Nuclear reactors are used for many purposes. The most significant current use is for the generation of electrical power (see nuclear power). Research reactors are used for radioisotope production and for beamline experiments with free neutrons. Historically, the first use of nuclear reactors was the production of weapons grade plutonium for nuclear weapons. Another military use is submarine / ship propulsion (Though this involves a much smaller nuclear reactor than the one used in a nuclear power plant).
Currently all commercial nuclear reactors are based on nuclear fission, and are considered by some to be a safe and pollution-free method of generating electricity. Conversely, some consider nuclear reactors problematic for their potential safety and health risks. Fusion power is an experimental technology based on nuclear fusion instead of fission. There are other devices in which nuclear reactions occur in a controlled fashion, including radioisotope thermoelectric generators and atomic batteries, which generate heat and power by exploiting passive radioactive decay, as well as Farnsworth-Hirsch fusors, in which controlled nuclear fusion is used to produce neutron radiation.
Although mankind has only tamed nuclear power recently, the first nuclear reactors were naturally occurring.[1] Fifteen natural fission reactors have so far been found in three separate ore deposits at the Oklo mine in Gabon, West Africa. First discovered in 1972 by French physicist Francis Perrin, they are collectively known as the Oklo Fossil Reactors. These reactors ran for approximately 150 million years, averaging 100 kW of power output during that time. The concept of a natural nuclear reactor was theorized as early as 1956 by Paul Kuroda at the University of Arkansas [2]
Enrico Fermi and Leó Szilárd, while both were at the University of Chicago, were the first to build a nuclear pile and demonstrate a controlled chain reaction on December 2, 1942. In 1955 they shared U.S. Patent 2,708,656 for the nuclear reactor.
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 generator was produced for the first time at Experimental Breeder Reactor-I (EBR-1) located near Arco, Idaho. On June 26, 1954, at 5:30 pm, the world's first nuclear power plant to generate electricity began operations at Obninsk, Kaluga Oblast, USSR. It produced 5 megawatts (electrical), enough to power 2,000 homes. [3][4].
The world's first commercial scale nuclear power station, Calder Hall,in England, began generation on October 17, 1956 [5] Another early power reactor was the Shippingport Reactor in Pennsylvania (1957).
Even before the 1979 Three Mile Island accident, new orders for nuclear plants in the U.S. had ceased for economic reasons primarily related to greatly extended construction times. As of 2004, no new nuclear plants have been ordered in the USA since 1978 [6], although that may change by 2010 (see Future of the industry below).
Unlike the Three Mile Island accident, the 1986 Chernobyl accident did not increase regulations affecting Western reactors. This was because the Chernobyl reactors were known to be an unsafe design, using the RBMK, without containment buildings and operated unsafely, and the West had little to learn from them [7]. There was however political fallout: Italy held a referendum the next year in 1987[8], the results of which led to a shutdown of the country's four nuclear power plants.
The Chernobyl accident raised awareness about the possible geographical range of nuclear events, with contamination from Ukraine easily crossing national borders and spreading over significant parts of Europe. As a consequence an international organisation to promote safety awareness and professional development on operators in nuclear facilities was created: WANO; World Association of Nuclear Operators.
In 1992 the Turkey Point Nuclear Generating Station in Florida, USA, was hit directly by Hurricane Andrew. Over $90 million of damage was done, largely to a water tank and to a smokestack of one of the fossil-fueled units on-site, but the containment buildings were undamaged [9][10].
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, 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.
[edit] Nuclear power in electricity production
Nuclear power from a reactor is typically utilized to produce electricity. The production of electricity is usually accomplished by somewhat standard methods that involve using heat from the nuclear reaction to power steam turbines. Nuclear power is attractive in that relatively small amounts of fuel are used to produce vast amounts of energy with no or much smaller production of free pollutants, such as greenhouse gas.
Nuclear power is controversial since it produces radioactive waste and runs the risk of nuclear meltdown. Such events, though unlikely with proper precautions, are typically viewed as catastrophic and can produce far reaching detrimental effects, such as widespread radiation contamination. Modern reactor designs and the relatively low enrichment of nuclear reactor fuel make it essentially impossible for a nuclear explosion to occur (the Chernobyl accident was neither a modern reactor design nor was it a nuclear explosion).
[edit] The future of the industry
As of 2006, Watts Bar 1, which came on-line in 1997, was the last U.S. commercial nuclear reactor to go on-line. This is often quoted as evidence of a successful worldwide campaign for nuclear power phase-out. However, political resistance to nuclear power has only ever been successful in parts of Europe, in New Zealand, in the Philippines, and in the United States. Even in the US and throughout Europe, investment in research and in the nuclear fuel cycle has continued, and some experts predict that electricity shortages, fossil fuel price increases and concern over greenhouse gas emissions will renew the demand for nuclear power plants.
Many countries remain active in developing nuclear power, including Japan, China and India, all actively developing both fast and thermal technology, South Korea and the United States, developing thermal technology only, and South Africa and China, developing versions of the Pebble Bed Modular Reactor (PBMR). Finland and France actively pursue nuclear programs; Finland has a new European Pressurized Reactor under construction by Areva. Japan has an active nuclear construction program with new units brought on-line in 2005. In the U.S., three consortia responded in 2004 to the U.S. Department of Energy's solicitation under the Nuclear Power 2010 Program and were awarded matching funds - the Energy Policy Act of 2005 authorized subsidies for up to six new reactors, and authorized the Department of Energy to build a reactor based on the Generation IV Very-High-Temperature Reactor concept to produce both electricity and hydrogen. As of the early 21st century, nuclear power is of particular interest to both China and India to serve their rapidly growing economies - both are developing fast breeder reactors. See also future energy development. In the energy policy of the United Kingdom it is recognized that there is a likely future energy supply shortfall, which may have to be filled by either new nuclear plant construction or maintaining existing plants beyond their programmed lifetime.
On September 22, 2005 it was announced that two sites in the U.S. had been selected to receive new power reactors (exclusive of the new power reactor scheduled for INL) - see Nuclear Power 2010 Program.
It is possible that the first new nuclear power plant to be built in the United States since the 1970s may be installed in the remote town of Galena, Alaska. The town's City Council approved the idea, and Toshiba proposed to install its model 4S "nuclear battery" in Galena free of charge as a test.
- See also: nuclear power phase-out and nuclear energy policy
[edit] 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.
- Fast reactors use fast neutrons to sustain the fission chain reaction. They are characterized by a lack of moderating material. Initiating the chain reaction requires enriched uranium (and/or enrichment with plutonium 239), due to the lower probability of absorbtion of a fast neutron by U-235 (as compared to a moderated, thermal neutron). Fast breeder reactors are capable of enriching Uranium during the fission chain reaction (by converting fertile U-238 to Pu-239) which allows an operational fast reactor to generate more fissile material than it consumes. Thus, a breeder reactor, once running, can be re-fueled with natural or even depleted uranium.
Some early power stations were fast reactors, as are some Russian naval propulsion units. Construction of prototypes is continuing (see fast breeder or generation IV reactors). Overall, fast reactors are less common than thermal reactors in most applications. All fast neutron reactors that have been used for power generation have been liquid metal cooled reactors, but research continues in gas cooled reactors.
- Thermal (slow) reactors use slow or thermal neutrons. These are characterized by moderating materials that slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, until they are thermalized. 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. 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. The first plutonium production reactors were thermal reactors using a graphite moderator. Some thermal power reactors are more thermalised than others; Graphite (e.g. Russian reaktor bolshoy moshchnosti kanalniy RBMK reactors) and heavy water moderated plants (e.g. Canadian CANada Deuterium Uranium CANDU reactors) tend to be more thoroughly thermalised than pressurized water reactors PWRs and boiling water reactors BWRs, which use light water (normal water) as the moderator. Due to the extra thermalization, these types can use natural uranium/unenriched fuel.
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 reactor heated steam pressure vessels. Pressure vessels balance out pressure transients in the primary loops that occur when reactor power changes. Pressure vessels also have a small role as primary coolant make-up sources. Pressure vessels are almost always lined up to reactors and are only isolated from reactors during special maintenance or tests.
- Pressurised channels are used by the RBMK and CANDU reactors. Channel-type reactors can be refuelled under load. This has advantages discussed under CANDU reactor.
- Gas-cooled reactors are cooled by a circulating inert gas, usually helium. Nitrogen and carbon dioxide have also been used. Utilization 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.
Further details on the classification of Nuclear reactors can be found at Classification of Nuclear Reactors.
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