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 fission, moderation and of the theoretical possibility of a nuclear chain reaction, early experimental results rapidly showed that natural uranium could only undergo a sustained chain reaction using graphite or heavy water as a moderator. While the world's first reactors (CP-1, X10 etc.) were successfully reaching criticality, uranium enrichment began to develop from theoretical concept to practical applications in order to meet the goal of the Manhattan Project, to build a nuclear explosive.
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. 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.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. It is however the first aqueous homogeneous reactor and the first reactor using enriched uranium as fuel and ordinary water as a moderator.
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. 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. 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. 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. 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 and was the world's first light-water reactor.
Pressurized water reactors
Immediately after the end of World War II the United States Navy started a program under the direction of Captain (later Admiral) Hyman Rickover, with the goal of nuclear propulsion for ships. It developed the first pressurized water reactors in the early 1950s, and led to the successful deployment of the first nuclear submarine, the .
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 Samuel Untermyer II led the effort to develop the BWR at the US National Reactor Testing Station (now the Idaho National Laboratory) in a series of tests called the BORAX experiments.
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.National Research Council (U.S.). Committee on Future Nuclear Power, Nuclear power: technical and institutional options for the future National Academies Press, 1992, page 122 Along with the SECURE reactor, 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.
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; due to these factors, they make up the vast majority of civil nuclear reactors and naval propulsion reactors in service throughout the world as of 2009. LWRs can be subdivided into three categories – pressurized water reactors (PWRs), boiling water reactors (BWRs), and supercritical water reactors (SCWRs). The SCWR remains hypothetical as of 2009; it is a Generation IV design that is still a light-water reactor, but it is only partially moderated by light water and exhibits certain characteristics of a fast neutron reactor.
The leaders in national experience with PWRs, offering reactors for export, are the United States (which offers the passively safe AP1000, a Westinghouse design, as well as several smaller, modular, passively safe PWRs, such as the Babcock & Wilcox MPower, and the NuScale MASLWR), the Russian Federation (offering both the VVER-1000 and the VVER-1200 for export), the Republic of France (offering the AREVA EPR for export), and Japan (offering the Mitsubishi Advanced Pressurized Water Reactor for export); in addition, both the People's Republic of China and the Republic of Korea are both noted to be rapidly ascending into the front rank of PWR-constructing nations as well, with the Chinese being engaged in a massive program of nuclear power expansion, and the Koreans currently designing and constructing their second generation of indigenous designs. The leaders in national experience with BWRs, offering reactors for export, are the United States and Japan, with the alliance of General Electric (of the US) and Hitachi (of Japan), offering both the Advanced Boiling Water Reactor (ABWR) and the Economic Simplified Boiling Water Reactor (ESBWR) for construction and export; in addition, Toshiba offers an ABWR variant for construction in Japan, as well. West Germany was also once a major player with BWRs. The other types of nuclear reactor in use for power generation are the heavy water moderated reactor, built by Canada (CANDU) and the Republic of India (AHWR), the advanced gas cooled reactor (AGCR), built by the United Kingdom, the liquid metal cooled reactor (LMFBR), built by the Russian Federation, the Republic of France, and Japan, and the graphite-moderated, water-cooled reactor (RBMK or LWGR), found exclusively within the Russian Federation and former Soviet states.
Though electricity generation capabilities are comparable between all these types of reactor, due to the aforementioned features, and the extensive experience with operations of the LWR, it is favored in the vast majority of new nuclear power plants. In addition, light-water reactors make up the vast majority of reactors that power naval nuclear-powered vessels. Four out of the five great powers with nuclear naval propulsion capacity use light-water reactors exclusively: the British Royal Navy, the Chinese People's Liberation Army Navy, the French Marine nationale, and the United States Navy. Only the Russian Federation's Navy has used a relative handful of liquid-metal cooled reactors in production vessels, specifically the Alfa class submarine, which used lead-bismuth eutectic as a reactor moderator and coolant, but the vast majority of Russian nuclear-powered boats and ships use light-water reactors exclusively. The reason for near exclusive LWR use aboard nuclear naval vessels is the level of inherent safety built into these types of reactors. Since light water is used as both a coolant and a neutron moderator in these reactors, if one of these reactors suffers damage due to military action, leading to a compromise of the reactor core's integrity, the resulting release of the light-water moderator will act to stop the nuclear reaction and shut the reactor down. This capability is known as a negative void coefficient of reactivity.
Currently offered LWRs include the following: ABWR AP1000 APR-1400 CPR-1000 EPR VVER
LWR statistics As of 2022, 379 out of 441 power reactors worldwide were light water reactors and 45 out of 51 under construction were of the light water reactor design. 306 reactors were Pressurized Light-Water Moderated and Cooled Reactors, 61 were Boiling Light-Water Cooled and Moderated Reactors, and 12 were Light-Water Cooled, Graphite Moderated Reactors. Their total combined net electrical capacity was 361,888 MW.
Reactor design
The light-water reactor produces heat by controlled nuclear fission. The nuclear reactor core is the portion of a nuclear reactor where the nuclear reactions take place. It mainly consists of nuclear fuel and control elements. The pencil-thin nuclear fuel rods, each about 12 feet (3.7 m) long, are grouped by the hundreds in bundles called fuel assemblies. Inside each fuel rod, pellets of uranium, or more commonly uranium oxide, are stacked end to end. The control elements, called control rods, are filled with pellets of substances like hafnium or cadmium that readily capture neutrons. When the control rods are lowered into the core, they absorb neutrons, which thus cannot take part in the chain reaction. On the converse, when the control rods are lifted out of the way, more neutrons strike the fissile uranium-235 or plutonium-239 nuclei in nearby fuel rods, and the chain reaction intensifies. All of this is enclosed in a water-filled steel pressure vessel, called the reactor vessel.
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.
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 heavy water reactors used in Canada.
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.