Jump to content

Nuclear reactor

From Wikipedia, the free encyclopedia
This is an old revision of this page, as edited by RTC (talk | contribs) at 04:35, 11 October 2002. The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

A nuclear reactor is an apparatus in which nuclear fission chain reactions are initiated, controlled, and sustained at a contained rate. They are used to create electric current for power, or isotopes.

Thermal 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. 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).

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.

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 to transfer the heat energy from reactor to turbine in the electricity generation process. In other reactor designs the heat energy is transferred by 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.

Types of reactors

[this content will need to be integrated properly into the article]

Pressurised Water Reactor (PWR)

This is the most common type, with over 230 in use for power generation and a further several hundred in naval propulsion. The design originated as a submarine power plant. It uses ordinary water as both coolant and moderator. The design is distinguished by having a primary cooling circuit which flows through the core of the reactor under very high pressure, and a secondary circuit in which steam is generated to drive the turbine.

A PWR has fuel assemblies of 200-300 rods each, arranged vertically in the core, and a large reactor would have about 150-250 fuel assemblies with 80-100 tonnes of uranium. Water in the reactor core reaches about 325°C, hence it must be kept under about 150 times atmospheric pressure to prevent it boiling. Pressure is maintained by steam in a pressuriser (see diagram). In the primary cooling circuit the water is also the moderator, and if any of it turned to steam the fission reaction would slow down. This negative feedback effect is one of the safety features of the type. The secondary shutdown system involves adding boron to the primary circuit.

The secondary circuit is under less pressure and the water here boils in the heat exchangers which are thus steam generators. The steam drives the turbine to produce electricity, and is then condensed and returned to the heat exchangers in contact with the primary circuit.

Boiling Water Reactor (BWR)

This design has many similarities to the PWR, except that there is only a single circuit in which the water is at lower pressure (about 75 times atmospheric pressure) so that it boils in the core at about 285°C. The reactor is designed to operate with 12-15% of the water in the top part of the core as steam, and hence with less moderating effect and thus efficiency there.

The steam passes through drier plates (steam separators) above the core and then directly to the turbines, which are thus part of the reactor circuit. Since the water around the core of a reactor is always contaminated with traces of radionuclides, it means that the turbine must be shielded and radiological protection provided during maintenance. The cost of this tends to balance the savings due to the simpler design. Most of the radioactivity in the water is very short-lived (mostly N-16, with a 7 second half life) , so the turbine hall can be entered soon after the reactor is shut down.

A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750 assemblies in a reactor core, holding up to 140 tonnes of uranium. The secondary control system involves restricting water flow through the core so that steam in the top part means moderation is reduced.

Pressurised Heavy Water Reactor (PHWR or CANDU)

The CANDU reactor design has been developed since the 1950s in Canada. It uses natural uranium (0.7% U-235) oxide as fuel, hence needs a more efficient moderator, in this case heavy water (D2O).

The moderator is in a large tank called a calandria, penetrated by several hundred horizontal pressure tubes which form channels for the fuel, cooled by a flow of heavy water under high pressure in the primary cooling circuit, reaching 290°C. As in the PWR, the primary coolant generates steam in a secondary circuit to drive the turbines. The pressure tube design means that the reactor can be refuelled progressively without shutting down, by isolating individual pressure tubes from the cooling circuit.

A CANDU fuel assembly consists of a bundle of 37 half metre long fuel rods (ceramic fuel pellets in zircaloy tubes) plus a support structure, with 12 bundles lying end to end in a fuel channel. Control rods penetrate the calandria vertically, and a secondary shutdown system involves adding gadolinium to the moderator. The heavy water moderator circulating through the body of the calandria vessel also yields some heat.

Advanced Gas cooled Reactor (AGR)

These are the second generation of British gas-cooled reactors, using graphite moderator and carbon dioxide as coolant. The fuel is uranium oxide pellets, enriched to 2.5-3.5%, in stainless steel tubes. The carbon dioxide circulates through the core, reaching 650°C and then past steam generator tubes outside it, but still inside the concrete and steel pressure vessel. Control rods penetrate the moderator and a secondary shutdown system involves injecting nitrogen to the coolant.

The AGR was developed from the Magnox reactor, also graphite moderated and CO2 cooled, and a number of these are still operating in UK. They use natural uranium fuel in metal form.

Light water graphite moderated reactor (RBMK)

This is a Soviet design, developed from plutonium production reactors. It employs long (7 metre) vertical pressure tubes running through graphite moderator, and is cooled by water, which is allowed to boil in the core at 290°C, much as in a BWR. Fuel is low-enriched uranium oxide made up into fuel assemblies 3.5 metres long. With moderation largely due to the fixed graphite, excess boiling simply reduces the cooling and neutron absorbtion without inhibiting the fission reaction, and a positive feedback problem can arise (such as at Chernobyl, which was a RBMK reactor).

Because the water used to remove heat from the core in a light-water reactor absorbs some of the free neutrons normally generated during operation of the reactor, the concentration of the naturally fissionable 235U isotope in uranium used to fuel light-water reactors must be increased above the level of natural uranium to assist in sustaining the nuclear chain reaction in the reactor core: the remainder of the uranium in the fuel is 238U. Increasing the concentration of 235U in nuclear fuel uranium above the level that occurs in natural uranium is accomplished through the process of enrichment, which is explained below.

The fuel core for a light-water nuclear power reactor can have up to 3,000 fuel assemblies. An assembly consists of a group of sealed fuel rods, each filled with UO2 pellets, held in place by end plates and supported by metal spacer-grids to brace the rods and maintain the proper distances between them. The fuel core can be thought of as a reservoir from which heat energy can be extracted through the nuclear chain reaction process. During the operation of the reactor, the concentration of 235U in the fuel is decreased as those atoms undergo nuclear fission to create heat energy. Some 238U atoms are converted to atoms of fissile 239Pu, some of which will, in turn, undergo fission and produce energy. The products created by the nuclear fission reactions are retained within the fuel pellets and these become neutron-absorbing products (called "poisons") that act to slow the rate of nuclear fission and heat production. As the reactor operation is continued, a point is reached at which the declining concentration of fissile nuclei in the fuel and the increasing concentration of poisons result in lower than optimal heat energy generation, and the reactor must be shut down temporarily and refueled.

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, discussed below.

More could be added about advanced reactor designs the PBMR has a web page for example.

The Pebble Bed Reactor

This technology, under development at places such as MIT, claims a dramatically higher level of safety and efficiency. Instead of water, it uses helium as the coolant, at very high temperature, to drive a turbine directly. This eliminates the difficult systems to manage steam, and increases the transfer efficiency to about 50%.

Instead of shutting down for weeks to replace fuel rods, pebbles are placed in a bin from which spent pellets are removed from the bottom and new ones added to the top (actually, each pellet goes through the cycle several times).

The pebbles are the size of tennis balls, and it takes 30,000? of them to fuel a reactor of 100? MW. Each pebble contains carbon and uranium, and is surrounded by hard silicon carbide. Even if the helium coolant were to leak away, it would take weeks before meltdown would even be a possibility. [add more here, particularly about the pilot projects, opposition from the public, and political and economic aspects]

The Nuclear Fuel Cycle

The nuclear fuel cycle consists of front end steps that lead to the preparation of uranium for use as fuel for reactor operation and back end steps that are necessary to safely manage, prepare, and dispose of the highly radioactive spent nuclear fuel.

Front End

Exploration: A deposit of uranium, discovered by geophysical techniques, is evaluated and sampled to determine the amounts of uranium materials that are extractable at specified costs from the deposit. Uranium reserves are the amounts of ore that are estimated to be recoverable at stated costs. Uranium in nature consists primarily of two isotopes, 238U and 235U. The numbers refer to the atomic mass for each isotope, or the number of protons and neutrons in the atomic nucleus. Naturally occurring uranium consists of approximately 99.28 percent 238U and 0.71 percent 235U. The atomic nucleus of 235U will nearly always fission when struck by a free neutron, and the isotope is therefore said to be a "fissile" isotope. The nucleus of a 238U atom on the other hand, rather than undergoing fission when struck by a free neutron, will nearly always absorb the neutron and yield an atom of the isotope 239U. This isotope then undergoes natural radioactive decay to yield 239Pu, which, like 235U, is a fissile isotope. The atoms of 238U are said to be fertile, because, through neutron irradiation in the core, some eventually yield atoms of fissile 239Pu.

Mining: Uranium ore can be extracted through conventional mining in open pit and underground methods similar to those used for mining other metals. In situ leach mining methods also are used to mine uranium in the United States. In this technology, uranium is leached from the in-place ore through an array of regularly spaced wells and is then recovered from the leach solution at a surface plant. Uranium ores in the United States typically range from about 0.05 to 0.3 percent uranium oxide (U3O8). Some uranium deposits developed in other countries are of higher grade and are also larger than deposits mined in the United States. Uranium is also present in very low grade amounts (50 to 200 parts per million) in some domestic phosphate-bearing deposits of marine origin. Because very large quantities of phosphate-bearing rock are mined for the production of wet-process phosphoric acid used in high analysis fertilizers and other phosphate chemicals, at some phosphate processing plants the uranium, although present in very low concentrations, can be economically recovered from the process stream.

Milling: Mined uranium ores normally are processed by grinding the ore materials to a uniform particle size and then treating the ore to extract the uranium by chemical leaching. The milling process commonly yields dry powder-form material consisting of natural uranium, "yellowcake," which is sold on the uranium market as U3O8.

Uranium conversion: Milled uranium oxide, U3O8, must be converted to uranium hexafluoride, UF6, which is the form required by most commercial uranium enrichment facilities currently in use. A solid at room temperature, UF6 can be changed to a gaseous form at moderately higher temperatures. The UF6 conversion product contains only natural, not enriched, uranium.

Enrichment: The concentration of the fissionable isotope, 235U (0.71 percent in natural uranium) is less than that required to sustain a nuclear chain reaction in light water reactor cores. Natural UF6 thus must be enriched in the fissionable isotope for it to be used as nuclear fuel. The different levels of enrichment required for a particular nuclear fuel application are specified by the customer: light-water reactor fuel normally is enriched up to about 4 percent 235U, but uranium enriched to lower concentrations also is required. Gaseous diffusion and gas centrifuge are the commonly used uranium enrichment technologies. The gaseous diffusion process consists of passing the natural UF6 gas feed under high pressure through a series of diffusion barriers (semiporous membranes) that permit passage of the lighter 235UF6 atoms at a faster rate than the heavier 238UF6 atoms. This differential treatment, applied across a large number of diffusion "stages," progressively raises the product stream concentration of 235U relative to 238U. In the gaseous diffusion technology, the separation achieved per diffusion stage is relatively low, and a large number of stages is required to achieve the desired level of isotope enrichment. Because this technology requires a large capital outlay for facilities and it consumes large amounts of electrical energy, it is relatively cost intensive. In the gas centrifuge process, the natural UF6 gas is spun at high speed in a series of cylinders. This acts to separate the 235UF6 and 238UF6 atoms based on their slightly different atomic masses. Gas centrifuge technology involves relatively high capital costs for the specialized equipment required, but its power costs are below those for the gaseous diffusion technology. New enrichment technologies currently being developed are the atomic vapor laser isotope separation (AVLIS) and the molecular laser isotope separation (MLIS). Each laser-based enrichment process can achieve higher initial enrichment (isotope separation) factors than the diffusion or centrifuge processes can achieve. Both AVLIS and MLIS will be capable of operating at high material throughput rates.

Fabrication: For use as nuclear fuel, enriched UF6 is converted into uranium dioxide (UO2) powder which is then processed into pellet form. The pellets are then fired in a high temperature sintering furnace to create hard, ceramic pellets of enriched uranium. The cylindrical pellets then undergo a grinding process to achieve a uniform pellet size. The pellets are stacked, according to each nuclear core's design specifications, into tubes of corrosion-resistant metal alloy. The tubes are sealed to contain the fuel pellets: these tubes are called fuel rods. The finished fuel rods are grouped in special fuel assemblies that are then used to build up the nuclear fuel core of a power reactor.

Back End

Interim Storage: After its operating cycle, the reactor is shut down for refueling. The fuel discharged at that time (spent fuel) is stored either at the reactor site or, potentially, in a common facility away from reactor sites. If on-site pool storage capacity is exceeded, it may be desirable to store aged fuel in modular dry storage facilities known as Independent Spent Fuel Storage Installations (ISFSI) at the reactor site or at a facility away from the site. The spent fuel rods are usually stored in water, which provides both cooling (the spent fuel continues to generate heat as a result of residual radioactive decay) and shielding (to protect the environment from residual ionizing radiation).

Reprocessing: Spent fuel discharged from light-water reactors contains appreciable quantities of fissile (U-235, Pu-239), fertile (U-238), and other radioactive materials. These fissile and fertile materials can be chemically separated and recovered from the spent fuel. The recovered uranium and plutonium can, if economic and institutional conditions permit, be recycled for use as nuclear fuel. Currently, plants in Europe are reprocessing spent fuel from utilities in Europe and Japan. Chemical processing of the spent fuel material to recover the remaining fractions of fissionable products, 235U and 239Pu, for use in fresh fuel assemblies is technically feasible. Reprocessing of spent commercial-reactor nuclear fuel is not permitted in the United States.

Waste Disposal: A current concern in the nuclear power field is the safe disposal and isolation of either spent fuel from reactors or, if the reprocessing option is used, wastes from reprocessing plants. These materials must be isolated from the biosphere until the radioactivity contained in them has diminished to a safe level. Under the Nuclear Waste Policy Act of 1982, as amended, the Department of Energy has responsibility for the development of the waste disposal system for spent nuclear fuel and high-level radioactive waste. Current plans call for the ultimate disposal of the wastes in solid form in licensed deep, stable geologic structures.

History

Enrico Fermi was the first to build a nuclear pile and demonstrate a controlled chain reaction. 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-1950's, 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 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. 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.

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 fuel is in the form of highly radioactive waste, which needs to be handled with great care and forethought due to the long half-lives of the waste. Critics of nuclear power also assert that any of the evironmental benefits are outweighed by costs related to safety 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 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.

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 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 that 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. Because spent nuclear fuels are radioactive they are pound for pound a more substantial problem than fossil fuel plant solid wastes. In volume spent fuels are a substantially lesser problem. See atomic waste

Economic Barriers

As a general rule natural gas-fired combined-cycle power plants cost much less to build than do steam-based coal-fired plants of the same capacity. Coal-fired power plants cost significantly less to build than do nuclear-based power plants of the same capacity. Moreover, it takes much less time to build a gas-fired combined-cycle power plant than it does to build a coal-fired power plant. Nuclear plants take much longer to complete than it takes to build coal-fired power plants. Because a power plant does not earn money during construction, the longer it takes to build a power plant, the higher will be the charges for interest during construction on borrowed construction funds. 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. This is indeed the case, with coal and nuclear experiencing roughly the same operating costs (operations and maintenance plus fuel costs). 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. 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 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 have been for peak supply, where the larger nuclear and coal plants are generally for the base line supply, which has not increased as much 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, 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.

Statistics


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

In 2001, there are 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.

See also: nuclear fission -- power plant -- nuclear fusion -- electricity generation -- nuclear physics -- Enrico Fermi -- Manhattan Project -- United States Naval reactor -- technology assessment

References and Links:

Retrieved from "https://en.wikipedia.org/w/index.php?title=Nuclear_reactor&oldid=373912"