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Encyclopedia > Nuclear reactors
Nuclear power station at , . The nuclear reactor is inside the dome-shaped containment building.
Nuclear power station at Leibstadt, Switzerland. The nuclear reactor is inside the dome-shaped containment building.

A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate (as opposed to a nuclear explosion, where the chain reaction occurs in a split second).

Currently all commercial nuclear reactors are based on nuclear fission. For experiments on reactors based on nuclear fusion, see fusion power.


Nuclear power can also be generated in a radioisotope thermoelectric generator, which produces heat through subcritical (i.e. less than critical mass) radioactive decay rather than fission in a near-critical mass. These generators have been used to power space probes and some lighthouses built by the Soviet Union.


Basic science

All commercial nuclear reactors produce heat through nuclear fission. In this process, the nucleus of an element such as uranium splits into two smaller atoms. This occurs naturally in radioactive elements, but it can be induced artificially by making some atoms absorb a neutron. This causes the nucleus to become unstable and makes it split apart very quickly.

The fission process for a uranium atom yields two smaller atoms, one to three fast-moving free neutrons, and energy. Uranium fission therefore releases more neutrons than it requires, and the reaction can become self sustaining if conditions are appropriate. This is called a chain reaction.

When a neutron is captured by a fissionable nucleus, it may cause fission immediately, or it may lead to an unstable species which undergoes fission a short time later. A mass of fissionable material is said to be a critical mass if each fission event leads to one or more fission events on average. A mass is said to be prompt critical if the immediate fission events are sufficient to carry on a chain reaction. A prompt critical mass will rapidly release an exponentially increasing amount of heat and cannot be controlled. Nuclear reactors are (with the exception of certain speculative subcritical reactors) designed to contain critical masses that are not prompt critical, so that control systems can react quickly enough to maintain a steady rate of heat production.

The neutrons released by fission are moving quickly. Such "fast neutrons" are not easily absorbed by fissionable nuclei. Some reactors are designed to work with these neutrons, but most reactors use a neutron moderator to slow these neutrons down so that they are more easily absorbed. Such neutrons are often slowed until they are in thermal equilibrium with the reactor core; as a result, they are called thermal neutrons (or slow neutrons).

The amount of heat produced by a reactor is a crucial parameter. It may be controlled by adjusting the amount of neutron moderator in the reactor core, control rods consisting of neutron absorbers may be used to control the output, or the physical arrangement of the fuel may be changed (often by thermal expansion or contraction). Many reactors use several methods, both for control and for emergency shutdown.

Reactor design

See also Nuclear_power_plant#Fission_reactors

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 from reactor to turbine in the electricity generation process. In other reactor designs, the heat may be transferred by light water, pressurized heavy water, helium, liquid sodium, or another 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 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.

Not all reactors need to be shut down for refueling; for example, pebble bed reactors and CANDU reactors allow fuel to be shifted through the reactor while it is running. In a CANDU reactor, this also allows individual fuel elements to be moved about within the reactor core to places that are best suited to the amount of U-235 in the fuel element.

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. Fission reactors can be divided roughly into two classes, depending on the energy of the neutrons that are used to sustain the fission chain reaction.

  • Thermal (slow) reactors use slow or thermal neutrons. These are characterised by having moderating materials which are intended to slow the neutrons until they approach the average kinetic energy of the surrounding particles, that is, until they are thermalised. Thermal neutrons have a far higher probability of fissioning U-235, and a lower probability of capture by U-238 than the faster neutrons that result from fission do. As well as the moderator, thermal reactors have fuel (fissionable material), containments, pressure vessels, shielding, and instrumentation to monitor and control the reactor's systems. Most power reactors are of this type, and the first plutonium production reactors were thermal reactors using graphite as the moderator. Some thermal power reactors are more thermalised than others; Graphite (ex. Russian RBMK reactors) and heavy water moderated plants (ex. Canadian CANDU reactors) tend to be more thoroughly thermalised than PWRs and BWRs, which use light water (normal water) as the moderator.
  • Fast reactors use fast neutrons to sustain the fission chain reaction, and are characterised by the lack of moderating material. They require highly enriched fuel (sometimes weapons-grade), or Plutonium in order to reduce the amount of U-238 that would otherwise capture fast neutrons. Some are capable of producing more fuel than they consume, usually by converting U-238 to Pu-239. Some early power stations were fast reactors, as are some Russian naval propulsion units, and construction of prototypes is continuing, see fast breeder, but overall the class has not achieved the success of thermal reactors in any application. An example of this type of reactor is the Fast Breeder Reactor (FBR).

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.

  • Pressure vessels holding steam heated by the reactor are used by most commercial and naval reactors. This serves as a layer of shielding and containment.
  • Pressurised channels are used by the RBMK and CANDU reactors. 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. Utilisation of the heat varies, depending on the reactor. 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 pebble bed reactor uses a gas-cooled design.

Since water serves as a moderator, it cannot be used as a coolant in a fast reactor. 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

Obsolescent 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 are under construction. The best-known radical new design is the Pebble Bed Modular Reactor (PBMR), a high temperature gas cooled reactor. Other possible designs exist on the drawing board, notably the energy amplifier, awaiting political support and funding. Some, such as the Integral Fast Reactor, have been cancelled due to a poltical climate unfavorable to nuclear power.

Nuclear fuel cycle

Main article: nuclear fuel cycle

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.

Uranium is sampled and mined as other metals are, via open-pit mining or leach mining. Raw uranium ore found in the United States ranges from 0.05% to 0.3% uranium oxide. Uranium ore is not rare; the largest probable resources, extractable at a cost of US$80 per kilogram or cheaper, are located in Australia, Kazakhstan, Canada, South Africa, Brazil, Namibia, Russia, and the United States.

The raw ore is then milled, where it is ground and chemically leached. The resulting powder of natural uranium oxide is called "yellowcake". The yellowcake powder is then converted to uranium hexafluoride to prepare for enrichment.

Since under 1% of the uranium found in nature is the easily fissionable U-235 isotope, the uranium must be enriched to about 4% U-235, usually by means of gaseous diffusion or gas centrifuge. The enriched result is then converted into uranium dioxide powder, which is pressed and fired onto pellet form. These pellets are stacked into tubes which are then sealed and called fuel rods. Many of these fuel rods are used in each nuclear reactor.

After its operating cycle, the reactor is shut down for refueling, and the spent fuel rods are stored in a pool of water which is usually located on-site. The water provides both cooling for the still-decaying uranium, and shielding from the continuing radioactivity.

Continuing storage of this high level nuclear waste is controversial; see below.


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 (see United States Naval reactor) to propel submarines and aircraft carriers. 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.

Even before the 1979 Three Mile Island accident, new orders for nuclear plants in the U.S. had ceased for economic reasons. As of 2004, no new nuclear plants have been ordered in the USA since 1978 (http://www.pbs.org/wgbh/pages/frontline/shows/reaction/maps/chart2.html).

Negative influence of the 1986 Chernobyl accident increased regulations which increased the costs of operating a reactor.

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 renew the demand for nuclear power plants.

As of 2004, 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). Finland and France actively pursue nuclear program and both have new reactors planed in very near future. As of the early 21st century, nuclear power is of particular interest to both China and India to serve their rapidly growing economies.

The first organization to develop utilitarian nuclear power, the U.S. Navy, is the only organization worldwide with a totally clean record. This is perhaps because of the stringent demands of Admiral Hyman G. Rickover, who was the driving force behind nuclear marine propulsion. The U.S. Navy has operated more nuclear reactors than any other entity, other than the Soviet Navy, with no publicly known major incidents. Two U.S. nuclear submarines, the USS Scorpion and Thresher, have been lost at sea, though for reasons not related to their reactors, and their wrecks are situated such that the risk of nuclear pollution is considered low.

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. However, reactors do release radioactive products - such as radioactive krypton gas (half life c. 19 days) - into the environment. The waste from highly radioactive spent fuels needs to be handled with great care and forethought due to the long half-lifes of the radioactive isotopes found in the waste. In addition, the nuclear industry produces a much greater volume of low-level radioactive waste in the form of contaminated items like clothing, hand tools, water purifier resins, and upon decomissioning the materials of which the reactor itself is built. In the United States, the NRC has repeatedly attempted to allow low-level materials to be handled as normal waste: landfilled, recycled into consumer items, etc. Much low-level waste releases very low levels of radioactivity, and is considered radioactive waste essentially because of its history. For example, according to the standards of the NRC, the radiation released by coffee is enough to treat it as low level 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 concentrated enough to build a bomb (most nuclear reactors run on 4% enriched uranium, while a bomb requires an estimated 90% enrichment), the technology used to enrich uranium could be used to make the 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 fissionables for their weapons. Nuclear material for bombs is generally made in special dedicated reactors that are quite different from commercial reactors.

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 which are used in medicine and food preservation, though the demand for these products can be satisfied by a relatively small number of plants.

The safe storage and disposal of nuclear waste is a difficult problem. Because of potential harm from radiation, 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 (which is thought to have been caused by a combination of a faulty reactor design, poorly trained operators, and a non-existent safety culture) is very small due to the care taken in designing adequate safety systems, and that nuclear industry overall has quite good safety record compared to other industries (Safety page (http://www.world-nuclear.org/info/inf06.htm)). Even in an accident such as Three Mile Island, the containment vessels were never breached, so that very little radiation was released into the environment. Opponents of nuclear power point out that nuclear wastes are not well protected and that they can be released in case of terrorist attack. Proponents of nuclear power however contend that nuclear wastes are well protected and as proof they state that there was no accident that involved any form of nuclear waste in civilian program worldwide. In addition they point on large studies carried out by NRC and other agencies that tested robustness of both Reactor and waste fuel storage and found that they should be able to sustain terrorist attack comparable to September 11 (see Resistance to terrorist attack (http://www.world-nuclear.org/news/resistance.htm)). Spent fuel is usually housed inside reactor containmen (see [Fuel Storage (http://www.world-nuclear.org/info/inf03.htm)).

Low-dose radiation released under normal operating conditions may also be a concern. 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. 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. The effects of long-term exposure to very low levels of radioactivity are also a matter of current dispute.

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 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). Nuclear reactors also regularly vent radioactive gases - which have too much volume to conveniently store - into the environment. In addition, the nuclear industry fuel cycle produces many tons of depleted uranium (uranium from which the easily fissile U235 element has been removed, leaving behind only U238). This material is much more concentrated than natural uranium ores, and must be disposed of. It can have commercial use in parts that have to be extra robust. They are used in Airplane production, for radiation shielding and similar things.

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. It is unclear that this material can be safeguarded over such a long period of time.

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. However, fear has been the single largest obstacle to the widespread use of Nuclear Power.

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. 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.

Finally, any company seeking to construct a nuclear reactor around the world (but most acutely in the US) must deal with NIMBY issues. Given the high profile of both Three Mile Island and Chernobyl, few municipalities would welcome a new nuclear reactor within their borders, and many have issued local ordinances prohibiting the development of nuclear power.

In attempt to encourage development of Nuclear Power in US Department of Energy DOE has offered interested parties to introduce France model for licensing and to share 50% of a construction expenses. Several applications were made but project is still in its infancy.

Nuclear Power plants usually tend to be most competitive in areas where no other resources are readily available. For example, the province of Ontario, Canada is already using all of its best sites for hydroelectric power, and has minimal supplies of fossil fuels, so a number of nuclear plants have been built there.

Nuclear proliferation

Critics of nuclear energy point out that nuclear technology is often dual-use, and much of the same materials and knowledge used in a civilian nuclear program can be used to develop nuclear weapons (see nuclear proliferation). To prevent this from happening, safeguards on nuclear technology were imposed through the Nuclear Non-Proliferation Treaty (NPT) and monitored by the International Atomic Energy Agency (IAEA). Several states did not sign the treaty, however, and were able to use international nuclear technology (often procured for civilian purposes) to develop nuclear weapons (India, Pakistan, Israel, and South Africa). Of those who have signed the treaty, many states have either claimed to or be accused of attempting to use supposedly civilian nuclear power plants towards weapons ends, including Iran and North Korea.

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

  • Report to the IAEA what nuclear materials they hold and their location.
  • Accept visits by IAEA auditors and inspectors to verify independently their material reports and physically inspect the nuclear materials concerned to confirm physical inventories of them.


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

In 2004, there were 104 (69 pressurized water reactors, 35 boiling water reactors) commercial nuclear generating units licensed to operate in the United States, producing a total of 97,400 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 2001, the nuclear share of electricity generation was 19%.

In France, as of 2002, 78% 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 formed 2 billion years ago in Oklo, Gabon, Africa. [1] (http://www.ans.org/pi/np/oklo) Such reactors can no longer form on Earth: radioactive decay over this immense time span has reduced the proportion of U-235 in naturally ocurring uranium to below the amount required to sustain a chain reaction.

List of atomic energy groups

External Links:

  • World Nuclear Association (http://www.world-nuclear.org/index.htm)

Related articles

See also

References and links

  Results from FactBites:
Nuclear reactor - Wikipedia, the free encyclopedia (3398 words)
A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate (as opposed to a nuclear explosion, where the chain reaction occurs in a split second).
The concept of a natural nuclear reactor was theorized as early as 1956 by Paul Kuroda at the University of Arkansas [1].
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.
nuclear reactor. The Columbia Encyclopedia, Sixth Edition. 2001-05 (923 words)
A nuclear reactor is sometimes called an atomic pile because a reactor using graphite as a moderator consists of a pile of graphite blocks with rods of uranium fuel inserted into it.
Reactors in which the uranium rods are immersed in a bath of heavy water are often referred to as “swimming-pool” reactors.
In a fusion reactor, the principal problem is the containment of the plasma fuel, which must be at a temperature of millions of degrees in order to initiate the reaction.
  More results at FactBites »



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