A pressurized heavy-water reactor ( PHWR ) is a nuclear reactor that uses heavy water ( deuterium oxide D 2 O) as its coolant and neutron moderator . PHWRs frequently use natural uranium as fuel, but sometimes also use very low enriched uranium . The heavy water coolant is kept under pressure to avoid boiling, allowing it to reach higher temperature (mostly) without forming steam bubbles, exactly as for a pressurized water reactor (PWR). While heavy water is very expensive to isolate from ordinary water (often referred to as light water in contrast to heavy water ), its low absorption of neutrons greatly increases the neutron economy of the reactor, avoiding the need for enriched fuel . The high cost of the heavy water is offset by the lowered cost of using natural uranium and/or alternative fuel cycles . As of the beginning of 2001, 31 PHWRs were in operation, having a total capacity of 16.5 GW(e), representing roughly 7.76% by number and 4.7% by generating capacity of all current operating reactors. CANDU and IPHWR are the most common type of reactors in the PHWR family.
98-612: The CANDU ( CANada Deuterium Uranium ) is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide ( heavy water ) moderator and its use of (originally, natural ) uranium fuel. CANDU reactors were first developed in the late 1950s and 1960s by a partnership between Atomic Energy of Canada Limited (AECL), the Hydro-Electric Power Commission of Ontario , Canadian General Electric , and other companies. There have been two major types of CANDU reactors,
196-433: A greater risk of nuclear proliferation versus comparable light-water reactors due to the low neutron absorption properties of heavy water, discovered in 1937 by Hans von Halban and Otto Frisch . Occasionally, when an atom of U is exposed to neutron radiation , its nucleus will capture a neutron , changing it to U . The U then rapidly undergoes two β decays — both emitting an electron and an antineutrino ,
294-439: A light-water moderator depends on the exact geometry and other design parameters of the reactor. One complication of this approach is the need for uranium enrichment facilities, which are generally expensive to build and operate. They also present a nuclear proliferation concern; the same systems used to enrich the U can also be used to produce much more "pure" weapons-grade material (90% or more U), suitable for producing
392-403: A lower neutron capture cross section than protium , this value isn't zero and thus part of the heavy water moderator will inevitably be converted to tritiated water . While tritium , a radioactive isotope of hydrogen, is also produced as a fission product in minute quantities in other reactors, tritium can more easily escape to the environment if it is also present in the cooling water, which
490-412: A nuclear weapon . This is not a trivial exercise by any means, but feasible enough that enrichment facilities present a significant 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 U, in which case there
588-422: 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 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
686-542: 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
784-507: 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
882-436: A calandria, is not pressurized and remains at much lower temperatures, making it much easier to fabricate. In order to prevent the heat from the pressure tubes from leaking into the surrounding moderator, each pressure tube is enclosed in a calandria tube. Carbon dioxide gas in the gap between the two tubes acts as an insulator. The moderator tank also acts as a large heat sink that provides an additional safety feature. In
980-483: A chain reaction with the small isolated U nuclei in the fuel, thus precluding criticality in natural uranium. Because of this, a light-water reactor will require that the U isotope be concentrated in its uranium fuel, as enriched uranium , generally between 3% and 5% U by weight (the by-product from this process enrichment process is known as depleted uranium , and so consisting mainly of U, chemically pure). The degree of enrichment needed to achieve criticality with
1078-423: A concentration of 90% uranium-235, and light water reactors require a concentration of roughly 3% uranium-235. Unenriched natural uranium is appropriate fuel for a heavy-water reactor , like a CANDU reactor . On rare occasions, earlier in geologic history when uranium-235 was more abundant, uranium ore was found to have naturally engaged in fission, forming natural nuclear fission reactors . Uranium-235 decays at
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#17330860265831176-408: A constant rate of fission over time. In order to keep the fission rate constant, the neutrons released by fission must produce an equal number of fissions in other fuel atoms. This balance is referred to as " criticality ." Neutrons released by nuclear fission are fairly energetic and don't readily get absorbed (or "captured") by the surrounding fissile material . In order to improve the capture rate,
1274-451: A conventional pressurized water reactor , refuelling the system requires to shut down the core and to open 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
1372-585: 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 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
1470-531: A faster rate ( half-life of 700 million years) compared to uranium-238, which decays extremely slowly (half-life of 4.5 billion years). Therefore, a billion years ago, there was more than double the uranium-235 compared to now. During the Manhattan Project , the name Tuballoy was used to refer to natural uranium in the refined condition; this term is still in occasional use. Uranium was also codenamed "X-Metal" during World War II. Similarly, enriched uranium
1568-733: 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
1666-425: 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
1764-596: 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
1862-690: A moderator roughly equals the temperature of the moderator) than in traditional designs, where the moderator normally is much hotter. The neutron cross section for fission is higher in U the lower the neutron temperature is, and thus lower temperatures in the moderator make successful interaction between neutrons and fissile material more likely. These features mean that a PHWR can use natural uranium and other fuels, and does so more efficiently than light water reactors (LWRs). CANDU type PHWRs are claimed to be able to handle fuels including reprocessed uranium or even spent nuclear fuel from "conventional" light water reactors as well as MOX fuel and there
1960-581: A non-CANDU 6 design was sold to India. The multi-unit design was used only in Ontario , Canada, and grew in size and power as more units were installed in the province, reaching ~880 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
2058-574: 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
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#17330860265832156-484: A pressure tube. One machine pushes in the new fuel, whereby the depleted fuel is pushed out and collected at the other end. A significant operational advantage of online refuelling is that a failed or leaking fuel bundle can be removed from the core once it has been located, thus reducing the radiation levels in the primary cooling loop. Each fuel bundle is a cylinder assembled from thin tubes filled with ceramic pellets of uranium oxide fuel (fuel elements). In older designs,
2254-445: 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
2352-529: 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
2450-419: A steam turbine with an electric generator attached to it (for a typical Rankine thermodynamic cycle ). The exhaust steam from the turbines is then cooled, condensed and returned as feedwater to the steam generator. The final cooling often uses cooling water from a nearby source, such as a lake, river, or ocean. Newer CANDU plants, such as the Darlington Nuclear Generating Station near Toronto , Ontario, use
2548-508: A suitable moderator due to overlooking impurities and thus made unsuccessful attempts using heavy water (which they correctly identified as an excellent moderator). The Soviet nuclear program likewise used graphite as a moderator and ultimately developed the graphite moderated RBMK as a reactor capable of producing both large amounts of electric power and weapons grade plutonium without the need for heavy water or - at least according to initial design specifications - uranium enrichment . Pu
2646-426: 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
2744-411: A wide variety of other materials such as plutonium and thorium . This was a major goal of the CANDU design; by operating on natural uranium the cost of enrichment is removed. This also presents an advantage in nuclear proliferation terms, as there is no need for enrichment facilities, which might also be used for weapons. In conventional light-water reactor (LWR) designs, the entire fissile core
2842-434: Is enough U in natural uranium to sustain criticality. One such moderator is heavy water , or deuterium-oxide. Although it reacts dynamically with the neutrons in a fashion similar 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 as
2940-429: 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
3038-420: Is a fissile material suitable for use in nuclear weapons . As a result, if the fuel of a heavy-water reactor is changed frequently, significant amounts of weapons-grade plutonium can be chemically extracted from the irradiated natural uranium fuel by nuclear reprocessing . In addition, the use of heavy water as a moderator results in the production of small amounts of tritium when the deuterium nuclei in
CANDU reactor - Misplaced Pages Continue
3136-427: 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 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
3234-611: 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
3332-454: 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 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
3430-469: 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,
3528-458: Is extracted for power. Most commercial reactor designs use normal water as the moderator. Water absorbs some of the neutrons, enough that it is not possible to keep the reaction going in natural uranium. CANDU replaces this "light" water with heavy water . Heavy water's extra neutron decreases its ability to absorb excess neutrons, resulting in a better neutron economy . This allows CANDU to run on unenriched natural uranium , or uranium mixed with
3626-438: Is intended to eventually replace the 37-element bundle. To allow the neutrons to flow freely between the bundles, the tubes and bundles are made of neutron-transparent zircaloy ( zirconium + 2.5% wt niobium ). 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
3724-475: 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 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
3822-449: Is normally accomplished by use of an on-power refuelling system. The increased rate of fuel movement through the reactor also results in higher volumes of spent fuel than in LWRs employing enriched uranium. Since unenriched uranium fuel accumulates a lower density of fission products than enriched uranium fuel, however, it generates less heat, allowing more compact storage. While deuterium has
3920-405: 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 ,
4018-524: 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
CANDU reactor - Misplaced Pages Continue
4116-551: 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
4214-489: Is ongoing research into the ability of CANDU type reactors to operate exclusively on such fuels in a commercial setting. (More on that in the article on the CANDU reactor itself) Pressurised heavy-water reactors do have some drawbacks. Heavy water generally costs hundreds of dollars per kilogram, though this is a trade-off against reduced fuel costs. The reduced energy content of natural uranium as compared to enriched uranium necessitates more frequent replacement of fuel; this
4312-407: Is only easily fissioned by the few energetic neutrons above ≈1.5–2 MeV . Since most of 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
4410-400: Is placed in a large pressure vessel . The amount of heat that can be removed by a unit of a coolant is a function of the temperature; by pressurizing the core, the water can be heated to much greater temperatures before boiling , thereby removing more heat and allowing the core to be smaller and more efficient. Building a pressure vessel of the required size is a significant challenge, and at
4508-518: 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
4606-400: Is still looking at marketing a 300 MW SMR in part due to projected demand due to climate change mitigation . The basic operation of the CANDU design is similar to other nuclear reactors. Fission reactions in the reactor core heat pressurized water in a primary cooling loop . A heat exchanger , also known as a steam generator , transfers the heat to a secondary cooling loop , which powers
4704-521: Is the case in those PHWRs which use heavy water both as moderator and as coolant. Some CANDU reactors separate out the tritium from their heavy water inventory at regular intervals and sell it at a profit, however. While with typical CANDU derived fuel bundles, the reactor design has a slightly positive Void coefficient of reactivity, the Argentina designed CARA fuel bundles used in Atucha I , are capable of
4802-494: 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
4900-582: 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 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
4998-461: The ordinary hydrogen or protium atoms in the water molecules are very close in mass to a single neutron, and so their collisions result in a very efficient transfer of momentum, similar conceptually to the collision of two billiard balls. However, as well as being a good moderator, ordinary water is also quite effective at absorbing neutrons. And so using ordinary water as a moderator will easily absorb so many neutrons that too few are left to sustain
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#17330860265835096-412: 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
5194-787: The plutonium for Operation Smiling Buddha , its first nuclear weapon test, by extraction from the spent fuel of a heavy-water research reactor known as the CIRUS reactor . Natural uranium Natural uranium ( NU or U nat ) is uranium with the same isotopic ratio as found in nature. It contains 0.711% uranium-235 , 99.284% uranium-238 , and a trace of uranium-234 by weight (0.0055%). Approximately 2.2% of its radioactivity comes from uranium-235, 48.6% from uranium-238, and 49.2% from uranium-234. Natural uranium can be used to fuel both low- and high-power nuclear reactors . Historically, graphite-moderated reactors and heavy water -moderated reactors have been fueled with natural uranium in
5292-592: The ACR reactor has ended. In 2017, a consultation with industry led Natural Resources Canada to establish a "SMR Roadmap" targeting the development of small modular reactors (SMRs). In response, SNC-Lavalin developed a 300 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
5390-542: 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
5488-709: 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
5586-452: 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
5684-399: The bundle had 28 or 37 half-meter-long fuel elements with 12–13 such assemblies lying end-to-end in a pressure tube. The newer CANFLEX bundle has 43 fuel elements, with two element sizes (so the power rating can be increased without melting the hottest fuel elements). It is about 10 centimetres (3.9 in) in diameter, 0.5 metres (20 in) long, weighs about 20 kilograms (44 lb), and
5782-497: 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
5880-503: 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
5978-410: 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
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#17330860265836076-429: 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
6174-491: The deuterium nuclei apart to produce extra neutrons. Both gammas produced directly by fission and by 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
6272-462: 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
6370-506: The first one transmuting the U into Np , and the second one transmuting the Np into Pu . Although this process takes place with natural uranium using other moderators such as ultra-pure graphite or beryllium, heavy water is by far the best. The Manhattan Project ultimately used graphite moderated reactors to produce plutonium, while the German wartime nuclear project wrongfully dismissed graphite as
6468-448: The fission process. U, on the other hand, can support a self-sustained chain reaction, but due to the low natural abundance of U, natural uranium cannot achieve criticality by itself. The trick to achieving criticality using only natural or low enriched uranium, for which there is no "bare" critical mass , is to slow down the emitted neutrons (without absorbing them) to the point where enough of them may cause further nuclear fission in
6566-428: 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
6664-400: 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, 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
6762-407: The fuel to undergo fission as well. This process is much more effective when 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
6860-403: The heavy water absorb neutrons, a very inefficient reaction. Tritium is essential for the production of boosted fission weapons , which in turn enable the easier production of thermonuclear weapons , including neutron bombs . This process is currently expected to provide (at least partially) tritium for ITER . The proliferation risk of heavy-water reactors was demonstrated when India produced
6958-445: 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
7056-500: 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. Pressurized heavy-water reactor The key to maintaining a nuclear chain reaction within a nuclear reactor is to use, on average, exactly one of the neutrons released from each nuclear fission event to stimulate another nuclear fission event (in another fissionable nucleus). With careful design of
7154-530: The moderator is the key to the PHWR (pressurized heavy water reactor) system, enabling the use of natural uranium as the fuel (in the form of ceramic UO 2 ), which means that it can be operated without expensive uranium enrichment facilities. The mechanical arrangement of the PHWR, which places most of the moderator at lower temperatures, is particularly efficient because the resulting thermal neutrons have lower energies ( neutron temperature after successive passes through
7252-439: The neutron energy moderation process from the uranium fuel itself, as U has a high probability of absorbing neutrons with intermediate kinetic energy levels, a reaction known as "resonance" absorption. This is a fundamental reason for designing reactors with separate solid fuel segments, surrounded by the moderator, rather than any geometry that would give a homogeneous mix of fuel and moderator. Water makes an excellent moderator;
7350-447: The neutron energy must be reduced, or "moderated", to be as low as possible. In practice, the lower energy limit is the energy where the neutrons are in thermal equilibrium with the moderator. When neutrons approach this lower energy limit, they are referred to as " thermal neutrons ." During moderation it helps to separate the neutrons and uranium, since U has a large affinity for intermediate-energy neutrons ("resonance" absorption), but
7448-432: The number available to react with the small amount of U in natural uranium, preventing criticality. In order to allow criticality, 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
7546-599: The original CANDU designs were dwindling due to the introduction of newer designs from other companies. AECL responded by cancelling CANDU 9 development and moving to the Advanced CANDU reactor (ACR) design. ACR failed to find any buyers; its last potential sale was for an expansion at Darlington, but this was cancelled in 2009. In October 2011, the Canadian Federal Government licensed the CANDU design to Candu Energy (a wholly owned subsidiary of SNC-Lavalin, now
7644-464: 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 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
7742-418: The point of using unenriched natural uranium fuel). Deuterium ("heavy hydrogen") already has the extra neutron that light hydrogen would absorb, reducing 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
7840-422: 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
7938-483: The preferred negative coefficient. While prior to India's development of nuclear weapons (see below), the ability to use natural uranium (and thus forego the need for uranium enrichment which is a dual use technology) was seen as hindering nuclear proliferation, this opinion has changed drastically in light of the ability of several countries to build atomic bombs out of plutonium, which can easily be produced in heavy water reactors. Heavy-water reactors may thus pose
8036-408: The pure metal (U) or uranium dioxide (UO 2 ) ceramic forms. However, experimental fuelings with uranium trioxide (UO 3 ) and triuranium octaoxide (U 3 O 8 ) have shown promise. The 0.72% uranium-235 is not sufficient to produce a self-sustaining critical chain reaction in light water reactors or nuclear weapons ; these applications must use enriched uranium . Nuclear weapons take
8134-411: 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
8232-526: 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,
8330-446: The reactor will produce 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
8428-538: The reactor's geometry, and careful control of the substances present so as to influence the reactivity , a self-sustaining chain reaction or " criticality " can be achieved and maintained. Natural uranium consists of a mixture of various isotopes , primarily U and a much smaller amount (about 0.72% by weight) of U . U can only be fissioned by neutrons that are relatively energetic, about 1 MeV or above. No amount of U can be made "critical" since it will tend to parasitically absorb more neutrons than it releases by
8526-421: 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
8624-407: The small amount of U which is available. ( U which is the bulk of natural uranium is also fissionable with fast neutrons.) This requires the use of a neutron moderator , which absorbs virtually all of the neutrons' kinetic energy , slowing them down to the point that they reach thermal equilibrium with surrounding material. It has been found beneficial to the neutron economy to physically separate
8722-422: The technology required to produce pressure vessels of the size required for commercial-scale heavy water moderated power reactors was thought to be very unlikely. In CANDU the fuel bundles of about 10 cm diameter are composed of many smaller metal tubes. The bundles are contained in pressure tubes within a larger vessel containing additional heavy water acting purely as a moderator. This larger vessel, known as
8820-410: 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
8918-638: The time of the CANDU's design, Canada's heavy industry lacked the requisite experience and capability to cast and machine reactor pressure vessels of the required size. This problem is amplified by natural uranium fuel's lower fissile density, which requires a larger reactor core. This issue was so major that even the relatively small pressure vessel originally intended for use in the NPD prior to its mid-construction redesign could not be fabricated domestically and had to be manufactured in Scotland instead. Domestic development of
9016-408: 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
9114-595: 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
9212-488: 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
9310-456: 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
9408-643: 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
9506-548: 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
9604-462: 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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