, Quebec , near Trois-Rivières , Quebec, was also an experimental version of CANDU, using a boiling light-water coolant and vertical pressure tubes, but was not considered successful and closed after seven years of fitful operation. Gentilly-2,

6106-550: 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

6192-421: The rest of the core, giving time to respond in an emergency. The independence of the neutrons' energies from the nuclear fuel used is what allows such fuel flexibility in a CANDU reactor, since every fuel bundle will experience the same environment and affect its neighbors in the same way, whether the fissile material is uranium-235, uranium-233 or plutonium . Canada developed the heavy-water-moderated design in

6278-496: 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 purely as a moderator. This larger vessel, known as a calandria, is not pressurized and remains at much lower temperatures, making it much easier to fabricate. In order to prevent

6364-410: The temperature of the fuel bundles increases to the point where they are mechanically unstable, their horizontal layout means that they will bend under gravity, shifting the layout of the bundles and reducing the efficiency of the reactions. Because the original fuel arrangement is optimal for a chain reaction, and the natural uranium fuel has little excess reactivity, any significant deformation will stop

6450-418: The tendency to capture neutrons. Deuterium has twice the mass of a single neutron (vs light hydrogen, which has about the same mass); the mismatch means that more collisions are needed to moderate the neutrons, requiring a larger thickness of moderator between the fuel rods. This increases the size of the reactor core and the leakage of neutrons. It is also the practical reason for the calandria design, otherwise,

6536-408: The total, so the overall price per kWh electricity is comparable. The next-generation Advanced CANDU reactor (ACR) mitigates these disadvantages by having light-water coolant and using a more compact core with less moderator. When first introduced, CANDUs offered much better capacity factor (ratio of power generated to what would be generated by running at full power, 100% of the time) than LWRs of

6622-618: 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. India

6708-419: 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. 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

6794-490: Was about 490 °C compared to the CANDU 6's nominal 310 °C; the higher temperature and thus thermodynamic efficiency offsets to some degree the fact that oils have about half the heat capacity of water. The higher temperatures also result in more efficient conversion to steam, and ultimately, electricity. WR-1 operated successfully for many years and promised a significantly higher efficiency than water-cooled versions. The successes at NPD and Douglas Point led to

6880-416: Was considered a major benefit in the 1960s as it appeared the demand for enrichment would outstrip the supply. By the 1970s it was clear that fuel supplies were not going to be a problem, and the use of unenriched fuel was no longer a major design goal. Using slight enrichment leads to higher burnup and more economical fuel cycles, offsetting the now-low costs of enrichment. The idea of using heavy water for

6966-460: Was criticized by Richard Osborne. The CANDU development effort has gone through four major stages over time. The first systems were experimental and prototype machines of limited power. These were replaced by a second generation of machines of 500 to 600 MW e (the CANDU 6), a series of larger machines of 900 MW e , and finally developing into the CANDU 9 and ACR-1000 effort. The first heavy-water-moderated design in Canada

7052-469: Was started in 1979 and shut down in 2003. As of 2018, it is undergoing decommissioning . It is located in Myōjin-chō, in the city of Tsuruga, Fukui . The name "Fugen" is derived from Fugen Bosatsu ( Samantabhadra ), a Buddhist deity. The reactor was the first in the world to use a full MOX fuel core. It had 772 assemblies, the most in the world. It has received the title of a historic landmark from

7138-666: Was the ZEEP , which started operation just after the end of World War II . ZEEP was joined by several other experimental machines, including the NRX in 1947 and NRU in 1957. These efforts led to the first CANDU-type reactor, the Nuclear Power Demonstration (NPD), in Rolphton, Ontario. It was intended as a proof-of-concept and rated for only 22  MW e , a very low power for a commercial power reactor. NPD produced

7224-430: Was the modern Advanced CANDU Reactor of the early 2000s, but development ended without an example being built. CANDU 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

7310-553: Was then Ontario Hydro sparked controversy in Ontario due to its plans to sell tritium to the United States. The plan, by law, involved sales to non-military applications only, but some speculated that the exports could have freed American tritium for the United States nuclear weapons program. Future demands appear to outstrip production, in particular the demands of future generations of experimental fusion reactors like ITER , with up to 10kg of tritium being required in order to start up

7396-462: Was therefore designed to use natural uranium. The CANDU includes a number of active and passive safety features in its design. Some of these are a side effect of the physical layout of the system. CANDU designs have a positive void coefficient , as well as a small power coefficient, normally considered bad in reactor design. This implies that steam generated in the coolant will increase the reaction rate, which in turn would generate more steam. This

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