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Encyclopedia > Pressurized water reactor

Pressurized water reactors (PWRs) (also VVER if of Russian design) are generation II nuclear power reactors that use ordinary water under high pressure as coolant and neutron moderator. The primary coolant loop is kept under high pressure to prevent the water from reaching film boiling, hence the name. PWRs are one of the most common types of reactors and are widely used all over the world. More than 230 of them are in use to generate electric power, and several hundred more for naval propulsion. They were originally designed by the Bettis Atomic Power Laboratory as a nuclear submarine power plant. WWER-10ff (also VVER-1000 as a direct translitteration from Russian ВВЭР-1000). ... A generation II reactor is a nuclear power reactor of one of several types developed from the first generation I reactors. ... Core of a small nuclear reactor used for research. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... This does not adequately cite its references or sources. ... The Leidenfrost effect is the phenomenon in which a liquid in near contact with a mass hotter than the liquids Leidenfrost point, which is higher than its boiling point, produces an insulating vapor layer which keeps it from boiling rapidly. ... Bettis Atomic Power Laboratory is a U. S. Government-owned, contractor-operated research and development facility located in West Mifflin, Pennsylvania, USA, focusing on the design and development of nuclear power for the U.S. Navy. ... USS Los Angeles A submarine is a specialized watercraft that can operate underwater. ...


Heat from small PWRs has been used for heating in polar regions, see Army Nuclear Power Program. The US Army Nuclear Power Program (ANPP) was a program to develop small PWR and BWR nuclear power reactors for use in remote sites. ...


The Three Mile Island accident occurred in a PWR manufactured by Babcock & Wilcox. Three Mile Island Nuclear Generating Station consisted of two pressurized water reactors manufactured by Babcock & Wilcox each inside its own containment building and connected cooling towers. ... The Babcock & Wilcox Company is an American firm engaged in the design, engineering, manufacture, service and construction of power generation and pollution control systems and equipment for utilities and industries. ...

Contents

Overview

A PWR works because the nuclear fuel in the reactor vessel is engaged in a chain reaction, which produces heat as the main goal of the entire setup. That heats the water in the primary coolant loop by thermal conduction through the fuel cladding. (The primary coolant loop is shown in the schematic as a red dashed line.) The hot water is pumped into a certain type of heat exchanger called steam generator, which allows the primary coolant to heat up the secondary coolant (shown as the loop steam generatorturbinecondenser). The transfer of heat is accomplished without mixing the two fluids since the primary coolant is necessarily radioactive, but it is desirable to avoid this for the secondary coolant. The steam formed in the steam generator is allowed to flow through a steam turbine, and the energy extracted by the turbine is used to drive an electric generator. In nuclear ships and submarines, the steam is fed through a steam turbine connected to a set of reduction gears to a shaft used for the propulsion. In a nuclear power station, the steam is fed through a steam turbine which drives a generator connected to the electric grid for distribution, as shown above. After passing through the turbine the secondary coolant (water-steam mixture) is cooled down and condensed in a condenser before being fed into the steam generator again. This reduces the pressure at the turbine outlet, which helps improve the thermal efficiency. Animated Diagram of a Pressurized Water Reactor. ... Nuclear Fuel Process A graph compairing nucleon number against binding energy Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. ... A schematic nuclear fission chain reaction. ... A heat exchanger is a device built for efficient heat transfer from one fluid to another, whether the fluids are separated by a solid wall so that they never mix, or the fluids are directly contacted. ... Steam generators are systems used to to convert water into steam from heat produced in a nuclear reactor core. ... Look up condenser in Wiktionary, the free dictionary. ...


Two things are characteristic for the pressurized water reactor (PWR) when compared with other reactor types:

  • In a PWR, there are two separate coolant loops (primary and secondary), which are both filled with ordinary water (also called light 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 the task (e.g. sodium in its liquid state).
  • The pressure in the primary coolant loop is typically 15-16 Megapascal, which is notably higher than in other nuclear reactors. As an effect of this, the water in the primary loop will not reach film boiling during normal operation and localized boiling will recondense promptly in the bulk fluid. By contrast, in a boiling water reactor the primary coolant is designed to boil.

Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... Light water, in the terminology of nuclear reactors, is ordinary water. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... A breeder reactor is a nuclear reactor that breeds fuel. ... The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Youngs modulus and tensile strength). ... Core of a small nuclear reactor used for research. ... The Leidenfrost effect is the phenomenon in which a liquid in near contact with a mass hotter than the liquids Leidenfrost point, which is higher than its boiling point, produces an insulating vapor layer which keeps it from boiling rapidly. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ...

PWR reactor design

Coolant

Rancho Seco PWR reactor hall and cooling tower (being decommissioned, 2004)

Borated water is used as primary coolant in a PWR and flows through the reactor at a temperature of roughly 315 °C (600 °F). The water remains liquid despite the high temperature due to the high pressure in the primary coolant loop (usually around 2200 psig [15 MPa, 150 atm]). The primary coolant loop is used to heat water in a secondary circuit that becomes saturated steam (in most designs 900 psia [6.2 MPa, 60 atm], 275 °C [530 °F]) for use in the steam turbine. Decommissioned PWR Nuclear Reactor, Rancho Seco, CA Taken by Pete Cassidy, 1998 File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Decommissioned PWR Nuclear Reactor, Rancho Seco, CA Taken by Pete Cassidy, 1998 File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Rancho Seco The Rancho Seco Nuclear Generating Station was a decommissioned nuclear power plant built by the Sacramento Municipal Utility District (SMUD) in Clay Station, California. ... The degree Celsius (symbol: °C) is an SI derived unit of temperature. ... Fahrenheit is a temperature scale named after the German physicist Daniel Gabriel Fahrenheit (1686–1736), who proposed it in 1724. ... Pounds-force per Square Inch (PSI) is a non-SI unit of pressure 1 psi approximately equals 6. ... The megapascal, symbol MPa is an SI unit of pressure. ... Standard atmosphere (symbol: atm) is a unit of pressure. ... Pounds-force per Square Inch (PSI) is a non-SI unit of pressure 1 psi approximately equals 6. ...


Although coolant flow rate in commercial PWRs is constant, it is not in nuclear reactors used on U.S. Navy ships.


Moderator

Pressurized water reactors, like thermal reactor designs, require the fast fission neutrons in the reactor to be slowed down (a process called moderation) in order to sustain its 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 is more dense (more collisions will occur). The use of water as a moderator is an important safety feature of PWRs, as any increase in temperature causes the water to expand and become less dense; thereby reducing the extent to which neutrons are slowed down and hence reducing the reactivity in the reactor. Therefore, if reactor activity increases beyond normal, 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. In contrast, the RBMK reactor design used at Chernobyl (using graphite instead of water as the moderator) greatly increases heat generation when coolant water temperatures increase, making them very unstable. This flaw in the RBMK reactor design is generally seen as one of several causes of the Chernobyl accident. A thermal reactor is the most common category of nuclear reactor. ... This does not adequately cite its references or sources. ...


Fuel

Main article: Nuclear fuel
PWR fuel bundle This fuel bundle is from a pressurized water reactor of the nuclear passenger and cargo ship NS Savannah. Designed and built by the Babcock and Wilcox Company.

The uranium used in PWR fuel is usually enriched several percent in 235U. After enrichment the uranium dioxide (UO2) powder is fired in a high-temperature, sintering furnace to create hard, ceramic pellets of enriched uranium dioxide. The cylindrical pellets are then put into tubes of a corrosion-resistant zirconium metal alloy (Zircaloy) which are backfilled with helium to aid heat conduction and detect leakages. The finished fuel rods are grouped in fuel assemblies, called fuel bundles, that are then used to build the core of the reactor. As a safety measure PWR designs do not contain enough fissile uranium to sustain a prompt critical chain reaction (i.e, substained only by prompt neutrons). Avoiding prompt criticality is important as a prompt critical chain reaction could very rapidly produce enough energy to damage or even melt the reactor (as is suspected to have occurred during the accident at the Chernobyl plant). 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 tonnes of uranium in all. Generally, the fuel bundles consist of fuel rods bundled 14x14 to 17x17. A PWR produces on the order of 900 to 1500 MWe. PWR fuel bundles are about 4 meters in length. Nuclear Fuel Process A graph compairing nucleon number against binding energy Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. ... Image File history File links Download high resolution version (1095x353, 50 KB) A nuclear fuel element for the cargo ship NS Savannah. ... Image File history File links Download high resolution version (1095x353, 50 KB) A nuclear fuel element for the cargo ship NS Savannah. ... NS Savannah, the first nuclear powered civilian ship NS Savannah, named for SS Savannah, the first steam-powered vessel to cross the Atlantic Ocean, was the first nuclear-powered cargo-passenger ship, one of only four nuclear-powered cargo ships ever built. ... This article or section does not cite its references or sources. ... In nuclear engineering, an assembly is prompt critical if for each nuclear fission event, one or more of the immediate or prompt neutrons released causes an additional fission event. ...


Refuelings for most commercial PWRs is on an 18-24 month cycle. Approximately one third of the core is replaced each refueling.

PWR reactor vessel

Image File history File links Download high resolution version (529x654, 20 KB) Summary This figure was obtained from the US Energy Information Agengy (EIA)Website at http://www. ... Image File history File links Download high resolution version (529x654, 20 KB) Summary This figure was obtained from the US Energy Information Agengy (EIA)Website at http://www. ...

Control

Generally, 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. Boron and control rods are used to maintain primary system temperature at 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 reactor to fission less and decrease in power. The operator could then add boric acid and/or insert control rods to decrease temperature to the desired point.


Reactivity adjustments to maintain 100% power as the fuel is burned up in most commercial PWR's is normally controlled by varying the concentration of boric acid dissolved in the primary reactor coolant. The boron readily absorbs neutrons and increasing or decreasing its concentration in the reactor coolant will therefore affect 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 top directly into the fuel bundles, are normally only used for power changes. In contrast, BWRs have no boron in the reactor coolant and control the reactor power by adjusting the reactor coolant flow rate. Boric acid, also called boracic acid or orthoboric acid or Acidum Boricum, is a mild acid often used as an antiseptic, insecticide, flame retardant, in nuclear power plants to control the fission rate of uranium, and as a precursor of other chemical compounds. ... A boiling water reactor (BWR) is a light water reactor design used in some nuclear power stations. ...


Due to design and fuel enrichment differences, naval nuclear reactors do not use boric acid.


Advantages

  • 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 reactors can be operated with a core containing less fissile material than is required for them to go prompt critical. This significantly reduces the chance that the reactor will run out of control and makes PWR designs relatively safe from criticality accidents.
  • Because PWR reactors use enriched uranium as fuel they can use ordinary water as a moderator rather than the much more expensive heavy water.
  • PWR turbine cycle loop is separate from the primary loop, so the water in the secondary loop is not contaminated by radioactive materials.

In nuclear engineering, an assembly is prompt critical if for each nuclear fission event, one or more of the immediate or prompt neutrons released causes an additional fission event. ... Heavy water is dideuterium oxide, or D2O or 2H2O. It is chemically the same as normal water, H2O, but the hydrogen atoms are of the heavy isotope deuterium, in which the nucleus contains a neutron in addition to the proton found in the nucleus of any hydrogen atom. ...

Disadvantages

  • The coolant water must be highly pressurized to remain liquid at high temperatures. This requires high strength piping and a heavy pressure vessel and hence increases construction costs. The higher pressure can increase the consequences of a Loss of Coolant Accident.
  • Most pressurized water reactors cannot be refueled while operating. This decreases the availability of the reactor- it has to go offline for comparably long periods of time (some weeks).
  • 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 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 radiation exposure.
  • Water absorbs neutrons making it necessary to enrich the uranium fuel, which increases the costs of fuel production. If heavy water is used it is possible to operate the reactor with natural uranium, but the production of heavy water requires large amounts of energy and is hence expensive.
  • Because water acts as a neutron moderator it is not possible to build a fast neutron reactor with a PWR design. For this reason it is not possible to build a fast breeder reactor with water coolant.
  • Because the reactor produces energy more slowly at higher temperatures, a sudden cooling of the reactor coolant could increase power production until safety systems shut down the reactor (OPΔT trip).

A Loss of Coolant Accident (LOCA) is a mode of failure for a nuclear reactor; in a nuclear reactor, the results of a LOCA could be catastrophic to the reactor, the facility that houses it, and the immediate vicinity around the reactor. ... Boric acid, also called boracic acid or orthoboric acid or Acidum Boricum, is a mild acid often used as an antiseptic, insecticide, flame retardant, in nuclear power plants to control the fission rate of uranium, and as a precursor of other chemical compounds. ... Shevchenko BN350 nuclear fast reactor and desalination plant situated on the shore of the Caspian Sea. ... The fast breeder or fast breeder reactor (FBR) is a fast neutron reactor designed to breed fuel by producing more fissile material than it consumes. ...

See also

The following is a list of pressurised water reactors. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ...

Next generation designs


  Results from FactBites:
 
BIGpedia - Pressurized water reactor - Encyclopedia and Dictionary Online (918 words)
In having this secondary loop the PWR differs from the boiling water reactor (BWR), in which the primary coolant is allowed to boil in the reactor core and drive a turbine directly.
Another advantage of using coolant water as a moderator in a pressurized water reactor is that the moderating effect decreases as a function of temperature due to the negative temperature coefficient of reactivity.
A disadvantage is that the reactor is succeptible to produce power at rates that result in damage to fuel cells in the event of introduction of cold water into the reactor or in the event the secondary system experiences a steam rupture.
Pressurized water reactor (328 words)
The pressurised water reactor (PWR) is the most common type of nuclear reactor, with over 230 in use for power generation and a further several hundred in naval propulsion.
One disadvantage to this type of reactor is that the reactor continues to generate heat from radioactive decay after the fission reaction is stopped, which can result in a nuclear meltdown if the reactor loses all coolant.
A pressured water reactor was involved in the accident at Three Mile Island.
  More results at FactBites »

 
 

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