Nuclear power plant

A nuclear power plant (NPP) is a thermal power station in which the heat source is one or more nuclear reactors generating nuclear power.

Nuclear power plants are base load stations, which work best when the power output is constant (although boiling water reactors can come down to half power at night). Their units range in power from about 40 MWe to over 1000 MWe. New units under construction in 2005 are typically in the range 600-1200 MWe.

As of 2006 there are 442 licensed nuclear power reactors in the world of which 441 are currently operational operating in 31 different countries [http://www.uic.com.au/reactors.htm. Together they produce about 17% of the world's electric power.

History

Electricity was generated for the first time by a nuclear reactor on December 20, 1951 at the EBR-I experimental station near Arco, Idaho in the United States. On June 27, 1954, the world's first nuclear power plant to generate electricity for a power grid started operations at Obninsk, USSR The worlds first commercial scale power station, Calder Hall in England opened in 17 October, 1956 [http://news.bbc.co.uk/onthisday/hi/dates/stories/october/17/newsid_3147000/3147145.stm.

For more history, see nuclear reactor and nuclear power.

For information on the Chernobyl accident which did not have a containment building, see that subject and RBMK and nuclear power.

Types of nuclear power plants

Nuclear power plants are classified according to the type of reactor used. However some installations have several independent units, and these may use different classes of reactor. In addition, some of the plant-types below in the future may have passively safe features.

Fission reactors

Fission power reactors generate heat by nuclear fission of fissile isotopes of uranium and plutonium.

They may be further divided into three classes:

Thermal reactors use a neutron moderator to slow or moderate neutrons so that they are more likely to produce another fission. Neutrons created by fission are high energy, or fast, and must have their energy decreased (be made thermal) by the moderator in order to efficiently maintain the chain reaction.
Fast reactors sustain the chain reaction without needing a neutron moderator. Because they use different fuel than thermal reactors, the neutrons in a fast reactor do not need to be moderated for an efficient chain reaction to occur.
Subcritical reactors use an outside source of neutrons rather than a chain reaction to produce fission. As of 2004 this was a theoretical concept, and no prototype had been proposed or built to generate electric power by this means, although some laboratory demonstrations and several feasibility studies had been conducted.

Thermal reactor classes

Light water reactor (LWR):
- Boiling water reactor (BWR)
- Pressurized water reactor (PWR)
- SSTAR, a sealed, shippable PWR-like reactor for small grids
Graphite-moderated:
- Magnox
- Advanced gas-cooled reactor (AGR)
- High temperature gas cooled reactor (HTGR)
- RBMK
- Pebble bed reactor (PBMR)
Heavy water-moderated:
- SGHWR
- CANDU

Fast reactors

Although some of the earliest nuclear power reactors were fast reactors, they have not as a class achieved the success of thermal reactors.

Fast reactors have the advantages that their fuel cycle can use all of the uranium in natural uranium, and also transmute the longer-lived radioisotopes in their waste to faster-decaying materials. For these reasons they are inherently more sustainable as an energy source than thermal reactors. See fast breeder reactor. Because most fast reactors have historically been used for plutonium production, they are associated with nuclear proliferation concerns.

More than twenty prototype fast reactors have been built in the USA, UK, USSR, France, Germany, Japan, and India, and as of 2004 one was under construction in China. These include:

EBR-I, 0.2 MWe, USA, 1951-1964.
Dounreay Fast Reactor, 14 MWe, UK, 1958-1977.
Enrico Fermi Nuclear Generating Station Unit 1, 94 MWe, USA, 1963-1972.
EBR-II, 20 MWe, USA, 1963-1994.
Phénix, 250 MWe, France, 1973-present.
BN-350, 150 MWe plus desalination, USSR/Kazakhstan, 1973-2000.
Prototype Fast Reactor, 250 MWe, UK, 1974-1994.
BN-600, 600 MWe, USSR/Russia, 1980-present.
Superphénix, 1200 MWe, France, 1985-1996.
FBTR, 13.2 MWe, India, 1985-present.
Monju, 300 MWe, Japan, 1994-present.
PFBR, 500 MWe, India, 1998-present.

(Electric output shown is the highest output configuration where several were used, dates shown are first criticality, and last criticality in the case of a plant that is now decommissioned.)

Fusion reactors

Main article: fusion power

Nuclear fusion offers the possibility of the release of very large amounts of energy with a minimal production of radioactive waste and improved safety. However, there remain considerable scientific, technical, and economic obstacles to the generation of commercial electric power using nuclear fusion. It is therefore an active area of research, with very large-scale facilities such as JET, ITER, and the Z machine.

Advantages and disadvantages

Advantages of nuclear power plants against other mainstream energy resources are:

No greenhouse gas emissions (during normal operation) - greenhouse gases are emitted only when the Emergency Diesel Generators are tested (the processes of uranium mining and of building and decommissioning power stations produce relitively small amounts)
Does not pollute the air - zero production of dangerous and polluting gases such as carbon monoxide, sulphur dioxide, aerosols, mercury, nitrogen oxides, particulates or photochemical smog
Small solid waste generation (during normal operation)
Low fuel costs - because so little fuel is needed
Large fuel reserves - again, because so little fuel is needed
Nuclear batteries - (see SSTAR)

However, the disadvantages include:

Risk of major accidents - an example of the worse possible situation is what happened to Chernobyl Nuclear Power Plant (which did not have a containment building)
Nuclear waste - high level radioactive waste produced can remain dangerous for thousands of years
Can help produce bombs - some designs of reactor can produce plutonium, aiding nuclear proliferation (see Magnox)
High initial costs
High maintenance costs
Security concerns
High cost of decommissioning plants

Controversy

Nuclear power is highly controversial, enough so that the building of new nuclear power stations has ceased in Europe (except in Finland and Ukraine). Almost all the advantages and disadvantages are disputed in some degree by the advocates for and against nuclear power.

The cost benefits of nuclear power are also in dispute. It is generally agreed that the capital costs of nuclear power are high and the cost of the necessary fuel is low compared to other fuel sources. Proponents claim that nuclear power has low running costs, opponents claim that the numerous safety systems required significantly increase running costs.

Disposal of spent fuel and other nuclear waste is claimed by some as an advantage of nuclear power, claiming that the waste is small in quantity compared to that generated by competing technologies, and the cost of disposal small compared to the value of the power produced. Others list it as a disadvantage, claiming that the environment cannot be adequately protected from the risk of future leakages from long-term storage.

Accident indemnification

The Vienna Convention on Civil Liability for Nuclear Damage puts in place an international framework for nuclear liability ^*. However states with a majority of the world's nuclear power plants, including the U.S., Russia, China and Japan, are not party to any international nuclear liability conventions.

In the U.S., insurance for nuclear or radiological incidents is covered (for facilities licensed through 2025) by the Price-Anderson Nuclear Industries Indemnity Act.

Per the Energy policy of the United Kingdom through its Nuclear Installations Act of 1965, liability is governed for nuclear damage for which a UK nuclear licensee is responsible. The Act requires compensation to be paid for damage up to a limit of £150 million by the liable operator for ten years after the incident. Between ten and thirty years afterwards, the Government meets this obligation. The Government is also liable for additional limited cross-border liability (about £300 million) under international conventions (Paris Convention on Third Party Liability in the Field of Nuclear Energy and Brussels Convention supplementary to the Paris Convention). ^*