Editing Pressurized water reactor (section)

== Design ==
[[File:PressurizedWaterReactor.gif|thumb|upright=1.8|Pictorial explanation of power transfer in a pressurized water reactor. Primary coolant is in orange and the secondary coolant (steam and later feedwater) is in blue.]]
[[File:HPR1000, reactor coolant system.png|thumb|Primary coolant system showing [[reactor pressure vessel]] (red), [[Steam generator (nuclear power)|steam generators]] (purple), [[Pressurizer_(nuclear_power)|Pressurizer]] (blue), and pumps (green) in the three coolant loop [[Hualong One]] design]]

[[Nuclear fuel]] in the [[reactor pressure vessel]] is engaged in a controlled [[Nuclear chain reaction|fission chain reaction]], which produces heat, heating the water in the primary coolant loop by thermal conduction through the fuel cladding.<ref name="energy.gov">{{Cite web |title=NUCLEAR 101: How Does a Nuclear Reactor Work? |author= |work=Energy.gov |date= |access-date=20 December 2022 |url= https://www.energy.gov/ne/articles/nuclear-101-how-does-nuclear-reactor-work}}</ref><ref>{{harvnb|Jacquemain|2015|pp=12,21}}</ref> The hot primary coolant is pumped into a [[heat exchanger]] called the [[Steam generator (nuclear power)|steam generator]], where it flows through several thousand small tubes.<ref>{{harvnb|Riznic|2017|p=3}}</ref> Heat is transferred through 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.<ref name="energy.gov"/>{{fv|date=January 2023}} Some common steam generator arrangements are u-tubes or single pass heat exchangers.{{Citation needed|date=September 2009}}

In a nuclear power station, 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 (heat transfer)|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 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.<ref>{{harvnb|Glasstone|Sesonske|1994|p=769}}</ref>

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 a shaft used for [[Nuclear marine propulsion|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.{{Citation needed|date=September 2009}}

Two things are characteristic for the pressurized water reactor (PWR) when compared with other reactor types: coolant loop separation from 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 reactor]]s use substances other than water for coolant and moderator (e.g. sodium in its liquid state as coolant or graphite as a moderator). The pressure in the primary coolant loop is typically {{convert|15|-|16|MPa|bar|lk=on}}, which is notably higher than in other [[nuclear reactor]]s, 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.<ref>{{harvnb|Duderstadt|Hamilton|1976|pp=91–92}}</ref>