Nuclear reactor


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.

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

   * Pressurized water reactor (PWR)
   * Boiling water reactor (BWR)
   * Pressurised Heavy Water Reactor (PHWR or CANDU)
   * Advanced gas-cooled Reactor (AGR)
   * Light water cooled graphite moderated reactor (RBMK)
   * D2G reactor

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.

   * Pebble Bed Reactor

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-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.

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-lives of the waste.

Critics of nuclear power also 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, 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 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. 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 experiencing 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, 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

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 occurred 1 500 000 000 years ago in Oklo, Gabon, Africa.