Editing Pressurized water reactor (section)

== Reactor ==
[[File:Reactorvessel.gif|thumb|upright=1.5|PWR [[reactor pressure vessel]]]]

=== Coolant ===

[[Water|Light water]] is used as the primary coolant in a PWR. Water enters through the bottom of the reactor's core at about {{convert|548|K|lk=in}} and is heated as it flows upwards through the reactor core to a temperature of about {{convert|588|K}}. The water remains liquid despite the high temperature due to the high pressure in the primary coolant loop, usually around 155 [[bar (unit)|bar]] (15.5&nbsp;[[megapascal|MPa]] 153&nbsp;[[Atmosphere (unit)|atm]], 2,250&nbsp;[[Pounds per square inch|psi]]).
The water in a PWR cannot exceed a temperature of {{convert|647|K}} or a pressure of 22.064&nbsp;MPa (3200 psi or 218 atm), because those are the [[critical point (thermodynamics)|critical point]] of water.<ref>International Association for the Properties of Water and Steam, 2007.</ref> [[Supercritical water reactor]]s are (as of 2022) only a proposed concept in which the coolant would never leave the [[supercritical fluid|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.

=== Pressurizer ===
{{Main|Pressurizer_(nuclear_power)|l1=Pressurizer}}

Pressure in 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 {{convert|155|bar|MPa}}, the pressurizer temperature is maintained at 345&nbsp;°C (653&nbsp;°F), which gives a subcooling margin (the difference between the pressurizer temperature and the highest temperature in the reactor core) of 30&nbsp;°C (54&nbsp;°F). As 345&nbsp;°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 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.<ref>{{harvnb|Glasstone|Sesonske|1994|p=767}}</ref>

=== Pumps ===

The coolant is pumped around the primary circuit by powerful pumps.<ref>{{harvnb|Tong|1988|p=175}}</ref> These pumps have a rate of ~100,000 gallons of coolant per minute. After picking up heat as it passes through 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&nbsp;MPa (60&nbsp;atm, 900&nbsp;[[Pounds per square inch|psia]]), 275&nbsp;°C (530&nbsp;°F) — for use in the steam turbine. The cooled primary coolant is then returned to the reactor vessel to be heated again.