View source for Pressurized water reactor

[[Category:Done 2018-08-03]] 
[[File:wattsbar.jpg|250px|thumb|Figure 1. The Watts Bar Nuclear Generating Station in Tennessee uses PWRs in its operation.<ref>Wikimedia Commons [Online], Available: https://upload.wikimedia.org/wikipedia/commons/c/ce/Watts_Bar-6.jpg</ref>]]
[[File:nuclearsub.jpg|250px|thumb|Figure 2. Nuclear submarines make use of the high power-to-weight ratio of PWRs in their operation.<Ref>Wikimedia Commons [Online], Available: https://upload.wikimedia.org/wikipedia/commons/a/ad/Delta-II_class_nuclear-powered_ballistic_missle_submarine_3.jpg</ref>]]
<onlyinclude>The '''pressurized water reactor''' ('''PWR''') is a type of [[nuclear reactor]] used to the [[electrical generation|generate electricity]] and propel nuclear submarines and naval vessels.<ref name=ine>J.R. Lamarsh and A.J. Baratta, "Power Reactors and Nuclear Steam Supply Systems" in ''Introduction to Nuclear Engineering'', 3rd ed., Upper Saddle River, NJ: Prentice Hall, 2001, ch.4, sec.5, pp. 136-185</ref> They make use of [[light water]] (ordinary water, as opposed to [[heavy water]]) as their coolant and [[neutron moderator]]. It is one of three types of light water reactors, with the others being the [[boiling water reactor]] and the [[supercritical water cooled reactor]].</onlyinclude>

It was originally designed for the U.S. Navy, however, it quickly grew to become the most widely used reactor in [[nuclear power plant]]s; with 297 in operation around the world as of 2018.<ref name = PRIS/> This makes them by far the most dominantly used reactor in the world, with the second most (the boiling water reactor) having only 80 in operation. Construction of PWRs diminished greatly after the [[Three Mile Island nuclear accident]], mainly as a result of [[public attitudes towards energy sources#Support for nuclear power|public support]] becoming weaker.

Their use on naval vessels and nuclear ships are of extreme importance to various militaries around the world.[[Nuclear power]] has a huge advantage over fuels like [[gasoline]] or [[diesel]] as it allows ships to run for very long periods without the need to refuel. PWRs make a good reactor for these ships since they have a high [[specific power]] (high power for their mass) due to their use of high [[pressure]]. This allows the reactors to be fairly compact, especially with the use of [[uranium enrichment|highly enriched]] [[uranium]].
[[File:Number of reactor types.png|400px|thumb|center|Figure 3. Percentage of Rector types worldwide.<ref name = PRIS>"PRIS - Reactor status reports - Operational & Long-Term Shutdown - By Type", Pris.iaea.org, 2018. [Online]. Available: https://pris.iaea.org/PRIS/WorldStatistics/OperationalReactorsByType.aspx. [Accessed: 13- Aug- 2018].</ref>]]

{| class="wikitable sortable"
|-
! Type !! Number of Reactors
|-
| PWR || 297 
|-
| [[BWR]] (Boiling Light-Water Cooled and Moderated Reactor)  || 75
|-
| PHWR	(Pressurized Heavy-Water Moderated and Cooled Reactor) || 49
|-
| LWGR	(Light-Water Cooled, Graphite Moderated Reactor) || 15
|-
| GCR	(Gas Cooled, Graphite Moderated Reactor)	 || 14
|-
| [[FBR]]	(Fast Breeder Reactor)|| 3
|-
|}

==Characteristics==

===Fuel=== 

Pressurized water reactors must use [[uranium enrichment|enriched uranium]] as their [[nuclear fuel]], because of their use of light water. This is because light water would absorb too many [[neutron]]s if natural [[uranium]] was used, so the [[nuclear fuel|fuel]] content of [[fissile]] Uranium-235 must be increased. This is done through a [[uranium enrichment]] process, in which the concentration of Uranium-235 is increased from 0.7% to around 4%.<ref>World Nuclear Association. (June 25 2015). ''Uranium Enrichment'' [Online], Available: http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Conversion-Enrichment-and-Fabrication/Uranium-Enrichment/</ref> 

The enriched uranium is packed into fuel rods which are assembled into a fuel bundle, as seen in Figure 3. There are about 200-300 rods in each bundle for a PWR, with a large reactor containing 150-250 bundles in their core.<ref name=wna>World Nuclear Association. (June 30 2015). ''Nuclear Power Reactors'' [Online], Available: http://www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Power-Reactors/Nuclear-Power-Reactors/</ref>  This corresponds to about five cubic meters of uranium, or 80-100 [[tonne]]s of uranium.<ref name=wna/> 

The bundles are arranged vertically in fuel tubes within the core. As the fuel is "burned" in the reactor, its [[density]] gradually increases, resulting in small voids to develop inside the fuel tube. These void spaces can cause a problem because high pressures could cause stress to the tubes, increasing the likelihood of a rupture. To avoid this problem, the tubes are pressurized with helium at about [[pascal|3.4 MPa]]. As [[fission]] gas products accumulate over the fuel's lifetime, the pressure gradually balances with the high pressure of the core.<ref name=ine/>

[[File:Nuclear_fuel_element.jpg|800px|thumb|center|Figure 3. A nuclear fuel bundle for a PWR.<ref>Wikimedia Commons [Online], Available: https://upload.wikimedia.org/wikipedia/commons/2/2d/Nuclear_fuel_element.jpg</ref>]]

===Coolant and Moderator=== 

As mentioned before, light water is used as the coolant and moderator for a pressurized water reactor. Light water is ''much'' more abundant than heavy water, as it makes up 99.99% of natural water.<ref>Jefferson Lab. (June 29 2015). ''Isotopes of the Element Hydrogen'' [Online], Available: http://education.jlab.org/itselemental/iso001.html</ref> 

Light water does not make as good of a moderator as heavy water or graphite as a result of its relatively high absorption of neutrons. However, its use as a moderator makes for an important safety feature; if there is a loss of coolant accident (LOCA), there will also be a loss of moderator causing the [[nuclear chain reaction]] to stop. Also if the moderating water overheats and becomes steam inside the bottom reactor core, there will be less moderator and therefore the chain reaction will stop. 

===Pressure, Temperatures and Water flow===
[[File:Utube.jpg|250px|thumb|Figure 4. The inverted U-tube bundle in the steam generator of a PWR.<ref>Wikimedia Commons [Online], Available: https://upload.wikimedia.org/wikipedia/commons/8/89/Nuclear_steam_generator.jpg</ref>]]

As the name implies, the water in the reactor is pressurized. This is due to the fact that as the pressure gets higher, the [[boiling point]] of water increases with it. This means that at high pressures the water can operate at extremely high [[temperature]]s without boiling to [[steam]]. This is important for the reactor as higher pressures allow for greater [[power]] output and higher [[thermal efficiency]].<ref>Encyclopaedia Britannica. (June 30 2015). ''Nuclear reactor'' [Online], Available: http://www.britannica.com/technology/nuclear-reactor/Containment-systems-and-major-nuclear-accidents#ref155186</ref> The pressure is maintained by the "pressurizer" (Figure 4), which acts to stabilize pressure changes caused by changes in electrical load.<Ref name=ine/>

Water enters the reactor at [[celsius|290°C]], and by the time it exits it is at around 325°C.<ref name=ine/> In order for it to remain a liquid at these temperatures, the pressure must be [[pascal|15 MPa]], or about 150 times [[atmospheric pressure]].<Ref name=wna/> By keeping the water in liquid form, the [[control rod]] system is simplified as they are able to be placed in from the top, rather than from the bottom like in a boiling water reactor. Therefore, if the power is lost in the plant, the electromagnetic system holding the rods will give out, and [[gravity]] will cause the rods to fall into the core, stopping the reaction.<ref name=ine/>

The hot water flowing from the reactor flows through inverted U-tubes (Figure 4) which acts as a [[heat exchanger]], heating up a secondary loop of water in what is called a "steam generator". This secondary loop is at a lower pressure so it is able to boil to [[steam]], which then passes through [[turbine]]s in order to [[generate electricity]]. Large reactors have up to ''4 steam generators'',<ref name=ine/> each of which may be larger than the reactor itself. 

The basic operation of a PWR can be seen below.

[[File:student-pwr.gif|800px|thumb|center|Figure 5. The basic cycle and water flow of a PWR.<ref>NRC. (June 30 2015). ''The Pressurized Water Reactor'' [Online], Available: http://www.nrc.gov/reading-rm/basic-ref/students/animated-pwr.html</ref>]]

==For Further Reading==
*[[Nuclear power]]
*[[Nuclear reactor]]
*[[Nuclear power plant]]
*[[Boiling water reactor]]
*[[Uranium]]
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==References==
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