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 contained rate. Nuclear reactors are used for providing heat for electricity generation, domestic and industrial heating, desalination, and naval propulsion, for providing neutron beams for research purposes, and for making radioactive isotopes.

Although the term 'nuclear reactor' could also refer to a power reactor that utilizes nuclear fusion, the term is used almost exclusively to refer to fission devices.

Table of contents
1 Types of reactors
2 Nuclear fuel cycle
3 History
4 Benefits and Disadvantages
5 Natural Nuclear Reactors
6 List of Atomic Energy Groups

Types of reactors

Although the majority of nuclear reactors exist to produce useful energy for the generation of electricity, some are used for research, the production of radioactive isotopes for medical and industrial use, and/or the production of plutonium for nuclear weapons. Since the beginning of atomic energy, several reactor technologies have been developed.

Technical differences

There are two basic types of reactors, differentiated by the energy spectrum (i.e., speed) of neutrons in the reactor.

  • Thermal (slow) reactors are composed of fuel (fissionable material), moderating materials to slow neutrons to low velocities (to prevent capture by U238), heavy-walled pressure vessels to house reactor components, shielding to protect personnel, systems to conduct heat away from the reactor, and instrumentation for monitoring and controlling the reactor's systems. Most nuclear reactors used for electric power generation are of this type. The first plutonium production reactors were thermal reactors using graphite as the moderator.
  • Fast reactors require highly enriched fuel (sometimes Weapons Grade), but no moderating material (the enrichment process removes most of the U238 that captures fast neutrons). This type of reactor is used in mobile applications, where space constraints are a major concern, as well as for the production of plutonium (see fast breeder).

Thermal power reactors can again be divided into two types, depending on whether they use pressurised fuel channels or a large pressure vessel. The RBMK and CANDU types use pressurised channels, while all other types to date have used a large pressure vessel. Channel-type reactors can be refuelled under load, which has advantages and disadvantages discussed under CANDU_reactor. The proposed pebble bed modular reactor can also be refueled under load.

Designs for fast power reactors to date have all been cooled by liquid metal. They have also been of two types, called pool and loop reactors.

To provide the power for a dynamo-electric machine, or electric generator, nuclear power plants rely on the process of 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 uranium atoms yields two smaller atoms, one to three fast-moving free neutrons, plus an amount of energy. Because more free neutrons are released from a uranium fission event than are required to initiate the event, the reaction can become self sustaining--a chain reaction --under controlled conditions, thus producing a tremendous amount of energy. 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 burning uranium fuel is collected in ordinary 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 superheated 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 a dynamo- electric machine, or 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 235U contained in the fuel assemblies at the beginning of the cycle. A higher percentage of 235U 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.

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).

Nuclear fuel cycle

All nuclear reactors depend on the availability of fissile material to operate. Thermal reactors generally depend on a refined and enriched product of uranium ore, however 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.


Enrico Fermi and Leo Szilard 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 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, 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 Chernobyl 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).

Benefits and Disadvantages

Proponents of nuclear power point out that the technology emits virtually no airborn 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.

Critics of nuclear power assert that any of the evironmental 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 decommisioning 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.

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 gasses 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 radiation 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 a substantially lesser problem than fossil fuel solid wastes. However, because spent nuclear fuels are radioactive, they are pound for pound a more substantial problem (see nuclear waste).

Economic Barriers

As a general rule nuclear power plants are significantly more expensive to build than steam-based coal-fired plants, which are themselves more expensive to build than natural gas-fired combined-cycle plants of similar capacity. A part of this additional cost is due to the fact that it takes significantly longer to build a nuclear plant than it does to build either a gas-fired plant or a coal-fired plant. Because a power plant does not earn money during construction, longer construction times translate directly into higher interest charges on borrowed construction funds.

All of these charges, taken together require that coal and especially nuclear based power plants, must demonstrate operating cost advantages over natural gas if they are to be commercially favored. In general, coal and nuclear plants experience roughly the same operating costs (operations and maintenance plus fuel costs), however nuclear and coal do differ in the source of their operating cost components. Nuclear has much lower fuel costs but much higher operating and maintenance costs than does coal. In recent times in the United States these operating cost advantages have not been sufficient for nuclear to overcome 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 90-95 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 face circumstances under which they 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.

Given the financial disadvantages of nuclear power, it is understandable that the nuclear industry also has sought to find additional benefits to using nuclear power. Additional benefits would translate into a willingness to pay higher prices for building nuclear based power generation, whether via direct charges or government subsidy. If all market conditions for generating power were otherwise equal, the difference that one might be willing to pay to build a new nuclear power plant would be a measure of perceived environmental gains. Because coal fired plants produce more airborn 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.

An additional issue to discuss is the fact that most additional gas fired plants are intended for peak supply, where the larger nuclear and coal plants are generally intended for baseline supply, which has not increased as rapidly as the peak demand.

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:

  • 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 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, 80% 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 1½ 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

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