Pressurized water reactor
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Pressurized water reactors (PWRs) (also VVER if of Russian design) are generation II nuclear power reactors that use ordinary water under high pressure as coolant and neutron moderator. The primary coolant loop is kept under high pressure to prevent the water from boiling, hence the name. PWRs are one of the most common types of reactors and are widely used all over the world. More than 230 of them are in use to generate electric power, and several hundred more for naval propulsion. They were originally designed by the Bettis Atomic Power Laboratory as a nuclear submarine power plant.
Heat from small PWRs has also been used for heating in polar regions, see Army Nuclear Power Program.
The Three Mile Island accident occurred in a PWR manufactured by Babcock & Wilcox.
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[edit] Overview
A PWR works because the nuclear fuel in the reactor vessel is engaged in a chain reaction, which produces heat as the main goal of the entire setup. That heats the water in the primary coolant loop by thermal conduction through the fuel cladding. (The primary coolant loop is shown in the schematic as a red dashed line.) The hot water is pumped into a certain type of heat exchanger called steam generator, which allows the primary coolant to heat up the secondary coolant (shown as the loop steam generator → turbine → condenser). The transfer of heat is accomplished without mixing the two fluids since the primary coolant is necessarily radioactive, but it is desirable to avoid this for the secondary coolant. The steam formed in the steam generator is allowed to flow through a steam turbine, and the energy extracted by the turbine is used to drive an electric generator. In nuclear ships and submarines, the steam is fed through a steam turbine connected to a set of reduction gears to a shaft used for the propulsion. In a nuclear power station, the steam is fed through a steam turbine which drives a generator connected to the electric grid for distribution, as shown above. After passing through the turbine the secondary coolant (water-steam mixture) is cooled down and condensed in a condenser before being fed into the steam generator again. This reduces the pressure at the turbine outlet, which helps improve the thermal efficiency.
Two things are characteristic for the pressurized water reactor (PWR) when compared with other reactor types:
- In a PWR, there are two separate coolant loops (primary and secondary), which are both filled with ordinary water (also called light water). A boiling water reactor, by contrast, has only one coolant loop, while more exotic designs such as breeder reactors use substances other than water (i.e., liquid metal as sodium) for the task.
- The pressure in the primary coolant loop is at typically 16 Megapascal, notably higher than in other nuclear reactors. As an effect of this, the gas laws guarantee that the primary coolant loop's water will never boil during the normal operation of the reactor. By contrast, in a boiling water reactor the primary coolant is allowed to boil and it feeds the turbine directly without the use of a secondary loop.
[edit] PWR reactor design
[edit] Coolant
Ordinary water is used as primary coolant in a PWR and flows through the reactor at a temperature of roughly 315 °C (600 °F). The water remains liquid despite the high temperature due to the high pressure in the primary coolant loop (usually around 2200 psig [15 MPa, 150 atm]). The primary coolant loop is used to heat water in a secondary circuit that becomes saturated steam (in most designs 900 psia [6.2 MPa, 60 atm], 275 °C [530 °F]) for use in the steam turbine.
Although coolant flow rate in commercial PWRs is constant, it is not in nuclear reactors used on U.S. Navy ships.
[edit] Moderator
Pressurized water reactors, like thermal reactor designs, require the fast fission neutrons in the reactor to be slowed down (a process called moderation) in order to sustain its chain reaction. In PWRs the coolant water is used as a 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 any increase in temperature causes the water to expand and become less dense; thereby reducing the extent to which neutrons are slowed down and hence reducing the reactivity in the reactor. Therefore, if reactor activity 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. In contrast, the RBMK reactor design used at Chernobyl (using graphite instead of water as the moderator) greatly increases heat generation when coolant water temperatures increase, making them very unstable. This flaw in the RBMK reactor design is generally seen as one of several causes of the Chernobyl accident.
[edit] Fuel
The uranium used in PWR fuel is usually enriched several percent in 235U. After enrichment the uranium dioxide (UO2) powder is fired in a high-temperature, sintering furnace to create hard, ceramic pellets of enriched uranium dioxide. The cylindrical pellets are then put into tubes of a corrosion-resistant zirconium metal alloy (Zircaloy) which are backfilled with helium to aid heat conduction and detect leakages. The finished fuel rods are grouped in fuel assemblies, called fuel bundles, that are then used to build the core of the reactor. As a safety measure PWR designs do not contain enough fissile uranium to sustain a prompt critical chain reaction (i.e, substained only by prompt neutron). Avoiding prompt criticality is important as a prompt critical chain reaction could very rapidly produce enough energy to damage or even melt the reactor (as is suspected to have occurred during the accident at the Chernobyl plant). 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 tonnes of uranium in all. Generally, the fuel bundles consist of fuel rods bundled 14x14 to 17x17. A PWR produces on the order of 900 to 1500 MWe. PWR fuel bundles are about 4 meters in length.
[edit] Control
Reactor power in most commercial PWR's is normally controlled by varying the concentration of boric acid dissolved in the primary reactor coolant. The 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 top directly into the fuel bundles, are normally only used for startup and shut down operations. In contrast, BWRs have no boron in the reactor coolant and control the reactor power by adjusting the reactor coolant flow rate. This is an advantage for the BWR design because boric acid is very corrosive and the complex charging and letdown system is not required. However, as a backup to control-blade insertion for the reactor shutdown, most commercial BWRs do have an emergency shutdown system which involves injecting a highly concentrated boric acid solution into the primary coolant circuit. CANDU reactors also inject boron as a backup means to shut down the nuclear chain reaction.
Power in most naval nuclear reactors is regulated by the steam demand.
[edit] Advantages
- PWR reactors are very stable due to their tendency to produce less power as temperatures increase, this helps reduce the chance of losing control of the chain reaction.
- PWR reactors can be operated with a core containing less fissile material than is required for them to go prompt critical. This significantly reduces the chance that the reactor will run out of control and makes PWR designs very safe.
- Because PWR reactors use enriched uranium as fuel they can use ordinary water as a moderator rather than the much more expensive heavy water.
- PWR has two coolant loops, so the water in the secondary loop is not contaminated by radioactive materials.
[edit] Disadvantages
- The coolant water must be heavily pressurized to remain liquid at high temperatures. This puts strong requirements on the piping and pressure vessel and hence increases construction costs.
- Most pressurized water reactors cannot be refuelled while operating. This limits the efficiency of the reactor and also means it has to go offline for comparably long periods of time (some weeks).
- The very hot water coolant with boric acid dissolved in it is corrosive to steel, causing radioactive corrosion products to circulate in the primary coolant loop. This not only limits the lifetime of the reactor, but the systems that filter out the corrosion products add significantly to the overall cost of the reactor and radiation exposure.
- Water absorbs neutrons making it necessary to enrich the uranium fuel, which increases the costs of fuel production. If heavy water is used it is possible to operate the reactor with natural uranium, but the production of heavy water requires large amounts of energy and is hence expensive.
- Because water acts as a neutron moderator it is not possible to build a fast neutron reactor with a PWR design. For this reason it is not possible to build a fast breeder reactor with water coolant.
- Because the reactor produces energy more slowly at higher temperatures, a sudden cooling of the reactor coolant could increase power production until safety systems shut down the reactor (OTΔT trip).
[edit] See also
[edit] Next generation designs
- European Pressurized Reactor (EPR)
- Westinghouse Advanced Passive 1000 (AP1000)
