Light-water reactor
The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron reactors are the most common type of nuclear reactor, and light-water reactors are the most common type of thermal-neutron reactor.
There are three varieties of light-water reactors: the pressurized water reactor (PWR), the boiling water reactor (BWR), and (most designs of) the supercritical water reactor (SCWR).
History
Early concepts and experiments
After the discoveries of
In May 1944, the first grams of enriched uranium ever produced reached criticality in the low power (LOPO) reactor at Los Alamos, which was used to estimate the critical mass of U235 to produce the atomic bomb.[1] LOPO cannot be considered as the first light-water reactor because its fuel was not a solid uranium compound cladded with corrosion-resistant material, but was composed of uranyl sulfate salt dissolved in water.[2] It is however the first aqueous homogeneous reactor and the first reactor using enriched uranium as fuel and ordinary water as a moderator.[1]
By the end of the war, following an idea of Alvin Weinberg, natural uranium fuel elements were arranged in a lattice in ordinary water at the top of the X10 reactor to evaluate the neutron multiplication factor.[3] The purpose of this experiment was to determine the feasibility of a nuclear reactor using light water as a moderator and coolant, and clad solid uranium as fuel. The results showed that, with a lightly enriched uranium, criticality could be reached.[4] This experiment was the first practical step toward the light-water reactor.
After World War II and with the availability of enriched uranium, new reactor concepts became feasible. In 1946, Eugene Wigner and Alvin Weinberg proposed and developed the concept of a reactor using enriched uranium as a fuel, and light water as a moderator and coolant.[3] This concept was proposed for a reactor whose purpose was to test the behavior of materials under neutron flux. This reactor, the Material Testing Reactor (MTR), was built in Idaho at INL and reached criticality on March 31, 1952.[5] For the design of this reactor, experiments were necessary, so a mock-up of the MTR was built at ORNL, to assess the hydraulic performances of the primary circuit and then to test its neutronic characteristics. This MTR mock-up, later called the Low Intensity Test Reactor (LITR), reached criticality on February 4, 1950[6] and was the world's first light-water reactor.[7]
Pressurized water reactors
Immediately after the end of
The Soviet Union independently developed a version of the PWR in the late 1950s, under the name of VVER. While functionally very similar to the American effort, it also has certain design distinctions from Western PWRs.
Boiling water reactor
Researcher
PIUS reactor
PIUS, standing for Process Inherent Ultimate Safety, was a Swedish design designed by ASEA-ATOM. It is a concept for a light-water reactor system.[8] Along with the SECURE reactor,[9] it relied on passive measures, not requiring operator actions or external energy supplies, to provide safe operation. No units were ever built.
OPEN100
In 2020, the Energy Impact Center announced publication of an open-sourced engineering design of a pressurized water reactor capable of producing 300 MWth/100 MWe of energy called OPEN100.[10]
Overview
The family of nuclear reactors known as light-water reactors (LWR), cooled and moderated using ordinary water, tend to be simpler and cheaper to build than other types of nuclear reactors[
The leaders in national experience with PWRs, offering reactors for export, are the United States (which offers the passively safe
Though
- Currently offered LWRs include the following
LWR statistics
This section needs to be updated.(July 2015) |
Data from the International Atomic Energy Agency in 2009:[11]
Reactors in operation. | 359 |
Reactors under construction. | 27 |
Number of countries with LWRs. | 27 |
Generating capacity ( gigawatts ).
|
328.4 |
Reactor design
The light-water reactor produces heat by controlled
In the boiling water reactor, the heat generated by fission turns the water into steam, which directly drives the power-generating turbines. But in the pressurized water reactor, the heat generated by fission is transferred to a secondary loop via a heat exchanger. Steam is produced in the secondary loop, and the secondary loop drives the power-generating turbines. In either case, after flowing through the turbines, the steam turns back into water in the condenser.[12]
-
Animated diagram of a boiling water reactor
-
Animated diagram of a pressurized water reactor
The water required to cool the condenser is taken from a nearby river or ocean. It is then pumped back into the river or ocean, in warmed condition. The heat can also be dissipated via a cooling tower into the atmosphere. The United States uses LWR reactors for electric power production, in comparison to the
Control
Control rods are usually combined into control rod assemblies — typically 20 rods for a commercial pressurized water reactor assembly — and inserted into guide tubes within a fuel element. A control rod is removed from or inserted into the central core of a nuclear reactor in order to control the number of neutrons which will split further uranium atoms. This in turn affects the thermal power of the reactor, the amount of steam generated, and hence the electricity produced. The control rods are partially removed from the core to allow a chain reaction to occur. The number of control rods inserted and the distance by which they are inserted can be varied to control the reactivity of the reactor.
Usually there are also other means of controlling reactivity. In the PWR design a soluble neutron absorber, usually boric acid, is added to the reactor coolant allowing the complete extraction of the control rods during stationary power operation ensuring an even power and flux distribution over the entire core. Operators of the BWR design use the coolant flow through the core to control reactivity by varying the speed of the reactor recirculation pumps. An increase in the coolant flow through the core improves the removal of steam bubbles, thus increasing the density of the coolant/moderator with the result of decreasing power.
Coolant
The light-water reactor also uses ordinary water to keep the reactor cooled. The cooling source, light water, is circulated past the reactor core to absorb the heat that it generates. The heat is carried away from the reactor and is then used to generate steam. Most reactor systems employ a cooling system that is physically separate from the water that will be boiled to produce pressurized steam for the
Many other reactors are also light-water cooled, notably the
Fuel
The use of ordinary water makes it necessary to do a certain amount of enrichment of the uranium fuel before the necessary criticality of the reactor can be maintained. The light-water reactor uses
The enriched UF6 is converted into
The finished fuel rods are grouped in special fuel assemblies that are then used to build up the nuclear fuel core of a power reactor. The metal used for the tubes depends on the design of the reactor –
Pressurized water reactor fuel consists of cylindrical rods put into bundles. A uranium oxide ceramic is formed into pellets and inserted into zirconium alloy tubes that are bundled together. The zirconium alloy tubes are about 1 cm in diameter, and the fuel cladding gap is filled with
In boiling water reactors, the fuel is similar to PWR fuel except that the bundles are "canned"; that is, there is a thin tube surrounding each bundle. This is primarily done to prevent local density variations from affecting
Moderator
A neutron moderator is a medium which reduces the velocity of
The light-water reactor uses ordinary
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 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 PWRs very stable. In event of a loss-of-coolant accident, the moderator is also lost and the active fission reaction will stop. Heat is still produced after the chain reaction stops from the radioactive byproducts of fission, at about 5% of rated power. This "decay heat" will continue for 1 to 3 years after shut down, whereupon the reactor finally reaches "full cold shutdown". Decay heat, while dangerous and strong enough to melt the core, is not nearly as intense as an active fission reaction. During the post shutdown period the reactor requires cooling water to be pumped or the reactor will overheat. If the temperature exceeds 2200 °C, cooling water will break down into hydrogen and oxygen, which can form a (chemically) explosive mixture. Decay heat is a major risk factor in LWR safety record.
See also
- Nuclear power
- Heavy water reactor
- List of nuclear reactors
- Light-water reactor sustainability
- Breeder Reactor
References
- ^ a b "Federation of American Scientists - Early reactor" (PDF). Retrieved 2012-12-30.
- ^ It also can be noted that as LOPO was designed to operate at zero power, and no means for cooling were necessary, so ordinary water served solely as a moderator.
- ^ a b "ORNL - An Account of Oak Ridge National Laboratory's Thirteen Nuclear Reactors" (PDF). p. 7. Retrieved 2012-12-28.
... Afterwards, responding to Weinberg's interest, the fuel elements were arranged in lattices in water and the multiplication factors determined. ...
- ^ "ORNL - History of the X10 Graphite Reactor". Archived from the original on 2012-12-11. Retrieved 2012-12-30.
- ^ "INEEL - Proving the principle" (PDF). Archived from the original (PDF) on 2012-03-05. Retrieved 2012-12-28.
- ^ "INEL - MTR handbook Appendix F (historical backgroup)" (PDF). p. 222. Archived from the original (PDF) on 2006-09-30. Retrieved 2012-12-31.
- ^ "DOE oral history presentation program - Interview of LITR operator transcript" (PDF). p. 4. Archived from the original (PDF) on 2013-05-14.
... We were so nervous because there had never been a reactor fueled with enriched uranium go critical before. ...
- ISBN 0-309-04395-6page 122
- ^ "GDM Marketing". Archived from the original on 2018-02-17. Retrieved 2018-02-16.
- ^ Proctor, Darrell (February 25, 2020). "Tech Guru's Plan—Fight Climate Change with Nuclear Power". Power Magazine. Retrieved October 6, 2021.
- ^ "IAEA - LWR". Archived from the original on 2009-02-25. Retrieved 2009-01-18.
- ^ "European Nuclear Society - Light water reactor". Archived from the original on 2017-12-05. Retrieved 2009-01-18.
- ^ "Light Water Reactors". Retrieved 2009-01-18.