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Nuclear reactor

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Core of a small nuclear reactor used for research.
File:Nuclear powerplant-01.jpg
A nuclear power station. The nuclear reactors are inside the two cylindrical containment buildings in the foreground—behind are the cooling towers (venting water vapor).

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). 

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Currently all commercial nuclear reactors are based on nuclear fission, and are considered problematic by some for their safety and health risks. Conversely, some consider nuclear power to be a safe and pollution-free method of generating electricity. 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, which generate heat and power by passive radioactive decay, and Farnsworth-Hirsch fusors, in which controlled nuclear fusion is used to produce neutron radiation.


Applications

History

In 1972, French physicist Francis Perrin declared that nature had beaten humans to the punch by creating the world’s first nuclear reactors. Indeed nature had a four-billion-year head start. Sixteen natural fission reactors have been found in three different ore deposits at the Oklo mine in Gabon, West Africa. These are collectively known as the Oklo Fossil Reactors

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.

File:Calderhall.jpeg
Calder Hall unit 1, the worlds first commercial scale nuclear power station.

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 27, 1954, the world's first nuclear power plant to generate electricity began operations at Obninsk, Kaluga Oblast, Russia, according to the Uranium Institute (London, England)[1]. The worlds first commercial scale nuclear power station, Calder Hall, began generation on 17 October, 1956 [2]. 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 [3], although that may change by 2010 (see Future of the industry below).

Surprisingly, and 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 [4]. There was however political fallout: Italy held a referendum the next year in 1987, the results of which led to a shutdown the country's four nuclear power plants [5].

In 1992 the Turkey Point Nuclear Generating Station 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 [6] [7].

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.

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 AREVA plant under construction. 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.

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, nuclear energy policy.

Types of reactors

NC State's PULSTAR Reactor is a 1 MW pool-type research reactor with 4% enriched, pin-type fuel consisting of UO2 pellets in zircaloy cladding.
The control room of NC State's Pulstar Nuclear Reactor.

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 characterized 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 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 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 (e.g. Canadian CANDU reactors) tend to be more thoroughly thermalised than PWRs and BWRs, which use light water (normal water) as the moderator (due to the extra thermalization, these types can use natural uranium/unenriched fuel).
  • Fast reactors use fast neutrons to sustain the fission chain reaction, and are characterized 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

Obsolete types still in service

Other types of reactors

Advanced reactors

More than a dozen advanced reactor designs are in various stages of development.[8] Some are evolutionary from the PWR, BWR and PHWR designs above, some are more radical departures. The former include the Advanced Boiling Water Reactor (ABWR), two of which are now operating with others are under construction, and the planned passively safe ESBWR and AP1000 units (see Nuclear Power 2010 Program). The best-known radical new design is the Pebble Bed Modular Reactor (PBMR), a High Temperature Gas Cooled Reactor (HTGCR). The Clean And Environmentally Safe Advanced Reactor (CAESAR) is a nuclear reactor concept that uses steam as a moderator - this design is still in development. Possible designs of subcritical reactors exist on the drawing board, notably the energy amplifier, awaiting political support and funding. Some, such as the Integral Fast Reactor (IFR), have been cancelled due to a political climate unfavorable to nuclear power.

Generation IV reactors

Even more-advanced reactors are also on the drawing boards. These are the Generation IV reactors[9], which are divided into six overall design classes.

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 (see MOX). 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.

Under 1% of the uranium found in nature is the easily fissionable U-235 isotope and as a result most reactor designs require enriched fuel. Enrichment involves increasing the percentage of U-235 and is usually done 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.

Most BWR and PWR commercial reactors use uranium enriched to about 4% U-235, many research reactors use highly enriched, or weapons grade uranium, while some commercial reactors with a high neutron economy do not require the fuel to be enriched at all.

Fueling of nuclear reactors

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. Although in practice, it is the buildup of reaction poisons in nuclear fuel that determines the lifetime of nuclear fuel in a reactor; long before all possible fissions have taken place, the buildup of long-lived neutron absorbing fission products damps out the chain reaction. 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, RBMK reactors,molten salt reactors, Magnox 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.

Waste management

The final stage of the nuclear fuel cycle is the management of the still highly radioactive, "spent" fuel, which constitutes the most problematic component of the nuclear waste stream. After fifty years of nuclear power the question of how to deal with this material remains fraught with safety concerns and technical problems, and one of the most important lines of criticism of the industry is based on the long-term risks and costs associated with dealing with the waste.

Management of the spent fuel can include various combinations of storage, reprocessing, and disposal. In practice storage has been the primary modality so far. Typically 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. After a few decades some on-site storage involves moving the now cooler, less radioactive fuel to a dry-storage facility, where the fuel is stored in steel and concrete containers which are monitored carefully.

Another, more permanent method of disposal of high-level nuclear waste calls for the material to be buried deep underground in certain geological formations. The Canadian government, for example, is seriously considering this method of disposal, known as the Deep Geological Disposal concept. Under the current plan, a vault is to be dug 500 to 1000 meters below ground, under the Canadian Shield, one of the most stable landforms on the planet. The vaults are to be dug inside geological formations known as batholiths, formed about a billion years ago. The used fuel bundles will be encased in a corrosion-resistant container, and further surrounded by a layer of buffer material, possibly of a special kind of clay (bentonite clay). The case itself is designed to last for thousands of years, while the clay would further slow the corrosion rates of the container. The batholiths themselves are chosen for their low ground-water movement rates, geological stability, and low economic value. (See The Canadian Nuclear FAQ, Waste Management section, by Dr. Jeremy Whitlock)

The Finnish government has already started building a vault to store nuclear waste 500 to 1000 meters below ground, not far from the nuclear plant at Olkiluoto.

Storing high level nuclear waste above ground for a century or so is considered appropriate by many scientists. This allows for the material to be more easily observed and any problems detected and managed, while the decay over this time period significantly reduces the level of radioactivity and the associated harmful effects to the container material. It is also considered likely that over the next century newer materials will be developed which will not break down as quickly when exposed to a high neutron flux thus increasing the longevity of the container once it is permanently buried.

Reprocessing is attractive in principle because (1) it can recycle nuclear fuel and (2) it can prepare the waste material for disposal. Considerable experience with reprocessing in France however, has indicated that a one way fuel cycle based on extracting and processing fresh supplies of uranium and storing the spent fuel is more economical than reprocessing.

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. [22] Such reactors can no longer form on Earth: radioactive decay over this immense time span has reduced the proportion of U-235 in naturally occurring uranium to below the amount required to sustain a chain reaction.

The natural nuclear reactors formed when a uranium-rich mineral deposit became inundated with groundwater that acted as a neutron moderator, and a strong chain reaction took place. The water moderator would boil away as the reaction increased, slowing it back down again and preventing a meltdown. The fission reaction was sustained for hundreds of thousands of years.

These natural reactors are extensively studied by scientists interested in geologic radioactive waste disposal. They offer a case study of how radioactive isotopes migrate through the earth's crust. This is a significant area of controversy as opponents of geologic waste disposal fear that isotopes from stored waste could end up in water supplies or be carried into the environment.

See also


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