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Nuclear reactor
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Nuclear reactor

Nuclear power station at Leibstadt, Switzerland

A nuclear reactor is an apparatus in which nuclear fission chain reactions are initiated, controlled, and sustained at a steady rate. Nuclear reactors provide heat for electricity generation, domestic and industrial heating, desalination, and naval propulsion. They also have many research applications including providing a source of neutrons and creating various radioactive isotopes.

Although the term 'nuclear reactor' could also refer to a nuclear fusion reactor, the term normally refers only to nuclear fission devices.

Nuclear power can also be generated in a Radioisotope thermoelectric generator, which produces heat through subcritical radioactive decay rather than fission in a near-critical mass.

Table of contents
1 Basic science
2 Types of reactors
3 Nuclear fuel cycle
4 History
5 Benefits and disadvantages
6 Natural nuclear reactors
7 List of atomic energy groups
8 References and links

Basic science

To provide the power for an electric generator, nuclear power plants get heat from nuclear fission. In this process, the nucleus of a heavy fuel element such as uranium absorbs a slow-moving free neutron, becomes unstable, and then splits into two smaller atoms.

The fission process for a uranium atoms yields two smaller atoms, one to three fast-moving free neutrons, plus an amount of energy.

Uranium fission releases more neutrons than it requires. Therefore, the reaction can become self sustaining--an enhanced, controlled radioactivity, caused by a chain reaction.

The newly-released fast neutrons must be slowed down (moderated) before they can be absorbed by the next fuel atom. This slowing down process is caused by collisions of the neutrons with atoms of an introduced substance called a moderator.

In the vast majority of the world's nuclear power plants, heat energy generated by fissioning uranium fuel is collected in purified water and is carried away from the reactor's core either as steam in boiling water reactors or as superheated water in pressurized-water reactors.

In a pressurized-water reactor, the High Temperature water in the primary cooling loop is used to transfer heat energy to a secondary loop for the creation of steam.

In either a boiling-water or pressurized-water installation, steam under high pressure is the medium used to transfer the nuclear reactor's heat energy to a turbine that mechanically turns an electric generator.

Boiling-water and pressurized-water reactors are called light-water reactors, because they utilize ordinary water as the moderator. In all light-water reactors to date this water is also used to transfer the heat energy from reactor to turbine in the electricity generation process. In other reactor designs the heat energy may be transferred by light water, pressurized heavy water, gas, or another cooling substance.

The amount of energy in the reservoir of nuclear fuel is frequently expressed in terms of "full-power days," which is the number of 24-hour periods (days) a reactor is scheduled for operation at full power output for the generation of heat energy. The number of full power days in a reactor's operating cycle (between refueling outage times) is related to the amount of fissile uranium-235 (U-235) contained in the fuel assemblies at the beginning of the cycle. A higher percentage of U-235 in the core at the beginning of a cycle will permit the reactor to be run for a greater number of full power days.

At the end of the operating cycle, the fuel in some of the assemblies is "spent," and it is discharged and replaced with new (fresh) fuel assemblies. The fraction of the reactor's fuel core replaced during refueling is typically one-fourth for a boiling-water reactor and one-third for a pressurized-water reactor.

The amount of energy extracted from nuclear fuel is called its "burn up," which is expressed in terms of the heat energy produced per initial unit of fuel weight. Burn up is commonly expressed as megawatt days thermal per metric ton of initial heavy metal.

Types of reactors

A number of reactor technologies have been developed. There are two basic types of reactors. They use different speeds of neutrons in the reactor.

Thermal power reactors can again be divided into three types, depending on whether they use pressurised fuel channels, a large pressure vessel or gas cooling.

Most commercial and naval reactors use a large pressure vessel holding steam heated by the reactor. This serves as a layer of shielding and containment.

The RBMK and CANDU types use pressurised channels. Channel-type reactors can be refuelled under load, which has advantages and disadvantages discussed under CANDU_reactor.

Gas-cooled reactors are cooled by a circulating inert gas, usually Helium, but Nitrogen and Carbon Dioxide have also been used. Plans to utilize the heat vary. Some reactors run hot enough that the gas can directly power a gas turbine. Older designs usually run the gas through a heat exchanger to make steam for a steam turbine.

The most common modern gas-cooled design is a Pebble bed reactor. Pebble-bed reactors can be designed to be safe even if all equipment fails. Basically, as the core heats, the reactor generates less power. Since the fuel elements are ceramic, they are unaffected by the higher heat. Pebble bed reactors have been designed to use both slow and fast neutron technology, and also to breed power isotopes. All pebble bed modular reactorss designed to date can also be refueled under load.

Most designs for fast power reactors have been cooled by liquid metal, usually molten Sodium. They have also been of two types, called pool and loop reactors.

Current families of reactors

Obsolete types still in service

Advanced reactors

More than a dozen advanced reactor designs are in various stages of development. Some are evolutionary from the PWR, BWR and CANDU designs above, some are more radical departures. The former include the Advanced Boiling Water Reactor, two of which are now operating with others under construction. The best-known radical new design is the Pebble Bed Modular Reactor (PBMR), a high temperature gas cooled reactor.

Nuclear fuel cycle

All nuclear reactors need fissionable material to operate. Uranium is currently (2004) US$52/Kg ($26/lb), and has an energy density per unit of mass of about a million times that of oil. No shortage exists or is anticipated. If land-based reserves are exhausted, seawater has enough uranium to power the world's current industrial civilization for a few hundred thousand years. The Japanese have an active project to extract uranium from seawater, to reduce their dependence on imports for energy.

Thermal reactors generally depend on refined and enriched uranium. Some nuclear reactors can operate with a mixture of plutonium and uranium. The process by which uranium ore is mined, processed, enriched, used, possibly reprocessed and disposed of is known as the nuclear fuel cycle.

Reactors have also been constructed to breed thorium-232 into U-233. Thorium is about three times as abundant in the Earth's crust as uranium.

Reactor waste is poisonous, but also compact. A nuclear reactor generates just a couple of cubic meters of waste per gigawatt year. After 600 years, reactor waste is no more radioactive than natural ores. The longer-term health hazards are heavy-metal toxicity and low-level radiation, problems already managed by human societies in order to use other heavy metals.

The most significant problem with waste at this time (2004) is that because of regulatory delays and civil protests, most existing waste is stored in cooling pools next to power reactors, rather than in safe geological storage.


Enrico Fermi and Leó Szilárd were the first to build a nuclear pile and demonstrate a controlled chain reaction. In 1955 they shared a joint patent for the nuclear reactor, issued by the U.S. Patent Office.

The first nuclear reactors were used to generate plutonium for nuclear weapons. Additional reactors were used in the navy (United States Naval reactor ) In the mid-1950s, both the Soviet Union and western countries were expanding their nuclear research to include non-military uses of the atom. However, as with the military program, much of the non-military work was done in secret. On December 20, 1951, electric power from a nuclear powered generater was produced for the first time at Experimental Breeder Reactor-I (EBR-1) located near Arco, Idaho. On June 27, 1954, the world's first nuclear power plant generated electricity but no headlines--at least, not in the West. According to the Uranium Institute (London, England), the first reactor to generate electricity for commercial use was at Obninsk, Kaluga Oblast, Russia. The Shippingport reactor (in Pennsylvania) was the first commercial nuclear generator to become operational in the United States. The Shippingport reactor was ordered in 1953 and began commercial operation in 1957.

Lots of construction in 60s and 70s (oil crisis influenced) - need some numbers here

In the aftermath of the 1979 Three Mile Island accident, the U.S. nuclear market was the first to deteriorate. No new nuclear plants have been ordered in the USA since then.

Negative influence of the 1986 Chernobyl accident increasing regulations increased costs.

need dates, declining construction numbers, reference to legislation in US

In 1997, a total of 78 reactors were either under construction, planned, or indefinitely deferred. These units have a combined capacity of 67,484 MWe, approximately 25 percent of the total capacity already in existence. However, only 45 reactors were under construction worldwide. The remaining 33 units are either being planned or indefinitely deferred. Three U.S. units are not projected to come on-line. Some experts have predicted that Watts Bar 1, which came on-line in 1997, will be the last U.S. commercial nuclear reactor to go on-line. Other experts, however, predict that electricity shortages will revamp the demand for nuclear power plants.

need more recent figures

As of 2003, the immediate future of the industry in many countries still appeared uncertain, the most notable exceptions being Japan, China and India, all actively developing both fast and thermal technology, South Korea, developing thermal technology only, and South Africa, developing the Pebble Bed Modular Reactor (PBMR). As of the early 21st century, nuclear power is of particular interest to both China and India because their rapidly growing economies are requiring increasing amounts of power for which their current infrastructure makes difficult to supply.

Benefits and disadvantages

Proponents of nuclear power point out that the technology emits virtually no airborne pollutants, and overall far less waste material than fossil fuel based power plants. Of course the relatively smaller amount of waste is in the form of highly radioactive spent fuels, which need to be handled with great care and forethought due to the long half-lifes of the radioactive isotopes found in the waste.

Another concern is that civilian nuclear technology could be used to create fissile materials for use in nuclear weapons. This concern is known as nuclear proliferation, and is a major reactor design criterion. While the enriched uranium used in most nuclear reactors is not sufficiently concentrated enough to build a bomb, the technology used to enrich uranium could be used to make highly enriched uranium needed to build a bomb. In addition, breeder reactor designs such as CANDU can be used to generate plutonium for bomb making materials (it is believed that the nuclear programs of India and Pakistan used CANDU-like reactors to produce the fissinables for their weapons).

Critics of nuclear power assert that any of the environmental benefits are outweighed by safety concerns and by costs related to the actual construction and operation of nuclear power plants, including spent fuel disposition and plant retirement costs. Proponents of nuclear power maintain that nuclear energy is the only power source which explicitly factors the estimated cost of waste containment and plant decommissioning into its overall cost, and that the quoted cost of fossil fuel plants is deceptively low for this reason. Nuclear power does have very useful additional advantages such as the production of radioisotopes (used in medicine and food preservation), though the demand for these products can be satisfied by a relatively small number of plants.

The storage and disposal of nuclear waste is significant. Because of potential harm from radiation, the spent nuclear fuel must be stored in shielded basins of water, or in dry storage vaults or containers until its radioactivity decreases naturally ("decays") to safe levels. This can take days or thousands of years, depending on the type of fuel. Most waste is currently stored in temporary storage sites, requiring constant maintenance, while suitable permanent disposal methods are discussed. See the article on the nuclear fuel cycle for more information.

A large disadvantage for the use of nuclear reactors is the perceived threat of an accident or terrorist attack and resulting exposure to radiation. Proponents contend that the potential for a meltdown as in Chernobyl is very small due to the excessive care taken to design adequate safety systems. Even in an accident such as Three Mile Island, the containment vessels were never breached, so that very little radiation was exposed to environment.

Low-dose radiation released under normal operating conditions--fission reactors produce gases such as iodine-131 or krypton-85 which have to be stored on-site for several half-lives until they have decayed to levels officially regarded as safe--or during waste spills is also a concern, but proponents point out that the radioactive contamination released from a nuclear reactor under normal circumstances is less than the exposure from the waste of a coal-fired plant.

Environmental concerns

The emissions problems of fossil fuels go beyond the area of greenhouse gases to include acid gases (sulfur dioxide and nitrogen oxides), particulates, heavy metals (notably mercury, but also including radioactive materials), and solid wastes such as ash. Some of these including nitrogen oxides are also greenhouse gases. Nuclear power produces essentially none of these wastes beyond spent fuels, a unique solid waste problem. In volume spent fuels from nuclear power plants are roughly a million times smaller than fossil fuel solid wastes. However, because spent nuclear fuels are radioactive, they are pound for pound a more substantial problem (see nuclear waste).

As of 2003, the United States accumulated about 49,000 metric tons of spent nuclear fuel from nuclear reactors. After 10,000 years of radioactive decay, according to United States Environmental Protection Agency standards, the spent nuclear fuel no longer poses a threat to public health and safety.

The dangers of nuclear power must also be weighed against the dangers of other methods of electricity generation. See environmental concerns with electricity generation for discussion of this issue.

Economic barriers

In the U.S., a single nuclear power plant is significantly more expensive to build than a single steam-based coal-fired plant. A coal plant is itself more expensive to build than a single natural gas-fired combined-cycle plant. Although the cost per megawatt for a nuclear power plant is comparable to a coal-fired plant and less than a natural gas plant, the smallest nuclear power plant that can be built is much larger than the smallest natural gas power plant, making it possible for a utility to build natural gas plants in much smaller increments.

In the U.S., licensing, inspection and certification delays add large amounts of time and cost to the construction of a nuclear plant. These delays and costs are not present when building either gas-fired or coal-fired plants. Because a power plant does not earn money during construction, longer construction times translate directly into higher interest charges on borrowed construction funds.

In the U.S., these charges require that coal and nuclear power plants must operate less-expensively than natural gas plants in order to be built. In general, coal and nuclear plants have the same operating costs (operations and maintenance plus fuel costs), however nuclear and coal differ in the source of those costs. Nuclear has lower fuel costs but higher operating and maintenance costs than coal. In recent times in the United States these operating costs have not been low enough for nuclear to repay its high investment costs. Thus new nuclear reactors have not been built in the United States. Coal's operating cost advantages have only rarely been sufficient to encourage the construction of new coal based power generation. Around 90 to 95 percent of new power plant construction in the United States has been natural gas-fired. These numbers exclude capacity expansions at existing coal and nuclear units.

Both the nuclear and coal industries must reduce new plant investment costs and construction time. The burden is clearly higher on nuclear producers than on coal producers, because investment costs are higher for nuclear plants with no visible advantage in operating costs over coal. The burden on operating costs on nuclear power plants is also greater with operation and maintenance costs particularly important simply because operation and maintenance costs are a large portion of nuclear operating costs.

In Japan and France, construction costs and delays are significantly less because of streamlined government licensing and certification procedures. In France, one model of reactor was type-certified, using a safety engineering process similar to the process used to certify aircraft models for safety. That is, rather than licensing individual reactors, the regulatory agency certified a particular design and its construction process to produce safe reactors. This seems like good public policy, because of the good safety record of commercial aircraft. U.S. law permits type-licensing of reactors, but no type license has ever been issued by a U.S. nuclear regulatory agency.

Given the financial disadvantages of nuclear power in the U.S., it is understandable that the nuclear industry also has sought to find additional benefits to using nuclear power. Because coal fired plants produce more airborne emissions, clearly the price differential accepted between nuclear and coal based power would be greater than the acceptable difference between nuclear power and natural gas.

Most new gas fired plants are intended for peak supply. The larger nuclear and coal plants cannot quickly adjust their instantaneous power production, and are generally intended for baseline supply. The demand for baseline power has not increased as rapidly as the peak demand. Some new experimental reactors, notably pebble bed modular reactors, are specifically designed for peaking power.

Nuclear proliferation

Detractors for the use of nuclear energy point out that the use of nuclear technology could lead to the proliferation of nuclear weapons (see nuclear proliferation), although the International Atomic Energy Agency's safeguards system under the Nuclear Non-Proliferation Treaty has been an international success and has prevented weapons proliferation thus far. It has involved cooperation in developing nuclear energy for electricity generation, while ensuring that civil uranium, plutonium and associated plants did not allow weapons proliferation to occur as a result of this.

International nuclear safeguards are administered by the IAEA and were formally established under the NPT which requires nations to:

However, despite the fact that North Korea joined the Nuclear Non-Proliferation Treaty in 1985, the United States claims they have managed to obtain enough material to create anywhere from one to five bombs from their fission reactor at Yongbyon.

The United Nations is also investigating Iran, which is said to have violated the Nuclear Non-Proliferation Treaty despite its denial.

Therefore, the claims that the Nuclear Non-Proliferation Treaty is an international success are in dispute. Clearly it has not stopped Israel, India, Pakistan or North Korea from creating nuclear weapons. South Africa also might have created nuclear weapons but has since renounced its nuclear program.


In 2000, there were 438 commercial nuclear generating units throughout the world, with a total capacity of about 351 gigawatts.

In 2001, there were 104 (69 pressurized water reactors, 35 boiling water reactors) commercial nuclear generating units that are licensed to operate in the United States, producing 32,300 net megawatts (electric), which is approximately 20 percent of the nation's total electric energy consumption. The United States is the world's largest supplier of commercial nuclear power.

In France, 75.8% of all electric power comes from nuclear reactors.

Natural nuclear reactors

A natural nuclear fission reactor can occur under certain circumstances that mimic the conditions in a constructed reactor. The only known natural nuclear reactor on Earth's surface occurred 2 billion years ago in Oklo, Gabon, Africa. [1]

List of atomic energy groups

See also: nuclear fission -- nuclear fusion -- power plant -- Nuclear waste -- electricity generation -- nuclear physics -- Enrico Fermi -- Manhattan Project -- United States Naval reactor -- technology assessment -- List of nuclear accidents -- List of nuclear reactors

References and links