The pebble-bed reactor ( PBR ) is a design for a graphite- moderated , gas-cooled nuclear reactor . It is a type of very-high-temperature reactor (VHTR), one of the six classes of nuclear reactors in the Generation IV initiative .
109-534: The basic design features spherical fuel elements called pebbles. These tennis ball -sized elements (approx. 6.7 cm or 2.6 in in diameter) are made of pyrolytic graphite (which acts as the moderator), and contain thousands of fuel particles called tristructural-isotropic (TRISO) particles. These TRISO particles consist of a fissile material (such as U ) surrounded by a ceramic coating of silicon carbide for structural integrity and fission product containment. Thousands of pebbles are amassed to create
218-416: A reactor core . The core is cooled by a gas that does not react chemically with the fuel elements, such as helium , nitrogen or carbon dioxide . Other coolants such as FLiBe (molten Li(BeF 4 ) ) have been suggested. The pebble bed design is passively safe . Because the reactor is designed to handle high temperatures, it can cool by natural circulation and survive accident scenarios, which may raise
327-442: A (partially) closed nuclear fuel cycle . Water is a nontoxic, transparent, chemically unreactive (by comparison with e.g. NaK ) coolant that is liquid at room temperature which makes visual inspection and maintenance easier. It is also easy and cheap to obtain unlike heavy water or even nuclear graphite . Compared to reactors operating on natural uranium , PWRs can achieve a relatively high burnup . A typical PWR will exchange
436-540: A 211 MW e gross unit HTR-PM , which incorporates two 250 MW t reactors. As of 2021, four sites were being considered for a 6-reactor successor, the HTR-PM600. The reactor entered service in December 2023. In June 2004, it was announced that a new PBMR would be built at Koeberg , South Africa by Eskom , the government-owned electrical utility to operate at 940 °C (1,720 °F). The PBMR project
545-479: A CANDU reactor or any other heavy water reactor when ordinary light water is supplied to the reactor as an emergency coolant. Depending on burnup , boric acid or another neutron poison will have to be added to emergency coolant to avoid a criticality accident . PWRs are designed to be maintained in an undermoderated state, meaning that there is room for increased water volume or density to further increase moderation, because if moderation were near saturation, then
654-465: A PWR cannot exceed a temperature of 647 K (374 °C; 705 °F) or a pressure of 22.064 MPa (3200 psi or 218 atm), because those are the critical point of water. Supercritical water reactors are (as of 2022) only a proposed concept in which the coolant would never leave the supercritical state. However, as this requires even higher pressures than a PWR and can cause issues of corrosion, so far no such reactor has been built. Pressure in
763-405: A PWR design. Nuclear fuel in the reactor pressure vessel is engaged in a controlled fission chain reaction , which produces heat, heating the water in the primary coolant loop by thermal conduction through the fuel cladding. The hot primary coolant is pumped into a heat exchanger called the steam generator , where it flows through several thousand small tubes. Heat is transferred through
872-475: A PWR is not suitable for most industrial applications as those require temperatures in excess of 400 °C (752 °F). Radiolysis and certain accident scenarios which involve interactions between hot steam and zircalloy cladding can produce hydrogen from the cooling water leading to hydrogen explosions as a potential accident scenario. During the Fukushima nuclear accident a hydrogen explosion damaging
981-459: A PWR. It can, however, be used in a CANDU with only minimal reprocessing in a process called "DUPIC" - Direct Use of spent PWR fuel in CANDU. Thermal efficiency , while better than for boiling water reactors , cannot achieve the values of reactors with higher operating temperatures such as those cooled with high temperature gases, liquid metals or molten salts. Similarly process heat drawn from
1090-656: A given temperature set by the position of the control rods. In contrast, the Soviet RBMK reactor design used at Chernobyl, which uses graphite instead of water as the moderator and uses boiling water as the coolant, has a large positive thermal coefficient of reactivity. This means reactivity and heat generation increases when coolant and fuel temperatures increase, which makes the RBMK design less stable than pressurized water reactors at high operating temperature. In addition to its property of slowing down neutrons when serving as
1199-444: A heavy pressure vessel and hence increases construction costs. The higher pressure can increase the consequences of a loss-of-coolant accident . The reactor pressure vessel is manufactured from ductile steel but, as the plant is operated, neutron flux from the reactor causes this steel to become less ductile. Eventually the ductility of the steel will reach limits determined by the applicable boiler and pressure vessel standards, and
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#17327870879511308-474: A human error during a blockage of pebbles in a pipe. Trying to restart the pebbles' movement by increasing gas flow stirred up dust, always present in PBRs, which was then released, unfiltered, into the environment due to an erroneously open valve. In spite of the limited amount of radioactivity released (0.1 GBq Co , Cs , Pa ), a commission of inquiry was appointed. The radioactivity in
1417-402: A long history of use in reactors and other high temperature applications. For example, pyrolytic graphite is also used, unreinforced, to construct missile reentry nose-cones and large solid rocket nozzles. Its strength and hardness come from its anisotropic crystals. Pyrolytic carbon can burn in air when the reaction is catalyzed by a hydroxyl radical (e.g., from water). Infamous examples include
1526-511: A long shut-down for inspections. In August, 1989, the THTR company almost went bankrupt, but was rescued by the government. The unexpected high costs of THTR operation and the accident ended interest in THTR reactors. The government decided to terminate the THTR operation at the end of September, 1989. This particular reactor was built despite criticism at the design phase. Most of those design critiques by German physicists, and by American physicists at
1635-463: A loss of coolant flow. The company went out of business in December 2010. Tennis ball This is an accepted version of this page A tennis ball is a small, hollow ball used in games of tennis and real tennis . Tennis balls are fluorescent yellow in professional competitions , but in recreational play other colors are also used. Tennis balls are covered in a fibrous felt , which modifies their aerodynamic properties, and each has
1744-426: A moderator). The pressure in the primary coolant loop is typically 15–16 megapascals (150–160 bar ), which is notably higher than in other nuclear reactors , and nearly twice that of a boiling water reactor (BWR). As an effect of this, only localized boiling occurs and steam will recondense promptly in the bulk fluid. By contrast, in a boiling water reactor the primary coolant is designed to boil. Light water
1853-423: A moderator, water also has a property of absorbing neutrons, albeit to a lesser degree. When the coolant water temperature increases, the boiling increases, which creates voids. Thus there is less water to absorb thermal neutrons that have already been slowed by the graphite moderator, causing an increase in reactivity. This property is called the void coefficient of reactivity, and in an RBMK reactor like Chernobyl,
1962-486: A nitrogen and oxygen mixture than the sea level ambient air pressure. Yellow and white are the only colors approved by the ITF. Most balls produced are a fluorescent color known as "optic yellow", first introduced in 1972 following research demonstrating they were more visible on television. What color to call the ball is mildly controversial; one poll showed that a little less than half of people consider this color yellow, while
2071-562: A pressurized water reactor (although the first power plant connected to the grid was at Obninsk , USSR), on insistence from Admiral Hyman G. Rickover that a viable commercial plant would include none of the "crazy thermodynamic cycles that everyone else wants to build". The United States Army Nuclear Power Program operated pressurized water reactors from 1954 to 1974. Three Mile Island Nuclear Generating Station initially operated two pressurized water reactor plants, TMI-1 and TMI-2. The partial meltdown of TMI-2 in 1979 essentially ended
2180-542: A quarter to a third of its fuel load every 18-24 months and have maintenance and inspection, that requires the reactor to be shut down, scheduled for this window. While more uranium ore is consumed per unit of electricity produced than in a natural uranium fueled reactor, the amount of spent fuel is less with the balance being depleted uranium whose radiological danger is lower than that of natural uranium. The coolant water must be highly pressurized to remain liquid at high temperatures. This requires high strength piping and
2289-469: A reduction in density of the moderator/coolant could reduce neutron absorption significantly while reducing moderation only slightly, making the void coefficient positive. Also, light water is actually a somewhat stronger moderator of neutrons than heavy water, though heavy water's neutron absorption is much lower. Because of these two facts, light water reactors have a relatively small moderator volume and therefore have compact cores. One next generation design,
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#17327870879512398-453: A reinforced concrete containment structure. PBR waste volumes are much greater, but have similar radioactivity measured in becquerels per kilowatt-hour . The waste tends to be less hazardous and simpler to handle. Current US legislation requires all waste to be safely contained, requiring waste storage facilities. Pebble defects may complicate storage. Graphite pebbles are more difficult to reprocess due to their construction. In 2008,
2507-400: A report about safety aspects of Germany's AVR reactor and general PBR features drew attention. The claims are contested. The report cited: Report author Rainer Moormann , recommended that average hot helium temperatures be limited to 800 °C (1,470 °F) minus the uncertainty of the core temperatures (about 200 °C or 360 °F). Farrington Daniels originated the concept and
2616-584: A safety test using the German AVR reactor, all the control rods were removed, and coolant flow was halted. The fuel remained undamaged. PBRs are intentionally operated above the 250 °C (482 °F) annealing temperature of graphite, so that Wigner energy does not accumulate. This solves a problem discovered in the Windscale fire . One reactor (not a PBR) caught fire because of the release of energy stored as crystalline dislocations (Wigner energy) in
2725-567: A secondary system where steam is generated. The steam then drives turbines, which spin an electric generator. In contrast to a boiling water reactor (BWR), pressure in the primary coolant loop prevents the water from boiling within the reactor. All light-water reactors use ordinary water as both coolant and neutron moderator . Most use anywhere from two to four vertically mounted steam generators; VVER reactors use horizontal steam generators. PWRs were originally designed to serve as nuclear marine propulsion for nuclear submarines and were used in
2834-420: A shaft used for propulsion . Direct mechanical action by expansion of the steam can be used for a steam-powered aircraft catapult or similar applications. District heating by the steam is used in some countries and direct heating is applied to internal plant applications. Two things are characteristic for the pressurized water reactor (PWR) when compared with other reactor types: coolant loop separation from
2943-449: A single turbine producing 210 MW e , operating commercially since 2023. Other designs are under development by MIT , University of California at Berkeley , General Atomics (U.S.), Dutch company Romawa B.V., Adams Atomic Engines , Idaho National Laboratory , X-energy and Kairos Power. A pebble-bed power plant combines a gas-cooled core and a novel fuel packaging. The uranium , thorium or plutonium nuclear fuels are in
3052-415: A slight majority consider it green. Tennis balls are filled with air and are surfaced by a uniform felt-covered rubber compound. Tennis ball felts comprise wool, nylon, and cotton in a mixture surrounding the rubber edge. The felt delays flow separation in the boundary layer which reduces aerodynamic drag and gives the ball better flight properties. Often, the balls will have a number in addition to
3161-541: A water/steam ingress accident of 30 metric tons (30 long tons; 33 short tons), which led to contamination of soil and groundwater by strontium-90 and by tritium. The leak in the steam generator leading to this accident was probably caused by high core temperatures (see criticism section). A re-examination of this accident was announced by the local government in July 2010. The AVR was originally designed to breed uranium-233 from thorium-232 . A practical thorium breeder reactor
3270-448: A white curvilinear oval covering it. Modern tennis balls must conform to certain size, weight, deformation, and bounce criteria to be approved for regulation play. The International Tennis Federation (ITF) defines the official diameter as 6.54–6.86 cm (2.57–2.70 inches). Balls must have masses in the range 56.0–59.4 g (1.98–2.10 ounces). A tennis ball generally has 12 pounds per square inch (80 kPa; 0.8 atm) more of
3379-530: A wool-wrapped stomach of a sheep or goat and tied with rope . Those recovered from the hammer-beam roof of Westminster Hall during a period of restoration in the 1920s were found to have been manufactured from a combination of putty and human hair and were dated to the reign of Henry VIII . Other versions, using materials such as animal fur , rope made from animal intestines and muscles, and pine wood , were found in Scottish castles dating back to
Pebble-bed reactor - Misplaced Pages Continue
3488-517: Is a type of light-water nuclear reactor . PWRs constitute the large majority of the world's nuclear power plants (with notable exceptions being the UK, Japan and Canada). In a PWR, the primary coolant ( water ) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms. The heated, high pressure water then flows to a steam generator , where it transfers its thermal energy to lower pressure water of
3597-555: Is due to cooling system complexity, which is not a factor in PBRs. Conventional plants require extensive safety systems and redundant backups. Their reactor cores are dwarfed by cooling systems. Further, the core irradiates the water with neutrons causing the water and impurities dissolved in it to become radioactive. The high-pressure piping in the primary side eventually becomes embrittled and requires inspection and replacement. Some designs are throttled by temperature rather than control rods . Such reactors do not need to operate well at
3706-402: Is generated per unit of uranium ore even though a higher burnup can be achieved. Nuclear reprocessing can "stretch" the fuel supply of both natural uranium and enriched uranium reactors but is virtually only practiced for light water reactors operating with lightly enriched fuel as spent fuel from e.g. CANDU reactors is very low in fissile material. Because water acts as a neutron moderator, it
3815-432: Is more dense (more collisions will occur). The use of water as a moderator is an important safety feature of PWRs, as an increase in temperature may cause the water to expand, giving greater 'gaps' between the water molecules and reducing the probability of thermalization — thereby reducing the extent to which neutrons are slowed and hence reducing the reactivity in the reactor. Therefore, if reactivity increases beyond normal,
3924-401: Is not possible to build a fast-neutron reactor with a PWR design. A reduced moderation water reactor may however achieve a breeding ratio greater than unity, though this reactor design has disadvantages of its own. Spent fuel from a PWR usually has a higher content of fissile material than natural uranium. Without nuclear reprocessing , this fissile material cannot be used as fuel in
4033-527: Is on an 18–24 month cycle. Approximately one third of the core is replaced each refueling, though some more modern refueling schemes may reduce refuel time to a few days and allow refueling to occur on a shorter periodicity. In PWRs reactor power can be viewed as following steam (turbine) demand due to the reactivity feedback of the temperature change caused by increased or decreased steam flow. (See: Negative temperature coefficient .) Boron and cadmium control rods are used to maintain primary system temperature at
4142-475: Is run directly through a turbine . However, if the gas from the primary coolant can be made radioactive by the neutrons in the reactor, or a fuel defect could contaminate the power production equipment, it may be brought instead to a heat exchanger where it heats another gas or produces steam. The turbine exhaust is warm and may be used to heat buildings or in other applications. Pebble-bed reactors are gas-cooled, sometimes at low pressures. The spaces between
4251-470: Is tested for bounce by dropping it from a height of 254 cm (100 inches) onto concrete ; a bounce between 135 and 147 cm (53 and 58 inches) is acceptable if taking place at sea-level and 20 °C (68 °F) with relative humidity of 60%; high-altitude balls have different characteristics when tested at sea level. The ITF's "Play and Stay" campaign aims to increase tennis participation worldwide by improving how starter players are introduced to
4360-465: Is used as the primary coolant in a PWR. Water enters through the bottom of the reactor's core at about 548 K (275 °C; 527 °F) and is heated as it flows upwards through the reactor core to a temperature of about 588 K (315 °C; 599 °F). The water remains liquid despite the high temperature due to the high pressure in the primary coolant loop, usually around 155 bar (15.5 MPa 153 atm , 2,250 psi ). The water in
4469-593: The Pennsylvania Rubber Company released a hermetically sealed pressurized metal tube that held three balls with a churchkey to open the top. Beginning in the 1980s, plastic (from recycled PET ) cans with a full-top pull-tab seal and plastic lid fit three or four balls per can. Pressureless balls often come in net bags or buckets since they need not be pressure-sealed. Each year approximately 325 million balls are produced, which contributes roughly 20,000 tonnes (22,000 short tons) of waste in
Pebble-bed reactor - Misplaced Pages Continue
4578-480: The accidents at Windscale and Chernobyl—both graphite-moderated reactors. However, PBRs are cooled by inert gases to prevent fire. All designs have at least one layer of silicon carbide that serves as a fire break and seal. All kernels are precipitated from a sol-gel , then washed, dried and calcined. U.S. kernels use uranium carbide , while German (AVR) kernels use uranium dioxide . German-produced fuel-pebbles release about 1000 times less radioactive gas than
4687-452: The supercritical water reactor , is even less moderated. A less moderated neutron energy spectrum does worsen the capture/fission ratio for U and especially Pu, meaning that more fissile nuclei fail to fission on neutron absorption and instead capture the neutron to become a heavier nonfissile isotope, wasting one or more neutrons and increasing accumulation of heavy transuranic actinides, some of which have long half-lives. After enrichment,
4796-531: The uranium dioxide ( UO 2 ) powder is fired in a high-temperature, sintering furnace to create hard, ceramic pellets of enriched uranium dioxide. The cylindrical pellets are then clad in a corrosion-resistant zirconium metal alloy Zircaloy which are backfilled with helium to aid heat conduction and detect leakages. Zircaloy is chosen because of its mechanical properties and its low absorption cross section. The finished fuel rods are grouped in fuel assemblies, called fuel bundles, that are then used to build
4905-673: The 16th century. In the 18th century, 1.9 cm ( 3 ⁄ 4 in) strips of wool were wound tightly around a nucleus made by rolling several strips into a little ball. String was then tied in many directions around the ball, and a white cloth covering was sewn around the ball. In the early 1870s, lawn tennis arose in Britain through the pioneering efforts of Walter Clopton Wingfield and Harry Gem , often using Victorian lawns laid out for croquet . Wingfield marketed lawn tennis sets which included rubber balls imported from Germany. After Charles Goodyear invented vulcanised rubber,
5014-584: The Germans had been most successful in developing air-filled vulcanised rubber balls. These were light and coloured grey or red with no covering. John Moyer Heathcote suggested and tried the experiment of covering the rubber ball with flannel, and by 1882 Wingfield was advertising his balls as clad in stout cloth made in Melton Mowbray . Tennis balls were initially entirely made of rubber, but they were later refined by using flannel and stitching it around
5123-644: The National Laboratory level, went ignored until shutdown. Nearly every problem encountered by the THTR 300 reactor was predicted by the physicists who criticized it as "overly complex". In 2004 China licensed the AVR technology and developed a reactor for power generation. The 10 megawatt prototype is called the HTR-10 . It is a conventional helium-cooled, helium-turbine design. In 2021 the Chinese then built
5232-401: The U.S. equivalents, due to that construction method. The primary criticism of pebble-bed reactors is that encasing the fuel in graphite poses a hazard. Graphite can burn in the presence of air, which could happen if the reactor vessel is compromised. Fire could vaporize the fuel, which could then be released to the surroundings. Fuel kernels are coated with a layer of silicon carbide to isolate
5341-613: The US, they were originally designed at the Oak Ridge National Laboratory for use as a nuclear submarine power plant with a fully operational submarine power plant located at the Idaho National Laboratory . Follow-on work was conducted by Westinghouse Bettis Atomic Power Laboratory . The first purely commercial nuclear power plant at Shippingport Atomic Power Station was originally designed as
5450-458: The air inside the balls, preserving the pressure inside. When a tennis ball is unpackaged, its frequent use allows for air to escape from the ball. They can be tested to determine their bounce. Modern regulation tennis balls are kept under pressure (approximately two atmospheres) until initially used; balls intended for use at high altitudes have a lower initial pressure, and inexpensive practice balls are made without internal pressurization. A ball
5559-487: The bottom to the top about ten times over a period of years, and are tested after each pass. Expended pebbles are removed to the nuclear-waste area, replaced by a new pebble. When the reactor temperature rises, the atoms in the fuel move rapidly, causing Doppler broadening . The fuel then experiences a wider range of neutron speeds. Uranium-238 , which forms the bulk of the uranium, is much more likely to absorb fast or epithermal neutrons at higher temperatures. This reduces
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#17327870879515668-413: The brand name. This helps distinguish one set of balls from another of the same brand on an adjacent court. Tennis balls begin to lose their bounce as soon as the tennis ball can is opened. Tennis balls lose bounciness because the air inside the ball is pushing harder when a can is opened compared to when a ball is packaged. When packaged, the pressure in the can equally pushes the ball from the outside as
5777-426: The core of the reactor. A typical PWR has fuel assemblies of 200 to 300 rods each, and a large reactor would have about 150–250 such assemblies with 80–100 tons of uranium in all. Generally, the fuel bundles consist of fuel rods bundled 14 × 14 to 17 × 17. A PWR produces on the order of 900 to 1,600 MW e . PWR fuel bundles are about 4 meters in length. Refuelings for most commercial PWRs
5886-437: The core, which used to be filled with rubber. The tennis ball quickly switched to having a hollow core, using gas to pressurize the inside. Originally, tennis ball manufacturing was done by cutting vulcanized rubber sheets into a shape similar to that of a three-leaf clover. Before the formation of the rubber into a sphere (which was executed via machinery), chemicals that reacted to produce a gas were added to produce pressure into
5995-421: The correct geometry creates criticality . The pebbles are held in a vessel, and an inert gas (such as helium, nitrogen or carbon dioxide) circulates through the spaces between the fuel pebbles to carry heat away from the reactor. Pebble-bed reactors must keep the pebbles' graphite from burning in the presence of air if the reactor wall is breached (the flammability of the pebbles is disputed ). The heated gas
6104-420: The crack. During this examination it was revealed that the AVR was the world's most heavily beta-contaminated ( strontium-90 ) nuclear installation and that this contamination was present as dust (the worst form). Localized fuel temperature instabilities resulted in heavy vessel contamination by Cs-137 and Sr-90 . The reactor vessel was filled with light concrete in order to fix the radioactive dust and in 2012
6213-544: The desired point. In order to decrease power, the operator throttles shut turbine inlet valves. This would result in less steam being drawn from the steam generators. This results in the primary loop increasing in temperature. The higher temperature causes the density of the primary reactor coolant water to decrease, allowing higher neutron speeds, thus less fission and decreased power output. This decrease of power will eventually result in primary system temperature returning to its previous steady-state value. The operator can control
6322-399: The fast fission neutrons to be slowed (a process called moderation or thermalizing) in order to interact with the nuclear fuel and sustain the chain reaction. In PWRs the coolant water is used as a moderator by letting the neutrons undergo multiple collisions with light hydrogen atoms in the water, losing speed in the process. This "moderating" of neutrons will happen more often when the water
6431-559: The flawed RBMK control rods design. These design flaws, in addition to operator errors that pushed the reactor to its limits, are generally seen as the causes of the Chernobyl disaster . The Canadian CANDU heavy water reactor design have a slight positive void coefficient, these reactors mitigate this issues with a number of built-in advanced passive safety systems not found in the Soviet RBMK design. No criticality could occur in
6540-446: The form of a ceramic (usually oxides or carbides ) contained within spherical pebbles a little smaller than the size of a tennis ball and made of pyrolytic graphite, which acts as the primary neutron moderator . The pebble design is relatively simple, with each sphere consisting of the nuclear fuel, fission product barrier, and moderator (which in a traditional water reactor would all be different parts). Grouping sufficient pebbles in
6649-750: The form of rubber that is not easily biodegradable . Historically, tennis ball recycling has not existed. Balls from The Championships, Wimbledon are now recycled to provide field homes for the nationally threatened Eurasian harvest mouse . The gift of tennis balls offered to Henry in Shakespeare's Henry V is portrayed as the final insult which re-ignites the Hundred Years' War between England and France. John Webster also refers to tennis balls in The Duchess of Malfi . Pressurized water reactor A pressurized water reactor ( PWR )
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#17327870879516758-511: The fuel. THTR-300 suffered technical difficulties, and owing to these and political events in Germany, was closed after four years of operation. An incident on 4 May 1986, only a few days after the Chernobyl disaster, allowed a release of part of the radioactive inventory into the environment. Although the radiological impact was small, it had a disproportionate impact. The release was caused by
6867-416: The game. The ITF recommends a progression that focuses on a range of slower balls and smaller court sizes to introduce the game to adults and children effectively. The slowest balls, marked with red, or using half-red felt, are oversized and unpressurized or made from foam rubber. The next, in orange, are unpressurized normal-sized balls. The last, with green, are half pressured normal sized. Lawn tennis, as
6976-422: The graphite. The dislocations are caused by neutron passage through the graphite. Windscale regularly annealed the graphite to release accumulated Wigner energy. However, the effect was not anticipated, and since the reactor was cooled by ambient air in an open cycle, the process could not be reliably controlled, and led to a fire. Berkeley professor Richard A. Muller described PBRs as "in every way ... safer than
7085-433: The graphite. While silicon carbide is strong in abrasion and compression applications, it has less resistance to expansion and shear forces. Some fission products such as Xe have limited absorbance in carbon, so some fuel kernels could accumulate enough gas to rupture the silicon carbide. Some designs do not include a containment building, leaving reactors more vulnerable to attack. However, most are surrounded by
7194-578: The growth in new construction of nuclear power plants in the United States for two decades. Watts Bar unit 2 (a Westinghouse 4-loop PWR) came online in 2016, becoming the first new nuclear reactor in the United States since 1996. The pressurized water reactor has several new Generation III reactor evolutionary designs: the AP1000 , VVER-1200, ACPR1000+, APR1400, Hualong One , IPWR-900 and EPR . The first AP1000 and EPR reactors were connected to
7303-472: The heat into water to create steam and is helium-cooled. The HTMR-100 reactor produces output of 35 MWe. Adams Atomic Engines (AAE) design was self-contained so it could be adapted to extreme environments such as space, polar and underwater environments. Their design was for a nitrogen coolant passing directly though a conventional low-pressure gas turbine, and due to the rapid ability of the turbine to change speeds, it can be used in applications where instead of
7412-460: The heaters or emptying the pressurizer. Pressure transients in the primary coolant system manifest as temperature transients in the pressurizer and are controlled through the use of automatic heaters and water spray, which raise and lower pressurizer temperature, respectively. The coolant is pumped around the primary circuit by powerful pumps. These pumps have a rate of ~100,000 gallons of coolant per minute. After picking up heat as it passes through
7521-682: The hollow inside once the sphere was assembled. The switch to the modern method of joining two hemispheres was done to improve uniformity of wall thickness. Until 1972, tennis balls were white (or sometimes black). In 1972, the International Tennis Federation introduced yellow balls, as these were easier to see on television, and these quickly became generally popular. Wimbledon continued using white balls until 1986. Before 1925, tennis balls were packaged in wrapped paper and paperboard boxes. In 1925, Wilson-Western Sporting Goods Company introduced cardboard tubes. In 1926,
7630-678: The innovative design of the Benghazi burner by British desert troops in WWII. Commercial development came in the 1960s via the West German AVR reactor designed by Rudolf Schulten . This system was plagued with problems and the technology was abandoned. The AVR design was licensed to South Africa as the PBMR and China as the HTR-10 . The HTR-10 prototype was developed into China's HTR-PM demonstration plant, which connects two reactors to
7739-584: The modern game was originally known, was developed in the early 1870s as a new version of the courtly game of real tennis . England banned the importation of real tennis balls, playing cards , dice , and other goods in the Importation (No. 2) Act 1463 ( 3 Edw. 4 . c. 4). In 1480, Louis XI of France forbade the filling of tennis balls with chalk, sand, sawdust, or earth, and stated that they were to be made of good leather, well-stuffed with wool. Other early tennis balls were made by Scottish craftsmen from
7848-518: The most deployed type of reactor globally, allowing for a wide range of suppliers of new plants and parts for existing plants. Due to long experience with their operation they are the closest thing to mature technology that exists in nuclear energy. PWRs - depending on type - can be fueled with MOX-fuel and/or the Russian Remix Fuel (which has a lower Pu and a higher U content than "regular" U/Pu MOX-fuel) allowing for
7957-426: The name in 1947 at Oak Ridge. Rudolf Schulten advanced the idea in the 1950s. The crucial insight was to combine fuel, structure, containment, and neutron moderator in a small, strong sphere. The concept depended on the availability of engineered forms of silicon carbide and pyrolytic carbon that were strong. A 15 MW e demonstration reactor, Arbeitsgemeinschaft Versuchsreaktor ( experimental reactor consortium ),
8066-430: The neutron activity correspondingly. An entire control system involving high pressure pumps (usually called the charging and letdown system) is required to remove water from the high pressure primary loop and re-inject the water back in with differing concentrations of boric acid. The reactor control rods, inserted through the reactor vessel head directly into the fuel bundles, are moved for the following reasons: to start up
8175-480: The nucleus of a boron-10 atom which subsequently splits into a lithium-7 and tritium atom. Pressurized water reactors annually emit several hundred curies of tritium to the environment as part of normal operation. Natural uranium is only 0.7% uranium-235, the isotope necessary for thermal reactors. This makes it necessary to enrich the uranium fuel, which significantly increases the costs of fuel production. Compared to reactors operating on natural uranium, less energy
8284-421: The number of neutrons available to cause fission, and reduces power. Doppler broadening therefore creates a negative feedback: as fuel temperature increases, reactor power decreases. All reactors have reactivity feedback mechanisms. The pebble-bed reactor is designed so that this effect is relatively strong, inherent to the design, and does not depend on moving parts. This negative feedback creates passive control of
8393-544: The original design of the second commercial power plant at Shippingport Atomic Power Station . PWRs currently operating in the United States are considered Generation II reactors . Russia's VVER reactors are similar to US PWRs, but the VVER-1200 is not considered Generation II (see below). France operates many PWRs to generate the bulk of its electricity. Several hundred PWRs are used for marine propulsion in aircraft carriers , nuclear submarines and ice breakers . In
8502-419: The pebbles replace the piping in conventional reactors. Since there is no actual piping in the core and the coolant contains no hydrogen, embrittlement is not a failure concern. The preferred gas, helium, does not easily absorb neutrons or impurities. Therefore, compared to water, it is both more efficient and less likely to become radioactive. Much of the cost of a conventional, water-cooled nuclear power plant
8611-412: The pebbles, ensures that the pebbles are passively cooled. Even in the event that all supporting machinery fails, the reactor will not crack, melt, explode or spew hazardous wastes. It heats to a designed "idle" temperature, and stays there. At idle, the reactor vessel radiates heat, but the vessel and fuel spheres remain intact and undamaged. The machinery can be repaired or the fuel can be removed. In
8720-456: The power generation used primary coolant, it was reported that the AVR exposed its personnel to less than 1/5 as much radiation as a typical light water reactor. It was decommissioned on December 1, 1988, in the wake of the Chernobyl disaster and operational problems. During removal of the fuel elements it became apparent that the neutron reflector under the pebble-bed core had cracked during operation. Some hundred fuel elements remained stuck in
8829-667: The power grid in China in 2018. In 2020, NuScale Power became the first U.S. company to receive regulatory approval from the Nuclear Regulatory Commission for a small modular reactor with a modified PWR design. Also in 2020, the Energy Impact Center introduced the OPEN100 project, which published open-source blueprints for the construction of a 100 MW electric nuclear power plant with
8938-406: The present nuclear reactors". Most PBR designs include multiple reinforcing levels of containment to prevent contact between the radioactive materials and the biosphere: Pyrolytic graphite is the main structural material in pebbles. It sublimates at 4,000 °C (7,230 °F), more than double the design temperature of most reactors. It slows neutrons effectively, is strong, inexpensive, and has
9047-432: The pressure drop across the turbine, and hence the energy extracted from the steam, is maximized. Before being fed into the steam generator, the condensed steam (referred to as feedwater) is sometimes preheated in order to minimize thermal shock. The steam generated has other uses besides power generation. In nuclear ships and submarines, the steam is fed through a steam turbine connected to a set of speed reduction gears to
9156-518: The pressure vessel must be repaired or replaced. This might not be practical or economic, and so determines the life of the plant. Additional high pressure components such as reactor coolant pumps, pressurizer, and steam generators are also needed. This also increases the capital cost and complexity of a PWR power plant. The high temperature water coolant with boric acid dissolved in it is corrosive to carbon steel (but not stainless steel ); this can cause radioactive corrosion products to circulate in
9265-419: The pressurized steam is fed through a steam turbine which drives an electrical generator connected to the electric grid for transmission. After passing through the turbine the secondary coolant (water-steam mixture) is cooled down and condensed in a condenser . The condenser converts the steam to a liquid so that it can be pumped back into the steam generator, and maintains a vacuum at the turbine outlet so that
9374-414: The pressurizer temperature and the highest temperature in the reactor core) of 30 °C (54 °F). As 345 °C is the boiling point of water at 155 bar, the liquid water is at the edge of a phase change. Thermal transients in the reactor coolant system result in large swings in pressurizer liquid/steam volume, and total pressurizer volume is designed around absorbing these transients without uncovering
9483-441: The primary circuit is maintained by a pressurizer, a separate vessel that is connected to the primary circuit and partially filled with water which is heated to the saturation temperature (boiling point) for the desired pressure by submerged electrical heaters. To achieve a pressure of 155 bars (15.5 MPa), the pressurizer temperature is maintained at 345 °C (653 °F), which gives a subcooling margin (the difference between
9592-423: The primary coolant loop. This not only limits the lifetime of the reactor, but the systems that filter out the corrosion products and adjust the boric acid concentration add significantly to the overall cost of the reactor and to radiation exposure. In one instance, this has resulted in severe corrosion to control rod drive mechanisms when the boric acid solution leaked through the seal between the mechanism itself and
9701-406: The primary system. Due to the requirement to load a pressurized water reactor's primary coolant loop with boron, undesirable radioactive secondary tritium production in the water is over 25 times greater than in boiling water reactors of similar power, owing to the latter's absence of the neutron moderating element in its coolant loop. The tritium is created by the absorption of a fast neutron in
9810-405: The reaction process. Thus PBRs passively reduce to a safe power-level in an accident scenario. This is the design's main passive safety feature. The reactor is cooled by an inert, fireproof gas, which has no phase transitions—it is always in the gaseous phase. The moderator is solid carbon; it does not act as a coolant, or move, or change phase. Convection of the gas, driven by the heat of
9919-432: The reactor coolant and control the reactor power by adjusting the reactor coolant flow rate. PWR reactors are very stable due to their tendency to produce less power as temperatures increase; this makes the reactor easier to operate from a stability standpoint. PWR turbine cycle loop is separate from the primary loop, so the water in the secondary loop is not contaminated by radioactive materials. PWRs can passively scram
10028-455: The reactor core, the primary coolant transfers heat in a steam generator to water in a lower pressure secondary circuit, evaporating the secondary coolant to saturated steam — in most designs 6.2 MPa (60 atm, 900 psia ), 275 °C (530 °F) — for use in the steam turbine. The cooled primary coolant is then returned to the reactor vessel to be heated again. Pressurized water reactors, like all thermal reactor designs, require
10137-403: The reactor in case offsite power is lost to immediately stop the primary nuclear reaction. The control rods are held by electromagnets and fall by gravity when current is lost; full insertion safely shuts down the primary nuclear reaction. PWR technology is favoured by nations seeking to develop a nuclear navy; the compact reactors fit well in nuclear submarines and other nuclear ships. PWRs are
10246-465: The reactor vessel of 2,100 metric tons (2,100 long tons; 2,300 short tons) was to be moved to intermediate storage until a permanent solution is devised. The reactor buildings were to be dismantled and soil and groundwater decontaminated. AVR dismantling costs were expected to far exceed its construction costs. In August 2010, the German government estimated costs for AVR dismantling without consideration of
10355-428: The reactor, to shut down the primary nuclear reactions in the reactor, to accommodate short term transients, such as changes to load on the turbine, The control rods can also be used to compensate for nuclear poison inventory and to compensate for nuclear fuel depletion. However, these effects are more usually accommodated by altering the primary coolant boric acid concentration. In contrast, BWRs have no boron in
10464-431: The reduced moderation of neutrons will cause the chain reaction to slow down, producing less heat. This property, known as the negative temperature coefficient of reactivity, makes PWR reactors very stable. This process is referred to as 'Self-Regulating', i.e. the hotter the coolant becomes, the less reactive the plant becomes, shutting itself down slightly to compensate and vice versa. Thus the plant controls itself around
10573-426: The steady state operating temperature by addition of boric acid and/or movement of control rods. Reactivity adjustment to maintain 100% power as the fuel is burned up in most commercial PWRs is normally achieved by varying the concentration of boric acid dissolved in the primary reactor coolant. Boron readily absorbs neutrons and increasing or decreasing its concentration in the reactor coolant will therefore affect
10682-427: The steam system and pressure inside the primary coolant loop. In a PWR, there are two separate coolant loops (primary and secondary), which are both filled with demineralized/deionized water. A boiling water reactor, by contrast, has only one coolant loop, while more exotic designs such as breeder reactors use substances other than water for coolant and moderator (e.g. sodium in its liquid state as coolant or graphite as
10791-405: The temperature of the reactor to 1,600 °C (2,910 °F). Such high temperatures allow higher thermal efficiencies than possible in traditional nuclear power plants (up to 50%) while the gases do not dissolve contaminants or absorb neutrons as water does, so the core has less in the way of radioactive fluids . The concept was first suggested by Farrington Daniels in the 1940s, inspired by
10900-577: The test series and the resulting higher contamination of the containment structure. Pebble debris and graphite dust blocked some of the coolant channels in the bottom reflector, as was discovered during fuel removal after final shut-down. A failure of insulation required frequent reactor shut-downs for inspection, because the insulation could not be repaired. Metallic components in the hot gas duct failed in September 1988, probably due to thermal fatigue induced by unexpected hot gas currents. This failure led to
11009-454: The turbine's output being converted to electricity, the turbine itself could directly drive a mechanical device, for instance, a propeller aboard a ship. Like all high temperature designs, the AAE engine would have been inherently safe, as the engine naturally shuts down due to Doppler broadening , stopping heat generation if the fuel in the engine gets too hot in the event of a loss of coolant or
11118-439: The varying neutron profiles caused by partially withdrawn control rods. PBRs can use fuel pebbles made from various fuels in the same design (though perhaps not simultaneously). Proponents claim that pebble-bed reactors can use thorium, plutonium and natural unenriched uranium, as well as enriched uranium . In most stationary designs, fuel replacement is continuous. Pebbles are placed in a bin-shaped reactor. Pebbles travel from
11227-460: The vessel dismantling at 600 million € ( $ 750 million, which corresponded to 0.4 € ($ 0.55) per kWh of electricity generated by the AVR. A separate containment was erected for dismantling purposes, as seen in the AVR-picture. Following the experience with the AVR, Germany constructed a full scale power station (the thorium high-temperature reactor or THTR-300 rated at 300 MW), using thorium as
11336-505: The vicinity of the THTR-300 was finally found to result 25% from Chernobyl and 75% from THTR-300. The handling of this minor accident severely damaged the credibility of the German pebble-bed community, which lost support in Germany. The overly complex design of the reactor, which is contrary to the general concept of self-moderated thorium reactors designed in the U.S., also suffered from the unplanned high destruction rate of pebbles during
11445-409: The void coefficient is positive, and fairly large, making it very hard to regulate when the reaction begins to run away. The RBMK reactors also have a flawed control rods design in which during rapid scrams, the graphite reaction enhancement tips of the rods would displace water at the bottom of the reactor and locally increase reactivity there. This is called the "positive scram effect" that is unique to
11554-414: The walls of these tubes to the lower pressure secondary coolant located on the shell side of the exchanger where the secondary coolant evaporates to pressurized steam. This transfer of heat is accomplished without mixing the two fluids to prevent the secondary coolant from becoming radioactive. Some common steam generator arrangements are u-tubes or single pass heat exchangers. In a nuclear power station,
11663-526: Was built at the Jülich Research Centre in Jülich , West Germany . The goal was to gain operational experience with a high-temperature gas-cooled reactor. Construction costs of AVR were 115 million Deutschmark (1966), corresponding to a 2010 value of 180 million €. The unit's first criticality was on August 26, 1966. The facility ran successfully for 21 years. In 1978, the AVR suffered from
11772-437: Was considered valuable technology. However, the AVR's fuel design contained the fuel so well that the transmuted fuels were uneconomic to extract—it was cheaper to use mined and purified uranium. The AVR used helium coolant , has a low neutron cross-section . Since few neutrons are absorbed, the coolant remains less radioactive. It is practical to route the primary coolant directly to power generation turbines. Even though
11881-520: Was opposed by groups such as Koeberg Alert and Earthlife Africa , the latter of which sued Eskom. The reactor was never completed and the testing facility was decommissioned and placed in a "care and maintenance mode" to protect the IP and the assets. A Pretoria-based company, Stratek Global, created a variant of the PBMR reactor. The Stratek HTMR-100 reactor functions at 750 °C (1,380 °F). It directs
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