Editing Pressurized water reactor (section)

== Moderator ==
{{Main| Passive nuclear safety}}

Pressurized water reactors, like all [[thermal reactor]] designs, require 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 [[neutron moderator|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 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, 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 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 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, 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 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]].<ref>{{harvnb|Mosey|1990|pp=92–94}}</ref>

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 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 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, the [[supercritical water reactor]], is even less moderated. A less moderated neutron energy spectrum does worsen the capture/fission ratio for <sup>235</sup>U and especially <sup>239</sup>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.

=== Fuel ===
{{Main|Nuclear fuel}}

[[File:Nuclear fuel element.jpg|thumb|'''PWR fuel bundle''' This fuel bundle is from a pressurized water reactor of the nuclear passenger and cargo ship [[NS Savannah|NS ''Savannah'']]. Designed and built by [[Babcock & Wilcox]].]]

After enrichment, the [[uranium dioxide]] ({{chem|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.<ref>{{cite web|url=http://www.fusion.org.uk/techdocs/icfrm9_fortkard.pdf |title=Uses of Zirconium Alloys in Fusion Applications |last=Forty |first=C.B.A. |author2=P.J. Karditsas |publisher=EURATOM/UKAEA Fusion Association, Culham Science Center |access-date=2008-05-21 |url-status=dead |archive-url=https://web.archive.org/web/20090225155448/http://www.fusion.org.uk/techdocs/icfrm9_fortkard.pdf |archive-date=February 25, 2009 }}</ref> The finished fuel rods are grouped in fuel assemblies, called fuel bundles, that are then used to build 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&nbsp;×&nbsp;14 to 17&nbsp;×&nbsp;17. A PWR produces on the order of 900 to 1,600&nbsp;MW<sub>e</sub>. PWR fuel bundles are about 4&nbsp;meters in length.<ref>{{harvnb|Glasstone|Sesonske|1994|p=21}}</ref>

Refuelings for most commercial PWRs 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.<ref>{{harvnb|Duderstadt|Hamilton|1976|p=598}}</ref>

=== Control ===
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 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 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 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 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, [[boiling water reactor|BWR]]s have no boron in the reactor coolant and control the reactor power by adjusting the reactor coolant flow rate.