Pressurized Water Reactor (PWR) Vessel Explained
A pressurised water reactor ('pressurized water reactor' in American English) is a type of light water reactor used within a PWR nuclear power plant and also by nuclear submarines. Nuclear power plants are a type of thermal power plant because they generate electrical power using the heat (thermal energy) from a fission chain reaction. The fission reaction occurs within a reactor core, which is contained within a reactor vessel. The nuclear reactor can be considered the 'heart' of any nuclear power plant; it is housed within a containment building.
Pressurized Water Reactor Power Plant
Good to know - the 'reactor' refers to the reactor vessel and everything it contains.
Good to know - 'light water' is 'normal' water (the ordinary water you have at home). The distinction is made because some reactors use 'heavy water' (where hydrogen in the water molecules is partly or wholly replaced by the isotope deuterium).
Good to know - the terms 'power plant' and 'power station' are used interchangeably i.e. their meaning is the same. For example, a 'PWR power plant' is also a 'PWR power station'.
PWR Power Plant Schematic
Light Water Reactors
PWR power plants are a type of light water reactor power plant; they are also the most common type of nuclear power plant. The second most common type uses a light water reactor, but it is the boiling water reactor (BWR) type. Light water reactors are used as the primary heat source in approx. 375 of the world's approx. 436 nuclear power plants (approximations are necessary as the figure changes continuously).
PWR Power Plant Parts
PWR Pressure Vessel Design Features
A typical PWR vessel will operate at pressures of approx. 150 bar (2175 psi) and at a temperature range of between 290°C-330°C (554°F-626°F); these factors are dictated by the reactor coolant system, which flows through the vessel. Reactor vessels may be up to 14m in length, 5.5m in diameter, and weigh over 600 tonnes. The main pressure boundary (vessel body) is usually manufactured from steel, as this is a mechanically strong metal alloy.
Good to know - although 150 bar (2175 psi) may seem to be a lot of pressure, and 330°C (626°F) a high temperature, it is worth noting that a coal fired power station watertube boiler may operate at over 220 bar (3190 psi), and over 550°C (990°F).
PWR Reactor Vessel Cross-Section
What are the main parts of a PWR reactor vessel (basic)?
The reactor vessel is a cylindrical vessel with a hemispherical bottom and a removable hemispherical top head. The top head is removable to allow for refuelling of the reactor. There is one inlet nozzle (from the cold leg) and one outlet nozzle (to the hot leg) for each reactor coolant system loop. The reactor vessel is constructed of manganese molybdenum steel, and all surfaces that come into contact with reactor coolant are clad with stainless steel to increase corrosion resistance.
The core barrel slides down inside the reactor vessel and houses the fuel. Toward the bottom of the core barrel, there is a lower core support plate on which the fuel assemblies sit. The core barrel and all of the lower internals hang inside the reactor vessel from the internal support ledge. On the outside of the core barrel are irradiation specimen holders in which samples of the material used to manufacture the vessel will be placed. At periodic time intervals, some of these samples will be removed and tested to see how radiation from the fuel has affected the strength of the material.
Good to know - Uranium dioxide is compressed to form ceramic fuel pellets, which are inserted into rods to form 'fuel rods' (also called 'fuel pins'); a typical fuel rod may be 4m in length. Fuel rods are then assembled to form a fuel assembly. Fuel assemblies are installed neighbouring each other -with space left for coolant- to form the reactor core.
Pressurized Water Reactor Fuel Assembly
The upper internals package sits on top of the fuel; it contains the guide columns to guide the control rods when they are pulled or inserted from/to the fuel. The upper internals package also prevents the core from moving upwards within the vessel due to the force from the coolant flowing through the assemblies.
What are the main parts of a PWR reactor vessel (detailed)?
Hot Leg
Primary coolant is heated in the reactor. Coolant that has been heated by the reactor is referred to as ‘hot’. The hot primary coolant circuit is referred to as the ‘hot leg’. The hot leg is the part of the coolant circuit that extends from the reactor vessel primary coolant outlet, to the steam generator (SG) inlet. saVRee's 3D model reactor has two hot legs and consequently two primary coolant discharge nozzles. The temperature in the hot leg is usually referred to as 'Thot'.
Good to know - steam generators are the only piece of equipment within the power plant that is used to create steam.
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Steam Generator (SG)
Good to know - the terms 'primary coolant loop' and 'primary coolant circuit' are used interchangeably i.e. their meaning is the same; the same is true of the 'secondary coolant loop' and 'secondary coolant circuit'. Both terms may also be shortened to 'Primary Loop' and 'Secondary Loop', or 'Primary System' and 'Secondary System'.
Good to know - PWR power plants maintain the primary coolant loop under pressure to prevent the coolant boiling (the high pressure ensures that the water remains liquid even at elevated temperatures).
Cold Leg
Primary coolant that has transferred its heat energy to the secondary coolant circuit, is referred to as ‘cold’. The cold primary coolant circuit is referred to as the ‘cold leg’. The cold leg extends from the reactor coolant pump (RCP) discharge to the inlet of the reactor vesel. The transfer leg connects the steam generator discharge to the reactor coolant pump inlet. Primary coolant is circulated by a reactor coolant pump (RCP), or several reactor coolant pumps. The temperature in the cold leg is usually referred to as 'Tcold'.
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Reactor Coolant Pump (RCP)
Body/Shell
The body of the reactor vessel forms its largest pressure boundary; it contains the fuel rods and control rods. A typical reactor vessel body is cylindrical in shape with a removable head that allows for fuel loading. Reactor bodies may operate at pressures up to approx. 170 bar (2,465 psi) at 350⁰C (662⁰F).
Control Rod Drive (CRD)
Control rod drives are used to extend and retract control rods into, or out of, a reactor. Extending or retracting the control rods will respectively decrease or increase the fission reaction rate. Upon loss of power, the control rod drive disengages, and the control rods are lowered due to gravity; the fail-safe position of the control drive is thus that the control rods are fully engaged and the rate of fission restricted. Although the reactor may now be subcritical (below criticality), decay heat is still present even after the control rods are fully inserted. Decay heat is removed via natural circulation of the primary coolant fluid. There are many safety systems used within a PWR power plant, but the control rod drive, control rods, and boron concentration within the primary coolant, are the main means for controlling a reactor's chain reaction.
Core Barrel
The core barrel has a long cylindrical shape and is manufactured from corrosive resistant materials. Fuel assembles are fixed to the lower core plate, which is welded to the base of the core barrel. The weight of the barrel is transferred to the reactor body; it forms part of the lower core support structure. As primary coolant enters the reactor, it flows downwards between the body and the barrel, this space is referred to as the ‘downcomer’.
Fuel Assembly
Nuclear reactors use Uranium as nuclear fuel, although there are different isotopes of Uranium. Uranium isotope U238 forms the majority of the nuclear fuel (>90%), with a small amount of U235 (2-5%) and very small amount of U234 (<0.1%); U235 is the Uranium isotope that is used during the fission reaction.
Processed uranium (enriched Uranium) is encapsulated in ceramic cylindrical pellets. These pellets are inserted into long thin metal tubes called ‘fuel rods’. Fuel rods are bundled together to form a fuel assembly. A typical reactor may have several hundred assemblies depending upon the power plant’s megawatt (MW) capacity. Fuel rods are submerged in water to cool and moderate (reduce the number of neutrons produced by the fission reaction) the nuclear reaction. The cooling water used may be light water or heavy water, depending upon the reactor design.
Fuel Pellet Within Zirconium Alloy Fuel Rod
Good to know - 'light water' and 'heavy water' are termed 'moderators' due to their effect upon the fission reaction; they are also described simply as 'coolant' although they actually have a dual-purpose because they act as a neutron moderator and coolant.
Good to know - there are many nuclear reactor types, but relatively few commercial nuclear reactors types (those that can be used to generate electricity in exchange for money); the most common types are the PWR, BWR, and CANDU, reactors.
Control Rod
Control rods are an integral safety and control item for all nuclear reactors. A control rod consists of long, cylindrical shaped, rod, manufactured from neutron absorbing material (boron, cadmium etc.). Control rods are bundled together to form clusters, which are further grouped to form a bank/group. Control rods absorb neutrons, which reduces the number of neutrons available for fission, reducing the reactor's reactivity, thus reducing the amount of heat generated by the reactor. Control rods can be extended or retracted into the reactor using control rod drives. Extending or retracting the control rods will respectively decrease or increase the reactivity of the reactor.
Neutron Reflector / Absorber
Neutron reflectors may be of the core baffle or heavy reflector design. The core baffle design uses vertical, stainless-steel baffles, and horizontal formers, that have been installed around the reactor core. A heavy reflector consists of multiple stainless-steel slabs that have been stacked vertically around the reactor core. Both reflector designs reduce the effects of irradiation embrittlement on the reactor body by reducing neutron leakage (neutrons are reflected into the fuel assemblies, which increases the reactor’s overall efficiency).
Vessel Head
The reactor vessel head is attached to the top of the reactor vessel body. Penetrations in the head are required to allow the control rod drives to attach to the control rods. Additional penetrations are required for the reactor level measurement probes.
Regulation
The field of nuclear energy is highly regulated and documented. If you would like to learn more about this topic, a good place to start is the nuclear regulatory commission website (although there are many more sites that serve as excellent sources of information).
Additional Resources
https://en.wikipedia.org/wiki/Nuclear_reactor_core
https://www.world-nuclear.org/nuclear-essentials/how-does-a-nuclear-reactor-work.aspx
https://www.energy.gov/ne/articles/nuclear-101-how-does-nuclear-reactor-work