|
A nuclear reactor is an apparatus in which nuclear
fission chain reactions are initiated, controlled, and sustained at
a steady rate. Nuclear reactors provide heat for electricity generation, domestic and industrial heating, desalination, and naval propulsion. They
also have many research applications including providing a source of neutrons and creating various radioactive
isotopes.
Although the term 'nuclear reactor' could also refer to a nuclear fusion
reactor, the term normally refers only to nuclear fission devices.
Nuclear power can also be generated in a Radioisotope thermoelectric generator, which produces heat through subcritical
radioactive decay rather than fission in a near-critical mass.
Basic science
To provide the power for an electric generator,
nuclear power plants get heat from 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 a uranium atoms yields two smaller atoms, one to three fast-moving free neutrons, plus an amount of energy.
Uranium fission releases more neutrons than it requires. Therefore, the reaction can become self sustaining--an enhanced,
controlled radioactivity, caused by a chain reaction.
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 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 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
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 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
A number of reactor technologies have been developed. There are two basic types of reactors. They use different speeds of
neutrons in the reactor.
- Thermal (slow) reactors use slow neutrons. Most power reactors are this type. They have moderating materials to slow neutrons to low velocities to prevent capture of the neutrons by U-238. They also have fuel (fissionable material),
containments, pressure vessels, shielding, and instrumentation to monitoring and control the reactor's systems. The first
plutonium production
reactors were thermal reactors using graphite as the moderator.
- Fast reactors use fast neutrons. They require highly enriched fuel (sometimes weapons-grade), but no
moderating material. The fuel is purified so that it lacks the U-238 that would otherwise capture 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 three types, depending on whether they use pressurised fuel channels, a large
pressure vessel or gas cooling.
Most commercial and naval reactors use a large pressure vessel holding steam heated by the reactor. This serves as a layer of
shielding and containment.
The RBMK and CANDU types use
pressurised channels. 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. Plans to utilize the heat vary. 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 most common modern gas-cooled design is a Pebble bed
reactor. Pebble-bed reactors can be designed to be safe even if all equipment fails. Basically, as the core heats, the
reactor generates less power. Since the fuel elements are ceramic, they are unaffected by the higher heat. Pebble bed reactors
have been designed to use both slow and fast neutron technology, and also to breed power isotopes. All pebble bed modular reactors designed to date can also be refueled under
load.
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
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), a high temperature gas cooled reactor.
Nuclear fuel cycle
All nuclear reactors need fissionable material to operate. Uranium is currently (2004) US$52/Kg ($26/lb), and has an energy
density per unit of mass of about a million times that of oil. No shortage exists or is anticipated. If land-based reserves are
exhausted, seawater has enough uranium to power the world's current industrial civilization for a few hundred thousand years. The
Japanese have an active project to extract uranium from seawater, to reduce their dependence on imports for energy.
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.
Reactors have also been constructed to breed thorium-232 into U-233. Thorium is
about three times as abundant in the Earth's crust as uranium.
Reactor waste is poisonous, but also compact. A nuclear reactor generates just a couple of cubic meters of waste per gigawatt
year. After 600 years, reactor waste is no more radioactive than natural ores. The longer-term health hazards are heavy-metal
toxicity and low-level radiation, problems already managed by human societies in order to use other heavy metals.
The most significant problem with waste at this time (2004) is that because of regulatory delays and civil protests, most
existing waste is stored in cooling pools next to power reactors, rather than in safe geological storage.
History
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
(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, 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.
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 the 1986 Chernobyl accident 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). As of the early 21st century, nuclear power is of particular interest to both China and India because their
rapidly growing economies are requiring increasing amounts of power for which their current infrastructure makes difficult to
supply.
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. 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.
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 sufficiently concentrated enough to build a bomb, the technology used to enrich uranium could be
used to make 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.
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 (used in medicine and food preservation), though the demand for
these products can be satisfied by a relatively small number of plants.
However, the cost of nuclear power does not factor in the cost of storing the waste. Because of potential harm from radiation,
the 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.
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 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--or during waste spills is also a concern, 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.
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 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).
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.
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. This seems like good public policy, because of the good safety record of commercial aircraft. 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.
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.
However, despite the fact that North Korea joined the Nuclear Non-Proliferation Treaty in 1985,
the United States claims they have managed to obtain enough material to
create anywhere from one to five bombs from their fission reactor at Yongbyon.
The United Nations is also investigating Iran, which is said to have violated the Nuclear Non-Proliferation Treaty despite its denial.
Therefore, the claims that the Nuclear Non-Proliferation Treaty is an international success are in dispute. Clearly it
has not stopped Israel, India, Pakistan or North Korea from creating
nuclear weapons. South Africa also might have created nuclear weapons but
has since renounced its nuclear program.
Statistics
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, 75.8% 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 2 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 -- List of nuclear accidents -- List of nuclear reactors
References and links
|