Hydroelectricity

Hydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy. Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse gas carbon dioxide (CO2) than fossil fuel powered energy plants. Worldwide, an installed capacity of 777 GWe supplied 2998 TWh of hydroelectricity in 2006.[1] This was approximately 20% of the world's electricity, and accounted for about 88% of electricity from renewable sources.[2]Contents [hide]



1 History
2 Generating methods
2.1 Conventional
2.2 Pumped-storage
2.3 Run-of-the-river
2.4 Tide
3 Sizes and capacities of hydroelectric facilities
3.1 Large and specialized industrial facilities
3.2 Small
3.3 Micro
3.4 Pico
4 Calculating the amount of available power
5 Advantages and disadvantages of hydroelectricity
5.1 Advantages
5.1.1 Economics
5.1.2 CO2 emissions
5.1.3 Other uses of the reservoir
5.2 Disadvantages
5.2.1 Ecosystem damage and loss of land
5.2.2 Flow shortage
5.2.3 Methane emissions (from reservoirs)
5.2.4 Relocation
5.2.5 Failure hazard
5.3 Comparison with other methods of power generation
6 World hydroelectric capacity
7 References
8 External links

History

Hydropower has been used since ancient times to grind flour and perform others tasks. In the mid-1770s, a French engineer Bernard Forest de Bélidor published Architecture Hydraulique which described vertical- and horizontal-axis hydraulic machines. In the late 1800s, the electrical generator was developed and could now be coupled with hydraulics.[3][4] The growing demand for the Industrial Revolution would drive development as well.[5] In 1878, the world's first house to be powered with hydroelectricity was Cragside in Northumberland, England. The old Schoelkopf Power Station No. 1 near Niagara Falls in the U.S. side began to produce electricity in 1881. The first Edison hydroelectric power plant - the Vulcan Street Plant - began operating September 30, 1882, in Appleton, Wisconsin, with an output of about 12.5 kilowatts.[6] By 1886 there was about 45 hydroelectric power plants in the U.S. and Canada. By 1889, there were 200 in the U.S.[3]

At the beginning of the twentieth century, a large number of small hydroelectric power plants were being constructed by commercial companies in the mountains that surrounded metropolitan areas. By 1920 as 40% of the power produced in the United States was hydroelectric, the Federal Power Act was enacted into law. The Act created the Federal Power Commission who's main purpose was to regulate hydroelectric power plants on federal land and water. As the power plants became larger, their associated dams developed additional purposes to include flood control, irrigation and navigation. Federal funding became necessary for large-scale development and federally owned corporations like the Tennessee Valley Authority (1933) and the Bonneville Power Administration (1937) were created.[5] Additionally, the Bureau of Reclamation which had began a series of western U.S. irrigation projects in the early 1900s was now constructing large hydroelectric projects such as the 1928 Boulder Canyon Project Act.[7] The U.S. Army Corps of Engineers was also involved in hydroelectric development, completing the Bonneville Dam in 1937 and being recognized by the Flood Control Act of 1936 as the premier federal flood control agency.[8]

Hydroelectric power plants continued to become larger throughout the twentieth century. After the Hoover Dam's 1,345 MW power plant became the world's largest hydroelectric power plant in 1936 it was soon eclipsed by the 6809 MW Grand Coulee Dam in 1942.[9] Brazil's and Paraguay's Itaipu Dam opened in 1984 as the largest, producing 14,000 MW but was surpassed in 2008 by the Three Gorges Dam in China with a production capacity of 22,500 MW. Hydroelectricity would eventually supply countries like Norway, Democratic Republic of the Congo, Paraguay and Brazil with over 85% of their electricity. The United States currently has over 2,000 hydroelectric power plants which supply 49% of its renewable electricity.[5]
Generating methods

A conventional hydroelectric dam in cross section.

A typical turbine and generator
Conventional
See also: List of conventional hydroelectric power stations

Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator. The power extracted from the water depends on the volume and on the difference in height between the source and the water's outflow. This height difference is called the head. The amount of potential energy in water is proportional to the head. To deliver water to a turbine while maintaining pressure arising from the head, a large pipe called a penstock may be used.
Pumped-storage
Main article: Pumped-storage hydroelectricity
See also: List of pumped-storage hydroelectric power stations

This method produces electricity to supply high peak demands by moving water between reservoirs at different elevations. At times of low electrical demand, excess generation capacity is used to pump water into the higher reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine. Pumped-storage schemes currently provide the the most commercially important means of large-scale grid energy storage and improve the daily capacity factor of the generation system.
Run-of-the-river
Main article: Run-of-the-river hydroelectricity
See also: List of run-of-the-river hydroelectric power stations

Run-of-the-river hydroelectric stations are those with comparably smaller reservoir capacities, thus making it impossible to store water.
Tide
Main article: Tide power
See also: List of tidal power stations

A tidal power plant makes use of the daily rise and fall of water due to tides; such sources are highly predictable, and if conditions permit construction of reservoirs, can also be dispatchable to generate power during high demand periods. Less common types of hydro schemes use water's kinetic energy or undammed sources such as undershot waterwheels.
Sizes and capacities of hydroelectric facilities
Large and specialized industrial facilities

The Three Gorges Dam, seen here from space, is the largest operating hydroelectric power stations at an installed capacity of 22,500 MW.
See also: List of largest power stations in the world

Although no official definition exist for the capacity range of large hydroelectric power stations, facilities from over a few hundred megawatts to more than 10 GW is generally considered large hydroelectric facilities. Currently, only three facilities over 10 GW (10,000 MW) are in operation worldwide; Three Gorges Dam at 22.5 GW, Itaipu Dam at 14 GW, and Guri Dam at 10.2 GW. Large-scale hydroelectric power stations are more commonly seen as the largest power producing facilities in the world, with some hydroelectric facilities capable of generating more than double the installed capacities of the current largest nuclear power stations.

While many hydroelectric projects supply public electricity networks, some are created to serve specific industrial enterprises. Dedicated hydroelectric projects are often built to provide the substantial amounts of electricity needed for aluminium electrolytic plants, for example. The Grand Coulee Dam switched to support Alcoa aluminium in Bellingham, Washington, United States for American World War II airplanes before it was allowed to provide irrigation and power to citizens (in addition to aluminium power) after the war. In Suriname, the Brokopondo Reservoir was constructed to provide electricity for the Alcoa aluminium industry. New Zealand's Manapouri Power Station was constructed to supply electricity to the aluminium smelter at Tiwai Point.

The construction of these large hydroelectric facilities and the changes it makes to the environment, are often too at very large scales, creating just as much damage to the environment as at helps it by being a renewable resource. Many specialized organizations, such as the International Hydropower Association, look into these matters on a global scale.
Small
Main article: Small hydro

Small hydro is the development of hydroelectric power on a scale serving a small community or industrial plant. The definition of a small hydro project varies but a generating capacity of up to 10 megawatts (MW) is generally accepted as the upper limit of what can be termed small hydro. This may be stretched to 25 MW and 30 MW in Canada and the United States. Small-scale hydroelectricity production grew by 28% during 2008 from 2005, raising the total world small-hydro capacity to 85 GW. Over 70% of this was in China (65 GW), followed by Japan (3.5 GW), the United States (3 GW), and India (2 GW).[10]

Small hydro plants may be connected to conventional electrical distribution networks as a source of low-cost renewable energy. Alternatively, small hydro projects may be built in isolated areas that would be uneconomic to serve from a network, or in areas where there is no national electrical distribution network. Since small hydro projects usually have minimal reservoirs and civil construction work, they are seen as having a relatively low environmental impact compared to large hydro. This decreased environmental impact depends strongly on the balance between stream flow and power production.
Micro
Main article: Micro hydro

A micro-hydro facility in Vietnam.

Micro hydro is a term used for hydroelectric power installations that typically produce up to 100 KW of power. These installations can provide power to an isolated home or small community, or are sometimes connected to electric power networks. There are many of these installations around the world, particularly in developing nations as they can provide an economical source of energy without purchase of fuel.[11] Micro hydro systems complement photovoltaic solar energy systems because in many areas, water flow, and thus available hydro power, is highest in the winter when solar energy is at a minimum.
Pico
Main article: Pico hydro

Pico hydro is a term used for hydroelectric power generation of under 5 KW. It is useful in small, remote communities that require only a small amount of electricity. For example, to power one or two fluorescent light bulbs and a TV or radio for a few homes.[12] Even smaller turbines of 200-300W may power a single home in a developing country with a drop of only 1 m (3 ft). Pico-hydro setups typically are run-of-the-river, meaning that dams are not used, but rather pipes divert some of the flow, drop this down a gradient, and through the turbine before being exhausted back to the stream.
Calculating the amount of available power
Main article: Hydropower

A simple formula for approximating electric power production at a hydroelectric plant is: P = ρhrgk, where
P is Power in watts,
ρ is the density of water (~1000 kg/m3),
h is height in meters,
r is flow rate in cubic meters per second,
g is acceleration due to gravity of 9.8 m/s2,
k is a coefficient of efficiency ranging from 0 to 1. Efficiency is often higher (that is, closer to 1) with larger and more modern turbines.

Annual electric energy production depends on the available water supply. In some installations the water flow rate can vary by a factor of 10:1 over the course of a year.
Advantages and disadvantages of hydroelectricity
Advantages

The Ffestiniog Power Station can generate 360 MW of electricity within 60 seconds of the demand arising.
Economics

The major advantage of hydroelectricity is elimination of the cost of fuel. The cost of operating a hydroelectric plant is nearly immune to increases in the cost of fossil fuels such as oil, natural gas or coal, and no imports are needed.

Hydroelectric plants also tend to have longer economic lives than fuel-fired generation, with some plants now in service which were built 50 to 100 years ago.[13] Operating labor cost is also usually low, as plants are automated and have few personnel on site during normal operation.

Where a dam serves multiple purposes, a hydroelectric plant may be added with relatively low construction cost, providing a useful revenue stream to offset the costs of dam operation. It has been calculated that the sale of electricity from the Three Gorges Dam will cover the construction costs after 5 to 8 years of full generation.[14]
CO2 emissions

Since hydroelectric dams do not burn fossil fuels, they do not directly produce carbon dioxide. While some carbon dioxide is produced during manufacture and construction of the project, this is a tiny fraction of the operating emissions of equivalent fossil-fuel electricity generation. One measurement of greenhouse gas related and other externality comparison between energy sources can be found in the ExternE project by the Paul Scherrer Institut and the University of Stuttgart which was funded by the European Commission.[15] According to this project, hydroelectricity produces the least amount of greenhouse gases and externality of any energy source.[16] Coming in second place was wind, third was nuclear energy, and fourth was solar photovoltaic.[16] The extremely positive greenhouse gas impact of hydroelectricity is found especially in temperate climates. The above study was for local energy in Europe; presumably similar conditions prevail in North America and Northern Asia, which all see a regular, natural freeze/thaw cycle (with associated seasonal plant decay and regrowth).
Other uses of the reservoir

Reservoirs created by hydroelectric schemes often provide facilities for water sports, and become tourist attractions themselves. In some countries, aquaculture in reservoirs is common. Multi-use dams installed for irrigation support agriculture with a relatively constant water supply. Large hydro dams can control floods, which would otherwise affect people living downstream of the project.
Disadvantages
Ecosystem damage and loss of land

Hydroelectric power stations that uses dams would submerge large areas of land due to the requirement of a reservoir.

Large reservoirs required for the operation of hydroelectric power stations result in submersion of extensive areas upstream of the dams, destroying biologically rich and productive lowland and riverine valley forests, marshland and grasslands. The loss of land is often exacerbated by the fact that reservoirs cause habitat fragmentation of surrounding areas.

Hydroelectric projects can be disruptive to surrounding aquatic ecosystems both upstream and downstream of the plant site. For instance, studies have shown that dams along the Atlantic and Pacific coasts of North America have reduced salmon populations by preventing access to spawning grounds upstream, even though most dams in salmon habitat have fish ladders installed. Salmon spawn are also harmed on their migration to sea when they must pass through turbines. This has led to some areas transporting smolt downstream by barge during parts of the year. In some cases dams, such as the Marmot Dam, have been demolished due to the high impact on fish.[17] Turbine and power-plant designs that are easier on aquatic life are an active area of research. Mitigation measures such as fish ladders may be required at new projects or as a condition of re-licensing of existing projects.

Generation of hydroelectric power changes the downstream river environment. Water exiting a turbine usually contains very little suspended sediment, which can lead to scouring of river beds and loss of riverbanks.[18] Since turbine gates are often opened intermittently, rapid or even daily fluctuations in river flow are observed. For example, in the Grand Canyon, the daily cyclic flow variation caused by Glen Canyon Dam was found to be contributing to erosion of sand bars. Dissolved oxygen content of the water may change from pre-construction conditions. Depending on the location, water exiting from turbines is typically much warmer than the pre-dam water, which can change aquatic faunal populations, including endangered species, and prevent natural freezing processes from occurring. Some hydroelectric projects also use canals to divert a river at a shallower gradient to increase the head of the scheme. In some cases, the entire river may be diverted leaving a dry riverbed. Examples include the Tekapo and Pukaki Rivers in New Zealand.
Flow shortage

Changes in the amount of river flow will correlate with the amount of energy produced by a dam. Lower river flows because of drought, climate change or upstream dams and diversions will reduce the amount of live storage in a reservoir therefore reducing the amount of water that can be used for hydroelectricity. The result of diminished river flow can be power shortages in areas that depend heavily on hydroelectric power.
Methane emissions (from reservoirs)

The Hoover Dam in United States is a large conventional dammed-hydro facility, with an installed capacity of up to 2,080 MW.
See also: Environmental impacts of reservoirs

Lower positive impacts are found in the tropical regions, as it has been noted that the reservoirs of power plants in tropical regions may produce substantial amounts of methane. This is due to plant material in flooded areas decaying in an anaerobic environment, and forming methane, a very potent greenhouse gas. According to the World Commission on Dams report,[19] where the reservoir is large compared to the generating capacity (less than 100 watts per square metre of surface area) and no clearing of the forests in the area was undertaken prior to impoundment of the reservoir, greenhouse gas emissions from the reservoir may be higher than those of a conventional oil-fired thermal generation plant.[20] Although these emissions represent carbon already in the biosphere, not fossil deposits that had been sequestered from the carbon cycle, there is a greater amount of methane due to anaerobic decay, causing greater damage than would otherwise have occurred had the forest decayed naturally.

In boreal reservoirs of Canada and Northern Europe, however, greenhouse gas emissions are typically only 2% to 8% of any kind of conventional fossil-fuel thermal generation. A new class of underwater logging operation that targets drowned forests can mitigate the effect of forest decay.[21]

In 2007, International Rivers accused hydropower firms for cheating with fake carbon credits under the Clean Development Mechanism, for hydropower projects already finished or under construction at the moment they applied to join the CDM. These carbon credits – of hydropower projects under the CDM in developing countries – can be sold to companies and governments in rich countries, in order to comply with the Kyoto protocol.[22]
Relocation

Another disadvantage of hydroelectric dams is the need to relocate the people living where the reservoirs are planned. In February 2008, it was estimated that 40-80 million people worldwide had been physically displaced as a direct result of dam construction.[23] In many cases, no amount of compensation can replace ancestral and cultural attachments to places that have spiritual value to the displaced population. Additionally, historically and culturally important sites can be flooded and lost.

Such problems have arisen at the Aswan Dam in Egypt between 1960 and 1980, the Three Gorges Dam in China, the Clyde Dam in New Zealand, and the Ilisu Dam in Turkey.
Failure hazard
Main article: Dam failure

Because large conventional dammed-hydro facilities hold back large volumes of water, a failure due to poor construction, terrorism, or other causes can be catastrophic to downriver settlements and infrastructure. Dam failures have been some of the largest man-made disasters in history. Also, good design and construction are not an adequate guarantee of safety. Dams are tempting industrial targets for wartime attack, sabotage and terrorism, such as Operation Chastise in World War II.

The Banqiao Dam failure in Southern China directly resulted in the deaths of 26,000 people, and another 145,000 from epidemics. Millions were left homeless. Also, the creation of a dam in a geologically inappropriate location may cause disasters like the one of the Vajont Dam in Italy, where almost 2000 people died, in 1963. [24]

Smaller dams and micro hydro facilities create less risk, but can form continuing hazards even after they have been decommissioned. For example, the small Kelly Barnes Dam failed in 1967, causing 39 deaths with the Toccoa Flood, ten years after its power plant was decommissioned in 1957. [25]
Comparison with other methods of power generation

Hydroelectricity eliminates the flue gas emissions from fossil fuel combustion, including pollutants such as sulfur dioxide, nitric oxide, carbon monoxide, dust, and mercury in the coal. Hydroelectricity also avoids the hazards of coal mining and the indirect health effects of coal emissions. Compared to nuclear power, hydroelectricity generates no nuclear waste, has none of the dangers associated with uranium mining, nor nuclear leaks. Unlike uranium, hydroelectricity is also a renewable energy source.

Compared to wind farms, hydroelectricity power plants have a more predictable load factor. If the project has a storage reservoir, it can be dispatched to generate power when needed. Hydroelectric plants can be easily regulated to follow variations in power demand.

Unlike fossil-fuelled combustion turbines, construction of a hydroelectric plant requires a long lead-time for site studies, hydrological studies, and environmental impact assessment. Hydrological data up to 50 years or more is usually required to determine the best sites and operating regimes for a large hydroelectric plant. Unlike plants operated by fuel, such as fossil or nuclear energy, the number of sites that can be economically developed for hydroelectric production is limited; in many areas the most cost effective sites have already been exploited. New hydro sites tend to be far from population centers and require extensive transmission lines. Hydroelectric generation depends on rainfall in the watershed, and may be significantly reduced in years of low rainfall or snowmelt. Long-term energy yield may be affected by climate change. Utilities that primarily use hydroelectric power may spend additional capital to build extra capacity to ensure sufficient power is available in low water years.
World hydroelectric capacity

World renewable energy share as at 2008, with hydroelectricity more than 50% of all renewable energy sources.

World renewable energy potential.
See also: List of countries by electricity production from renewable sources and Cost of electricity by source

The ranking of hydro-electric capacity is either by actual annual energy production or by installed capacity power rating. A hydro-electric plant rarely operates at its full power rating over a full year; the ratio between annual average power and installed capacity rating is the capacity factor. The installed capacity is the sum of all generator nameplate power ratings. Sources came from BP Statistical Review - Full Report 2009[26]

Brazil, Canada, Norway, Switzerland and Venezuela are the only countries in the world where the majority of the internal electric energy production is from hydroelectric power, while Paraguay not only produces 100% its electricity from hydroelectric dams, but exports 90% of its production to Brazil and to Argentina. Norway produces 98–99% of its electricity from hydroelectric sources.[27]
Ten of the largest hydroelectric producers as at 2009.[28][27]Country Annual hydroelectric
production (TWh) Installed
capacity (GW) Capacity
factor % of total
capacity
China 585.2 196.79 0.37 22.25
Canada 369.5 88.974 0.59 61.12
Brazil 363.8 69.080 0.56 85.56
United States 250.6 79.511 0.42 5.74
Russia 167.0 45.000 0.42 17.64
Norway 140.5 27.528 0.49 98.25
India 115.6 33.600 0.43 15.80
Venezuela 86.8 67.17
Japan 69.2 27.229 0.37 7.21
Sweden 65.5 16.209 0.46 44.34

Major hydroelectric projects over 5,000 MWName Capacity (MW) Country Construction Completion Ref
Red Sea Dam 50,000 Africa/Middle East Proposed
Grand Inga Dam 39,000 Congo DR 2014 2025
Three Gorges Dam 22,500 China 1994 2011 [29]
Baihetan Dam 13,050 China 2009 2015
Belo Monte Dam 11,233 Brazil Proposed
Wudongde Dam 7,500 China 2009 2015

Nuclear power

"Nuclear energy" redirects here. For other uses, see Nuclear binding energy and Nuclear Energy (sculpture).
"Atomic Power" redirects here. For the film, see Atomic Power (film).

The Ikata Nuclear Power Plant, a pressurized water reactor that cools by secondary coolant exchange with the ocean.


The Susquehanna Steam Electric Station, a boiling water reactor. The reactors are located inside the rectangular containment buildings towards the front of the cooling towers.

Three nuclear powered ships, (top to bottom) nuclear cruisers USS Bainbridge and USS Long Beach with USS Enterprise the first nuclear powered aircraft carrier in 1964. Crew members are spelling out Einstein's mass-energy equivalence formula E=mc² on the flight deck.


Nuclear power is produced by controlled (i.e., non-explosive) nuclear reactions. Commercial and utility plants currently use nuclear fission reactions to heat water to produce steam, which is then used to generate electricity.

In 2009, 13-14% of the world's electricity came from nuclear power.[1] Also, more than 150 naval vessels using nuclear propulsion have been built.Contents [hide]
1 Use
1.1 Nuclear fusion
1.2 Use in space
2 History
2.1 Origins
2.2 Early years
2.3 Development
3 Nuclear reactor technology
3.1 Flexibility of nuclear power plants
4 Life cycle
4.1 Conventional fuel resources
4.1.1 Breeding
4.1.2 Fusion
4.2 Solid waste
4.2.1 High-level radioactive waste
4.2.2 Low-level radioactive waste
4.2.3 Comparing radioactive waste to industrial toxic waste
4.3 Reprocessing
4.3.1 Depleted uranium
5 Economics
6 Accidents and safety
7 Environmental effects of nuclear power
7.1 Comparisons of life-cycle greenhouse gas emissions
8 Debate on nuclear power
9 Nuclear power organizations
9.1 Against
9.2 Supportive
10 Future of the industry
11 See also
12 References
13 Further reading
14 External links

Use




Historical and projected world energy use by energy source, 1980-2030, Source: International Energy Outlook 2007, EIA.

Nuclear power installed capacity and generation, 1980 to 2007 (EIA).

The status of nuclear power globally. Click image for legend.
See also: Nuclear power by country and List of nuclear reactors

As of 2005, nuclear power provided 6.3% of the world's energy and 15% of the world's electricity, with the U.S., France, and Japan together accounting for 56.5% of nuclear generated electricity.[2] In 2007, the IAEA reported there were 439 nuclear power reactors in operation in the world,[3] operating in 31 countries.[4] As of December 2009, the world had 436 reactors.[5] Since commercial nuclear energy began in the mid-1950s, 2008 was the first year that no new nuclear power plant was connected to the grid, although two were connected in 2009.[5][6]

Annual generation of nuclear power has been on a slight downward trend since 2007, decreasing 1.8% in 2009 to 2558 TWh with nuclear power meeting 13-14% of the world's electricity demand.[1] One factor in the nuclear power percentage decrease since 2007 has been the prolonged shutdown of large reactors at the Kashiwazaki-Kariwa Nuclear Power Plant in Japan following the Niigata-Chuetsu-Oki earthquake.[1]

The United States produces the most nuclear energy, with nuclear power providing 19%[7] of the electricity it consumes, while France produces the highest percentage of its electrical energy from nuclear reactors—80% as of 2006.[8] In the European Union as a whole, nuclear energy provides 30% of the electricity.[9] Nuclear energy policy differs between European Union countries, and some, such as Austria, Estonia, and Ireland, have no active nuclear power stations. In comparison, France has a large number of these plants, with 16 multi-unit stations in current use.

In the US, while the Coal and Gas Electricity industry is projected to be worth $85 billion by 2013, Nuclear Power generators are forecast to be worth $18 billion.[10]

Many military and some civilian (such as some icebreaker) ships use nuclear marine propulsion, a form of nuclear propulsion.[11] A few space vehicles have been launched using full-fledged nuclear reactors: the Soviet RORSAT series and the American SNAP-10A.

International research is continuing into safety improvements such as passively safe plants,[12] the use of nuclear fusion, and additional uses of process heat such as hydrogen production (in support of a hydrogen economy), for desalinating sea water, and for use in district heating systems.
Nuclear fusion

Nuclear fusion reactions are safer and generate less radioactive waste than fission. These reactions appear potentially viable, though technically quite difficult and have yet to be created on a scale that could be used in a functional power plant. Fusion power has been under intense theoretical and experimental investigation since the 1950s.
Use in space

Both fission and fusion appear promising for space propulsion applications, generating higher mission velocities with less reaction mass. This is due to the much higher energy density of nuclear reactions: some 7 orders of magnitude (10,000,000 times) more energetic than the chemical reactions which power the current generation of rockets.

Radioactive decay has been used on a relatively small (few kW) scale, mostly to power space missions and experiments.
History
Origins
See also: Nuclear fission#History

The pursuit of nuclear energy for electricity generation began soon after the discovery in the early 20th century that radioactive elements, such as radium, released immense amounts of energy, according to the principle of mass–energy equivalence. However, means of harnessing such energy was impractical, because intensely radioactive elements were, by their very nature, short-lived (high energy release is correlated with short half-lives). However, the dream of harnessing "atomic energy" was quite strong, even it was dismissed by such fathers of nuclear physics like Ernest Rutherford as "moonshine." This situation, however, changed in the late 1930s, with the discovery of nuclear fission.

In 1932, James Chadwick discovered the neutron, which was immediately recognized as a potential tool for nuclear experimentation because of its lack of an electric charge. Experimentation with bombardment of materials with neutrons led Frédéric and Irène Joliot-Curie to discover induced radioactivity in 1934, which allowed the creation of radium-like elements at much less the price of natural radium. Further work by Enrico Fermi in the 1930s focused on using slow neutrons to increase the effectiveness of induced radioactivity. Experiments bombarding uranium with neutrons led Fermi to believe he had created a new, transuranic element, which he dubbed Hesperium.

But in 1938, German chemists Otto Hahn[13] and Fritz Strassmann, along with Austrian physicist Lise Meitner[14] and Meitner's nephew, Otto Robert Frisch,[15] conducted experiments with the products of neutron-bombarded uranium, as a means of further investigating Fermi's claims. They determined that the relatively tiny neutron split the nucleus of the massive uranium atoms into two roughly equal pieces, contradicting Fermi. This was an extremely surprising result: all other forms of nuclear decay involved only small changes to the mass of the nucleus, whereas this process—dubbed "fission" as a reference to biology—involved a complete rupture of the nucleus. Numerous scientists, including Leo Szilard, who was one of the first, recognized that if fission reactions released additional neutrons, a self-sustaining nuclear chain reaction could result. Once this was experimentally confirmed and announced by Frédéric Joliot-Curie in 1939, scientists in many countries (including the United States, the United Kingdom, France, Germany, and the Soviet Union) petitioned their governments for support of nuclear fission research, just on the cusp of World War II.

Constructing the core of B-Reactor at Hanford Site during the Manhattan Project.

In the United States, where Fermi and Szilard had both emigrated, this led to the creation of the first man-made reactor, known as Chicago Pile-1, which achieved criticality on December 2, 1942. This work became part of the Manhattan Project, which built large reactors at the Hanford Site (formerly the town of Hanford, Washington) to breed plutonium for use in the first nuclear weapons, which were used on the cities of Hiroshima and Nagasaki. A parallel uranium enrichment effort also was pursued.

After World War II, the prospects of using "atomic energy" for good, rather than simply for war, were greatly advocated as a reason not to keep all nuclear research controlled by military organizations. However, most scientists agreed that civilian nuclear power would take at least a decade to master, and the fact that nuclear reactors also produced weapons-usable plutonium created a situation in which most national governments (such as those in the United States, the United Kingdom, Canada, and the USSR) attempted to keep reactor research under strict government control and classification. In the United States, reactor research was conducted by the U.S. Atomic Energy Commission, primarily at Oak Ridge, Tennessee, Hanford Site, and Argonne National Laboratory.

Work in the United States, United Kingdom, Canada, and USSR proceeded over the course of the late 1940s and early 1950s. Electricity was generated for the first time by a nuclear reactor on December 20, 1951, at the EBR-I experimental station near Arco, Idaho, which initially produced about 100 kW. Work was also strongly researched in the US on nuclear marine propulsion, with a test reactor being developed by 1953. (Eventually, the USS Nautilus, the first nuclear-powered submarine, would launch in 1955.) In 1953, US President Dwight Eisenhower gave his "Atoms for Peace" speech at the United Nations, emphasizing the need to develop "peaceful" uses of nuclear power quickly. This was followed by the 1954 Amendments to the Atomic Energy Act which allowed rapid declassification of U.S. reactor technology and encouraged development by the private sector.
Early years

Calder Hall nuclear power station in the United Kingdom was the world's first nuclear power station to produce electricity in commercial quantities.[16]

The Shippingport Atomic Power Station in Shippingport, Pennsylvania was the first commercial reactor in the USA and was opened in 1957.

On June 27, 1954, the USSR's Obninsk Nuclear Power Plant became the world's first nuclear power plant to generate electricity for a power grid, and produced around 5 megawatts of electric power.[17][18]

Later in 1954, Lewis Strauss, then chairman of the United States Atomic Energy Commission (U.S. AEC, forerunner of the U.S. Nuclear Regulatory Commission and the United States Department of Energy) spoke of electricity in the future being "too cheap to meter".[19] Strauss was referring to hydrogen fusion[20][21]- which was secretly being developed as part of Project Sherwood at the time - but Strauss's statement was interpreted as a promise of very cheap energy from nuclear fission. The U.S. AEC itself had issued far more conservative testimony regarding nuclear fission to the U.S. Congress only months before, projecting that "costs can be brought down... [to]... about the same as the cost of electricity from conventional sources..." Significant disappointment would develop later on, when the new nuclear plants did not provide energy "too cheap to meter."

In 1955 the United Nations' "First Geneva Conference", then the world's largest gathering of scientists and engineers, met to explore the technology. In 1957 EURATOM was launched alongside the European Economic Community (the latter is now the European Union). The same year also saw the launch of the International Atomic Energy Agency (IAEA).

The world's first commercial nuclear power station, Calder Hall in Sellafield, England was opened in 1956 with an initial capacity of 50 MW (later 200 MW).[16][22] The first commercial nuclear generator to become operational in the United States was the Shippingport Reactor (Pennsylvania, December 1957).

One of the first organizations to develop nuclear power was the U.S. Navy, for the purpose of propelling submarines and aircraft carriers. It has an unblemished record in nuclear safety,[citation needed] perhaps because of the stringent demands of Admiral Hyman G. Rickover, who was the driving force behind nuclear marine propulsion as well as the Shippingport Reactor (Alvin Radkowsky was chief scientist at the U.S. Navy nuclear propulsion division, and was involved with the latter). The U.S. Navy has operated more nuclear reactors than any other entity, including the Soviet Navy,[citation needed][dubious – discuss] with no publicly known major incidents. The first nuclear-powered submarine, USS Nautilus (SSN-571)), was put to sea in December 1954.[23] Two U.S. nuclear submarines, USS Scorpion and USS Thresher, have been lost at sea. These vessels were both lost due to malfunctions in systems not related to the reactor plants.[citation needed] The sites are monitored and no known leakage has occurred from the onboard reactors. The United States Army also had a nuclear power program, beginning in 1954. The SM-1 Nuclear Power Plant, at Ft. Belvoir, Virginia, was the first power reactor in the US to supply electrical energy to a commercial grid (VEPCO), in April 1957, before Shippingport.
Development

History of the use of nuclear power (top) and the number of active nuclear power plants (bottom).

Installed nuclear capacity initially rose relatively quickly, rising from less than 1 gigawatt (GW) in 1960 to 100 GW in the late 1970s, and 300 GW in the late 1980s. Since the late 1980s worldwide capacity has risen much more slowly, reaching 366 GW in 2005. Between around 1970 and 1990, more than 50 GW of capacity was under construction (peaking at over 150 GW in the late 70s and early 80s) — in 2005, around 25 GW of new capacity was planned. More than two-thirds of all nuclear plants ordered after January 1970 were eventually cancelled.[23] A total of 63 nuclear units were canceled in the USA between 1975 and 1980.[24]

Washington Public Power Supply System Nuclear Power Plants 3 and 5 were never completed.

During the 1970s and 1980s rising economic costs (related to extended construction times largely due to regulatory changes and pressure-group litigation)[25] and falling fossil fuel prices made nuclear power plants then under construction less attractive. In the 1980s (U.S.) and 1990s (Europe), flat load growth and electricity liberalization also made the addition of large new baseload capacity unattractive.

The 1973 oil crisis had a significant effect on countries, such as France and Japan, which had relied more heavily on oil for electric generation (39% and 73% respectively) to invest in nuclear power.[26][27] Today, nuclear power supplies about 80% and 30% of the electricity in those countries, respectively.

A general movement against nuclear power arose during the last third of the 20th century, based on the fear of a possible nuclear accident as well as the history of accidents, fears of radiation as well as the history of radiation of the public, nuclear proliferation, and on the opposition to nuclear waste production, transport and lack of any final storage plans. Protest movements against nuclear power first emerged in the USA in the late 1970s and spread quickly to Europe and the rest of the world. Anti-nuclear power groups emerged in every country that has had a nuclear power programme. Some of these anti-nuclear power organisations are reported to have developed considerable expertise on nuclear power and energy issues.[28] In 1992, the chairman of the Nuclear Regulatory Commission said that "his agency had been pushed in the right direction on safety issues because of the pleas and protests of nuclear watchdog groups".[29]

Health and safety concerns, the 1979 accident at Three Mile Island, and the 1986 Chernobyl disaster played a part in stopping new plant construction in many countries,[30][31] although the public policy organization Brookings Institution suggests that new nuclear units have not been ordered in the U.S. because of soft demand for electricity, and cost overruns on nuclear plants due to regulatory issues and construction delays.[32]

Unlike the Three Mile Island accident, the much more serious Chernobyl accident did not increase regulations affecting Western reactors since the Chernobyl reactors were of the problematic RBMK design only used in the Soviet Union, for example lacking "robust" containment buildings.[33] Many of these reactors are still in use today. However, changes were made in both the reactors themselves (use of low enriched uranium) and in the control system (prevention of disabling safety systems) to reduce the possibility of a duplicate accident.

An international organization to promote safety awareness and professional development on operators in nuclear facilities was created: WANO; World Association of Nuclear Operators.

Opposition in Ireland and Poland prevented nuclear programs there, while Austria (1978), Sweden (1980) and Italy (1987) (influenced by Chernobyl) voted in referendums to oppose or phase out nuclear power. In July 2009, the Italian Parliament passed a law that canceled the results of an earlier referendum and allowed the immediate start of the Italian nuclear program.[34]
Nuclear reactor technology
Main article: Nuclear reactor technology

Cattenom Nuclear Power Plant.

Just as many conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear power plants convert the energy released from the nucleus of an atom, typically via nuclear fission.

When a relatively large fissile atomic nucleus (usually uranium-235 or plutonium-239) absorbs a neutron, a fission of the atom often results. Fission splits the atom into two or more smaller nuclei with kinetic energy (known as fission products) and also releases gamma radiation and free neutrons.[35] A portion of these neutrons may later be absorbed by other fissile atoms and create more fissions, which release more neutrons, and so on.[36]

This nuclear chain reaction can be controlled by using neutron poisons and neutron moderators to change the portion of neutrons that will go on to cause more fissions.[36] Nuclear reactors generally have automatic and manual systems to shut the fission reaction down if unsafe conditions are detected.[37]

A cooling system removes heat from the reactor core and transports it to another area of the plant, where the thermal energy can be harnessed to produce electricity or to do other useful work. Typically the hot coolant will be used as a heat source for a boiler, and the pressurized steam from that boiler will power one or more steam turbine driven electrical generators.[38]

There are many different reactor designs, utilizing different fuels and coolants and incorporating different control schemes. Some of these designs have been engineered to meet a specific need. Reactors for nuclear submarines and large naval ships, for example, commonly use highly enriched uranium as a fuel. This fuel choice increases the reactor's power density and extends the usable life of the nuclear fuel load, but is more expensive and a greater risk to nuclear proliferation than some of the other nuclear fuels.[39]

A number of new designs for nuclear power generation, collectively known as the Generation IV reactors, are the subject of active research and may be used for practical power generation in the future. Many of these new designs specifically attempt to make fission reactors cleaner, safer and/or less of a risk to the proliferation of nuclear weapons. Passively safe plants (such as the ESBWR) are available to be built[40] and other designs that are believed to be nearly fool-proof are being pursued.[41] Fusion reactors, which may be viable in the future, diminish or eliminate many of the risks associated with nuclear fission.[42]
Flexibility of nuclear power plants

It is often claimed that nuclear stations are inflexible in their output, implying that other forms of energy would be required to meet peak demand. While that is true for certain reactors, this is no longer true of at least some modern designs.[43]

Nuclear plants are routinely used in load following mode on a large scale in France.[44]

Boiling water reactors normally have load-following capability, implemented by varying the recirculation water flow.
Life cycle

The nuclear fuel cycle begins when uranium is mined, enriched, and manufactured into nuclear fuel, (1) which is delivered to a nuclear power plant. After usage in the power plant, the spent fuel is delivered to a reprocessing plant (2) or to a final repository (3) for geological disposition. In reprocessing 95% of spent fuel can be recycled to be returned to usage in a power plant (4).
Main article: Nuclear fuel cycle

A nuclear reactor is only part of the life-cycle for nuclear power. The process starts with mining (see Uranium mining). Uranium mines are underground, open-pit, or in-situ leach mines. In any case, the uranium ore is extracted, usually converted into a stable and compact form such as yellowcake, and then transported to a processing facility. Here, the yellowcake is converted to uranium hexafluoride, which is then enriched using various techniques. At this point, the enriched uranium, containing more than the natural 0.7% U-235, is used to make rods of the proper composition and geometry for the particular reactor that the fuel is destined for. The fuel rods will spend about 3 operational cycles (typically 6 years total now) inside the reactor, generally until about 3% of their uranium has been fissioned, then they will be moved to a spent fuel pool where the short lived isotopes generated by fission can decay away. After about 5 years in a cooling pond, the spent fuel is radioactively and thermally cool enough to handle, and it can be moved to dry storage casks or reprocessed.
Conventional fuel resources
Main articles: Uranium market and Energy development - Nuclear energy

Uranium is a fairly common element in the Earth's crust. Uranium is approximately as common as tin or germanium in Earth's crust, and is about 35 times more common than silver. Uranium is a constituent of most rocks, dirt, and of the oceans. The fact that uranium is so spread out is a problem because mining uranium is only economically feasible where there is a large concentration. Still, the world's present measured resources of uranium, economically recoverable at a price of 130 USD/kg, are enough to last for "at least a century" at current consumption rates.[45][46] This represents a higher level of assured resources than is normal for most minerals. On the basis of analogies with other metallic minerals, a doubling of price from present levels could be expected to create about a tenfold increase in measured resources, over time. However, the cost of nuclear power lies for the most part in the construction of the power station. Therefore the fuel's contribution to the overall cost of the electricity produced is relatively small, so even a large fuel price escalation will have relatively little effect on final price. For instance, typically a doubling of the uranium market price would increase the fuel cost for a light water reactor by 26% and the electricity cost about 7%, whereas doubling the price of natural gas would typically add 70% to the price of electricity from that source. At high enough prices, eventually extraction from sources such as granite and seawater become economically feasible.[47][48]

Current light water reactors make relatively inefficient use of nuclear fuel, fissioning only the very rare uranium-235 isotope. Nuclear reprocessing can make this waste reusable and more efficient reactor designs allow better use of the available resources.[49]
Breeding
Main article: Breeder reactor

As opposed to current light water reactors which use uranium-235 (0.7% of all natural uranium), fast breeder reactors use uranium-238 (99.3% of all natural uranium). It has been estimated that there is up to five billion years’ worth of uranium-238 for use in these power plants.[50]

Breeder technology has been used in several reactors, but the high cost of reprocessing fuel safely requires uranium prices of more than 200 USD/kg before becoming justified economically.[51] As of December 2005, the only breeder reactor producing power is BN-600 in Beloyarsk, Russia. The electricity output of BN-600 is 600 MW — Russia has planned to build another unit, BN-800, at Beloyarsk nuclear power plant. Also, Japan's Monju reactor is planned for restart (having been shut down since 1995), and both China and India intend to build breeder reactors.

Another alternative would be to use uranium-233 bred from thorium as fission fuel in the thorium fuel cycle. Thorium is about 3.5 times as common as uranium in the Earth's crust, and has different geographic characteristics. This would extend the total practical fissionable resource base by 450%.[52] Unlike the breeding of U-238 into plutonium, fast breeder reactors are not necessary — it can be performed satisfactorily in more conventional plants. India has looked into this technology, as it has abundant thorium reserves but little uranium.
Fusion

Fusion power advocates commonly propose the use of deuterium, or tritium, both isotopes of hydrogen, as fuel and in many current designs also lithium and boron. Assuming a fusion energy output equal to the current global output and that this does not increase in the future, then the known current lithium reserves would last 3000 years, lithium from sea water would last 60 million years, and a more complicated fusion process using only deuterium from sea water would have fuel for 150 billion years.[53] Although this process has yet to be realized, many experts and civilians alike believe fusion to be a promising future energy source due to the short lived radioactivity of the produced waste, its low carbon emissions, and its prospective power output.
Solid waste
For more details on this topic, see Radioactive waste.
See also: List of nuclear waste treatment technologies

The most important waste stream from nuclear power plants is spent nuclear fuel. It is primarily composed of unconverted uranium as well as significant quantities of transuranic actinides (plutonium and curium, mostly). In addition, about 3% of it is fission products from nuclear reactions. The actinides (uranium, plutonium, and curium) are responsible for the bulk of the long-term radioactivity, whereas the fission products are responsible for the bulk of the short-term radioactivity.[54]
High-level radioactive waste
See also: High-level radioactive waste management and High-level waste

After about 5 percent of a nuclear fuel rod has reacted inside a nuclear reactor that rod is no longer able to be used as fuel (due to the build-up of fission products). Today, scientists are experimenting on how to recycle these rods so as to reduce waste and use the remaining actinides as fuel (large-scale reprocessing is being used in a number of countries).

A typical 1000-MWe nuclear reactor produces approximately 20 cubic meters (about 27 tonnes) of spent nuclear fuel each year (but only 3 cubic meters of vitrified volume if reprocessed).[55][56] All the spent fuel produced to date by all commercial nuclear power plants in the US would cover a football field to the depth of about one meter.[57]

Spent nuclear fuel is initially very highly radioactive and so must be handled with great care and forethought. However, it becomes significantly less radioactive over the course of thousands of years of time. After 40 years, the radiation flux is 99.9% lower than it was the moment the spent fuel was removed from operation, although the spent fuel is still dangerously radioactive at that time.[49] After 10,000 years of radioactive decay, according to United States Environmental Protection Agency standards the spent nuclear fuel will no longer pose a threat to public health and safety.[citation needed]

When first extracted, spent fuel rods are stored in shielded basins of water (spent fuel pools), usually located on-site. The water provides both cooling for the still-decaying fission products, and shielding from the continuing radioactivity. After a period of time (generally five years for US plants), the now cooler, less radioactive fuel is typically moved to a dry-storage facility or dry cask storage, where the fuel is stored in steel and concrete containers. Most U.S. waste is currently stored at the nuclear site where it is generated, while suitable permanent disposal methods are discussed.

As of 2007, the United States had accumulated more than 50,000 metric tons of spent nuclear fuel from nuclear reactors.[58] Permanent storage underground in U.S. had been proposed at the Yucca Mountain nuclear waste repository, but that project has now been effectively cancelled - the permanent disposal of the U.S.'s high-level waste is an as-yet unresolved political problem.[59]

The amount of high-level waste can be reduced in several ways, particularly Nuclear reprocessing. Even so, the remaining waste will be substantially radioactive for at least 300 years even if the actinides are removed, and for up to thousands of years if the actinides are left in.[citation needed] Even with separation of all actinides, and using fast breeder reactors to destroy by transmutation some of the longer-lived non-actinides as well, the waste must be segregated from the environment for one to a few hundred years, and therefore this is properly categorized as a long-term problem. Subcritical reactors or fusion reactors could also reduce the time the waste has to be stored.[60] It has been argued[who?] that the best solution for the nuclear waste is above ground temporary storage since technology is rapidly changing. Some people believe that current waste might become a valuable resource in the future[citation needed].

According to a 2007 story broadcast on 60 Minutes, nuclear power gives France the cleanest air of any industrialized country, and the cheapest electricity in all of Europe.[61] France reprocesses its nuclear waste to reduce its mass and make more energy.[62] However, the article continues, "Today we stock containers of waste because currently scientists don't know how to reduce or eliminate the toxicity, but maybe in 100 years perhaps scientists will... Nuclear waste is an enormously difficult political problem which to date no country has solved. It is, in a sense, the Achilles heel of the nuclear industry... If France is unable to solve this issue, says Mandil, then 'I do not see how we can continue our nuclear program.'"[62] Further, reprocessing itself has its critics, such as the Union of Concerned Scientists.[63]
Low-level radioactive waste
See also: Low-level waste

The nuclear industry also produces a huge volume of low-level radioactive waste in the form of contaminated items like clothing, hand tools, water purifier resins, and (upon decommissioning) the materials of which the reactor itself is built. In the United States, the Nuclear Regulatory Commission has repeatedly attempted to allow low-level materials to be handled as normal waste: landfilled, recycled into consumer items, et cetera.[citation needed] Most low-level waste releases very low levels of radioactivity and is only considered radioactive waste because of its history.[64]
Comparing radioactive waste to industrial toxic waste

In countries with nuclear power, radioactive wastes comprise less than 1% of total industrial toxic wastes, much of which remains hazardous indefinitely.[49] Overall, nuclear power produces far less waste material by volume than fossil-fuel based power plants. Coal-burning plants are particularly noted for producing large amounts of toxic and mildly radioactive ash due to concentrating naturally occurring metals and mildly radioactive material from the coal. A recent report from Oak Ridge National Laboratory concludes that coal power actually results in more radioactivity being released into the environment than nuclear power operation, and that the population effective dose equivalent from radiation from coal plants is 100 times as much as from ideal operation of nuclear plants.[65] Indeed, coal ash is much less radioactive than nuclear waste, but ash is released directly into the environment, whereas nuclear plants use shielding to protect the environment from the irradiated reactor vessel, fuel rods, and any radioactive waste on site.[66]
Reprocessing
For more details on this topic, see Nuclear reprocessing.

Reprocessing can potentially recover up to 95% of the remaining uranium and plutonium in spent nuclear fuel, putting it into new mixed oxide fuel. This produces a reduction in long term radioactivity within the remaining waste, since this is largely short-lived fission products, and reduces its volume by over 90%. Reprocessing of civilian fuel from power reactors is currently done on large scale in Britain, France and (formerly) Russia, soon will be done in China and perhaps India, and is being done on an expanding scale in Japan. The full potential of reprocessing has not been achieved because it requires breeder reactors, which are not yet commercially available. France is generally cited as the most successful reprocessor, but it presently only recycles 28% (by mass) of the yearly fuel use, 7% within France and another 21% in Russia.[67]
Unlike other countries, the US stopped civilian reprocessing from 1976 to 1981 as one part of US non-proliferation policy, since reprocessed material such as plutonium could be used in nuclear weapons: however, reprocessing is not allowed in the U.S.[68] In the U.S., spent nuclear fuel is currently all treated as waste.[69]

In February, 2006, a new U.S. initiative, the Global Nuclear Energy Partnership was announced. It is an international effort aimed to reprocess fuel in a manner making nuclear proliferation unfeasible, while making nuclear power available to developing countries.[70]
Depleted uranium
Main article: Depleted uranium

Uranium enrichment produces many tons of depleted uranium (DU) which consists of U-238 with most of the easily fissile U-235 isotope removed. U-238 is a tough metal with several commercial uses—for example, aircraft production, radiation shielding, and armor—as it has a higher density than lead. Depleted uranium is also useful in munitions as DU penetrators (bullets or APFSDS tips) "self sharpen", due to uranium's tendency to fracture along shear bands.[71][72]

There are concerns that U-238 may lead to health problems in groups exposed to this material excessively, such as tank crews and civilians living in areas where large quantities of DU ammunition have been used in shielding, bombs, missile warheads, and bullets. In January 2003 the World Health Organization released a report finding that contamination from DU munitions were localized to a few tens of meters from the impact sites and contamination of local vegetation and water was 'extremely low'. The report also states that approximately 70% of ingested DU will leave the body after twenty four hours and 90% after a few days.[73]
Economics
Main article: Economics of new nuclear power plants
See also: Relative cost of electricity generated by different sources

The economics of nuclear power plants are primarily influenced by the high initial investment necessary to construct a plant. In 2009, estimates for the cost of a new plant in the U.S. ranged from $6 to $10 billion. It is therefore usually more economical to run them as long as possible, or construct additional reactor blocks in existing facilities. In 2008, new nuclear power plant construction costs were rising faster than the costs of other types of power plants.[74][75]. A prestigious panel assembled for a 2003 MIT study of the industry found the following:
In deregulated markets, nuclear power is not now cost competitive with coal and natural gas. However, plausible reductions by industry in capital cost, operation and maintenance costs, and construction time could reduce the gap. Carbon emission credits, if enacted by government, can give nuclear power a cost advantage.
—The Future of Nuclear Power[76]

Comparative economics with other power sources are also discussed in the Main article above and in nuclear power debate.
Accidents and safety
Main articles: Nuclear safety and Nuclear and radiation accidents
See also: Nuclear safety systems and Design Basis Accident
Nine nuclear power plant accidents with more than US$300 million in property damage, to 2010[77][78][79]Date Location Description Cost
(in millions
2006 $)
December 7, 1975 Greifswald, East Germany Electrician's error causes fire in the main trough that destroys control lines and five main coolant pumps US$443
February 22, 1977 Jasłovske Bohunice, Czechoslovakia Severe corrosion of reactor and release of radioactivity into the plant area, necessitating total decommission US$1,700
March 28, 1979 Middletown, Pennsylvania, US Loss of coolant and partial core meltdown, see Three Mile Island accident and Three Mile Island accident health effects US$2,400
March 9, 1985 Athens, Alabama, US Instrumentation systems malfunction during startup, which led to suspension of operations at all three Browns Ferry Units US$1,830
April 11, 1986 Plymouth, Massachusetts, US Recurring equipment problems force emergency shutdown of Boston Edison's Pilgrim Nuclear Power Plant US$1,001
April 26, 1986 Chernobyl, Kiev, Ukraine Steam explosion and meltdown with 4,056 deaths (see Chernobyl disaster) necessitating the evacuation of 300,000 people from Kiev and dispersing radioactive material across Europe (see Chernobyl disaster effects) US$6,700
March 31, 1987 Delta, Pennsylvania, US Peach Bottom units 2 and 3 shutdown due to cooling malfunctions and unexplained equipment problems US$400
September 2, 1996 Crystal River, Florida, US Balance-of-plant equipment malfunction forces shutdown and extensive repairs at Crystal River Unit 3 US$384
February 1, 2010 Montpelier, Vermont, US Deteriorating underground pipes from the Vermont Yankee Nuclear Power Plant leak radioactive tritium into groundwater supplies US$10 [80]

Environmental effects of nuclear power

A 2008 synthesis of 103 studies, published by Benjamin K. Sovacool, determined that the value of CO2 emissions for nuclear power over the lifecycle of a plant was 66.08 g/kWh, based on the mean value of all the 103 studies. Comparative results for wind power, hydroelectricity, solar thermal power, and solar photovoltaic were 9-10 g/kWh, 10-13 g/kWh, 13 g/kWh and 32 g/kWh respectively.[81]
Main article: Environmental effects of nuclear power
Comparisons of life-cycle greenhouse gas emissions
Main article: Comparisons of life-cycle greenhouse gas emissions

Comparisons of life cycle analysis (LCA) of carbon dioxide emissions show nuclear power as comparable to renewable energy sources.[82][83] A conclusion that is disputed by others studies.[84]
Debate on nuclear power
Main article: Nuclear power debate
See also: Nuclear energy policy and Anti-nuclear movement

The nuclear power debate is about the controversy[85][86][87] which has surrounded the deployment and use of nuclear fission reactors to generate electricity from nuclear fuel for civilian purposes. The debate about nuclear power peaked during the 1970s and 1980s, when it "reached an intensity unprecedented in the history of technology controversies", in some countries.[88][89]

Proponents of nuclear energy contend that nuclear power is a sustainable energy source that reduces carbon emissions and increases energy security by decreasing dependence on foreign oil.[90] Proponents claim that nuclear power produces virtually no conventional air pollution, such as greenhouse gases and smog, in contrast to the chief viable alternative of fossil fuel. Proponents also believe that nuclear power is the only viable course to achieve energy independence for most Western countries. Proponents claim that the risks of storing waste are small and can be further reduced by using the latest technology in newer reactors, and the operational safety record in the Western world is excellent when compared to the other major kinds of power plants.[91]

Opponents believe that nuclear power poses many threats to people and the environment[92][93][94]. These threats include the problems of processing, transport and storage of radioactive nuclear waste, the risk of nuclear weapons proliferation and terrorism, as well as health risks and environmental damage from uranium mining.[95][96] They also contend that reactors themselves are enormously complex machines where many things can and do go wrong, and there have been serious nuclear accidents.[78][97] Critics do not believe that the risks of using nuclear fission as a power source can be offset through the development of new technology. They also argue that when all the energy-intensive stages of the nuclear fuel chain are considered, from uranium mining to nuclear decommissioning, nuclear power is not a low-carbon electricity source.[98][99][100]

Arguments of economics and safety are used by both sides of the debate.
Nuclear power organizations
Against
Main article: List of anti-nuclear power groups
Friends of the Earth International, a network of environmental organizations in 77 countries.[101]
Greenpeace International, a non-governmental environmental organization[102] with offices in 41 countries.[103]
Nuclear Information and Resource Service (International)
Sortir du nucléaire (Canada)
Sortir du nucléaire (France)
Pembina Institute (Canada)
Institute for Energy and Environmental Research (United States)
Supportive
Main article: List of nuclear power groups
World Nuclear Association, a confederation of companies connected with nuclear power production. (International)
International Atomic Energy Agency (IAEA)
Nuclear Energy Institute (United States)
American Nuclear Society (United States)
United Kingdom Atomic Energy Authority (United Kingdom)
EURATOM (Europe)
Atomic Energy of Canada Limited (Canada)
Future of the industry

Diablo Canyon Power Plant in San Luis Obispo County, California, USA
See also: List of prospective nuclear units in the United States, Nuclear energy policy, Nuclear renaissance, and Mitigation of global warming

As of 2007, Watts Bar 1, which came on-line in February 7, 1996, 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, even in the U.S. and throughout Europe, investment in research and in the nuclear fuel cycle has continued, and some nuclear industry experts[104] predict electricity shortages, fossil fuel price increases, global warming and heavy metal emissions from fossil fuel use, new technology such as passively safe plants, and national energy security will renew the demand for nuclear power plants.

According to the World Nuclear Association, globally during the 1980s one new nuclear reactor started up every 17 days on average, and by the year 2015 this rate could increase to one every 5 days.[105]

Brunswick Nuclear Plant discharge canal.

Many countries remain active in developing nuclear power, including China, India, Japan and Pakistan. 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). Several EU member states actively pursue nuclear programs, while some other member states continue to have a ban for the nuclear energy use. 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 loan guarantees 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 energy development). In the energy policy of the United Kingdom it is recognized that there is a likely future energy supply shortfall, which may have to be filled by either new nuclear plant construction or maintaining existing plants beyond their programmed lifetime.

There is a possible impediment to production of nuclear power plants as only a few companies worldwide have the capacity to forge single-piece reactor pressure vessels,[106] which are necessary in most reactor designs. Utilities across the world are submitting orders years in advance of any actual need for these vessels. Other manufacturers are examining various options, including making the component themselves, or finding ways to make a similar item using alternate methods.[107] Other solutions include using designs that do not require single-piece forged pressure vessels such as Canada's Advanced CANDU Reactors or Sodium-cooled Fast Reactors.

This graph illustrates the potential rise in CO2 emissions if base-load electricity currently produced in the U.S. by nuclear power were replaced by coal or natural gas as current reactors go offline after their 60 year licenses expire. Note: graph assumes all 104 American nuclear power plants receive license extensions out to 60 years.

China plans to build more than 100 plants,[108] while in the US the licenses of almost half its reactors have already been extended to 60 years,[109] and plans to build more than 30 new ones are under consideration.[110] Further, the U.S. NRC and the U.S. Department of Energy have initiated research into Light water reactor sustainability which is hoped will lead to allowing extensions of reactor licenses beyond 60 years, in increments of 20 years, provided that safety can be maintained, as the loss in non-CO2-emitting generation capacity by retiring reactors "may serve to challenge U.S. energy security, potentially resulting in increased greenhouse gas emissions, and contributing to an imbalance between electric supply and demand."[111] In 2008, the International Atomic Energy Agency (IAEA) predicted that nuclear power capacity could double by 2030, though that would not be enough to increase nuclear's share of electricity generation.[112]
See also Nuclear technology portal
Energy portal

Electricity generation
World energy resources and consumption
Nuclear optimism
Nuclear physics
German nuclear energy project
Light Water Reactor Sustainability Program
Linear no-threshold model
List of civilian nuclear accidents
List of military nuclear accidents
Anti-nuclear protests
Nuclear power debate
Nuclear weapons debate
Uranium mining debate
Nuclear power in the United States
Passive nuclear safety

Thermal power station

A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which either drives an electrical generator or does some other work, like ship propulsion. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this is known as a Rankine cycle. The greatest variation in the design of thermal power stations is due to the different fuel sources. Some prefer to use the term energy center because such facilities convert forms of heat energy into electrical energy.

Overhead Line Insulators

Shackle type insulators [8]. These are mostly applied to support line strain (tension), such as at changes of transmission line direction [6].
5) Post type. These may have thicker insulation and more discs than pin types and can be mounted via clamp [9] or pin method. They may be applied as a pin or strain type insulator, but rarely as a suspension type [10]. Since post-type insulators may also also act as a cantilever to support line weight, post-type insulators normally have a Maximum Design Cantilever Load (MDCL) rating [10].
6) Hewlett type. A variation of the disc type, but can take more mechanical strain due to internally insulated steel bolt interlocks holding discs together instead of cement. On the other hand, the Hewlett type has higher internal electrical stress due to its internal steel bolts.

6) Pot type [13], which are usually pin mounted and often used with telephone lines. Telephone utilities may use various types of insulators other than the above pot types . However, most telephone line insulator design variations appear very similar to the pot type.


1.2 Design
Overhead line insulators are designed to have both electrical insulation and mechanical strength. Highly insulative material is used (see section 1.3) and a recurring design theme are the “watershed” fins that discourage conductive water paths during rain and provides the required electrical leakage insulation distance [10].
Design and manufacturing care is taken to ensure smooth electrical stress loading and mechanical integrity of line insulators:

1) Insulation materials may only be drilled or cored parallel sided, and may only be hot-punched at forging temperatures [14].

2) Sharp radii of curvature shall be avoided to reduce electrical stress [14]. In practice, this also means that porcelain insulators will be glazed and free from rough particles and unevenness [15]. Glass insulators will be free from internal bubbles [15] that allow partial discharge and put the insulator at risk of explosion and failure.
3) Dimensions such as shed and creepage distances may be adjusted for service in high pollution environments (with or without rainwashing), areas of airborne sea salts, icing [10] and bird risk areas [16]. Extra creepage distances are used to avoid inadvertent flashover in such highly ionised atmospheres or areas with large bird sizes (e.g. Sudan, North America). 4) Dimensions of insulator couplings are material strength dependent and guidelines are specified in standards such as AS2947.3 [17] and IEC 120.5) Voltage and waveform tests, including parameters such as water and dampness are also specified in standards such as AS2947.4 [18].

6) For disc-type insulators, the cement adhesion strength should be strong enough to hold the contacted disc insulator material to the cap and pin even when the exposed disc perimeter is shattered or vandalised (Figure 23, section 2.9).


1.3 Materials
Overhead line insulators are mostly made of the following materials [3]:
1) Porcelain, which is widely used for all the abovementioned overhead line insulator types.
2) Glass, which may be used for disc and pin types. It’s thermal stability is consistent up to 538 degrees C [3].

3) Composite synthetics, which may be a combination of fibreglass, plastic and resin. These are sometimes used for the longrod and post type insulators and have been in service for more than 25 years [10]. When modern composite synthetics are used, often the insulative core consists of glass fibers in a resin-based matrix to achieve maximum tensile strength [19].

Figure 7

Synthetic composite insulator end-fitting, silicone rubber housing and FRP rod core.
(Source: Kobayashi et al, 2000)
The housing (Figure 7) that encloses a composite synthetic also forms the water-sheds [21] and may be hydrophobic (water repellent), which helps reduce leakage current. Some housings are designed to remain hydrophobic when polluted, giving composite synthetics a distinct advantage over porcelain types [23]. A rule of thumb operating temperature range spec for housing is -50 to 50 degrees Celsius [10].

4) Plasticised wood, also referred to as Polymer Concrete [20] has been used for post type insulators. Polymer Concrete has demonstrated thermal stability in excess of 300 degrees C. Since both these designs utilise organic material, there have been concerns about material lifespan and lack of UV resistance [21].

5) Coupling fittings for overhead line insulators (i.e. the ball-socket and clevis-tongue interlocks) are normally galvanised cast iron and forged or mild steel. Clevis and pins may be specified with a coating of hot-dipped galvanised zinc to protect the base metal against severe corrosion [25] , [15].6) For ease of load specification identification, each insulator is marked with its specified Electromechanical Failing Load and the name or trademark of the manufacturer in conformance with IEC 60383 [15].

1.1 Testing
1.4.1 Porcelain and glass insulators
In countries having IEC-based test standards, two types of tests [14] are required for porcelain and glass insulators: type tests and batch tests. For type tests, AS1154.1 requires three representative samples from each factory production specification run tested1. Type tests for porcelain and glass insulators are mainly voltage withstand tests [26].
For batch tests, AS1154.3 requires both voltage and mechanical loading tests, based on the amount of units per batch of customer orders.
Note: 1. A single factory production specification run is a factory production run using a consistent set of manufacturing specifications.
1.4.2 Composite synthetic insulators
Due to greater manufacturing variability in electrical and mechanical properties of composite synthetic insulators and difficulty in predicting polymer insulation performance [10], two additional tests are required to ensure design viability and quality consistency: (1) design tests and (2) routine tests [19].
The design, type, batch and routine tests for composite synthetic insulators specified in AS4435.1 appear more comprehensive and stringent than the tests specified for porcelain and glass in AS1154.3. Tests detailed in AS4435.1 are:
1) Design tests, which include sudden release loading, thermal-mechanical loading, water immersion, mechanical loading and dye penetration tests to detect material permeability.
2) Type tests, which include wet power testing and mechanical loading.
3) Batch (sample) tests, which include dimension, locking system and mechanical load testing.
4) Routine tests, which include visual inspection and mechanical testing.
The mechanical loading test consistently recurs at each stage of testing, underlining the importance of insulator mechanical integrity under both static and dynamic loading caused by wind, ice sloughing and fault current [27].
1.4.3 Design testsDesign test standards vary from manufacturer to manufacturer. In general, design tests will indicate specifications for holding tension1, failing load2, and nominated conductor tension3 for each insulator design. For composite synthetic insulators, holding tensions (MDWL) are typically 80-90% of failing load (Ultimate Strength), while nominated conductor tension (SML) are usually specified at 50% of holding tension [28].
Notes: 1. Also referred to as Maximum Design Withstand Load (MDWL) [10]
2. Also referred to as Ultimate Strength, which Is usually twice the MDWL [10]
3. Also referred to as Specified Mechanical Load (SML) [10]
Minimum design tests for composite synthetic insulators described in IEC 61109 clauses 5.1-5.4 are as follows [29]:
1) Tests on Interfaces and Connections of Metal Fittings, which include the following:
- Dry power frequency voltage test
- Sudden load release test
- Thermal-mechanical test
- Water immersion test
- Steep-front impulse voltage test
- Repeated dry power frequency test after the initial five tests

2) Assembled Core Load-Time Test, which consist of:
- Determination of the average failing load of the core of the assembled
insulator
- Control of the slope of the strength-time curve of the insulator

3) Test of Housing: Tracking and Erosion Test

4) Tests for Core Material, which consists of a:
- Dye penetration test
- Water diffusion test

Depending on application requirements, a more rigourous set of composite synthetic insulator design test standards may be specified as follows [22]:

1) Overall performance:
- UV durability test (ASTM G53)
- Ozone durability test (JIS K 6301)
- Durability of end-fitting interfaces (IEC 61109)
- Core load-time test (IEC 61109)
- Housing tracking and erosion test (IEC 61109)
- Core material test (IEC 61109)
- Flammability test to (IEC 60707)

2) Electrical performance of insulator:
- Power-frequency wet withstand voltage (IEC 60383)
- Lightning impulse wet withstand voltage (IEC 60383)
- Switching impulse wet withstand voltage (IEC 60383)
- Maximum withstand voltage of pollution to (JEC 170)
- Arc-withstand characteristics (IEC SC36B (Secretariat) 116)
- Corona characteristics (IEC 60437)
- TV interface test (for V string insulators only)

3) Mechanical performance of insulator:
- Tensile breakdown strength (IEC 61109, JIS C 3801)
- Tensile withstand load (IEC 61109)
- Bending characteristics (JIS C 3801)
- Bending breakdown strength (JIS C 3801)

The IEC 61109 durability of end-fittings test is particularly salient because end fittings are particularly prone to water ingress due to electromechanical defects. The insulator must endure 1000kV/microsecond steep-front voltage tests in positive and negative polarity, 25 times each without puncture of the end-fitting, housing or core material.

1.4.4 Type tests
Minimum type tests are covered in detail in IEC 61109 clauses 6.1-6.4 are [29]:
1) Dry Lightning Impulse Withstand Voltage Test
2) Wet Power Frequency Test
3) Wet Switching Impulse Test
4) Mechanical Load-Time Test
As in design testing, type testing may also be expanded and/or made more rigourous, depending on customer requirements.

1.4.5 Tests to simulate lifespan and wearing
For synthetic composite insulators, electrical aging simulation tests may be carried out according to IEC 61109 Annex C, which involves accelerated electrical stressing under a natural environment and measuring cumulative insulator charge, leakage current, hydrophobicity and surface conditions using scanning electron microscopy (SEM) and photoelectron spectrometry (XPS) [21].
In Japan, it was found that overhead line insulator failures increased during typhoons. Hence there have been studies on the links between increased leakage currents during typhoons and electrical aging due to “accelerated pollution” during the typhoon [21].
IEEE Std 987 provides helpful guidelines in designing tests that simulate insulator aging, including load-time tests.


1.5 Other relevant standards
Apart from the AS, JIS and IEC design, manufacturing and test standards referred to in sections 1.2-1.5, relevant BS, IEC and IEEE standards for line insulators and associated fittings commonly used are:

BS
BS 3288-1 :1997 - Insulator and conductor fittings for overhead power lines.
BS 3288-2 :1990 - Specification for a range of fittings
BS 3288-3 :1989 - Dimension of ball and socket coupling of string insulator unitBS 3288-4 :1989 - Locking devices for Dimension of ball and socket coupling of string insulator
unit: dimensions and test
IEC

IEC 60060 – 1 (1989-11) ‑ High Voltage Test Techniques: General definitions and test requirements

IEC 60060 – 1 (1989-11) ‑ High Voltage Test Techniques: Measuring system

IEC 60120 (1984-01) ‑ Dimensions of ball and socket coupling string insulator units

IEC 60305 (1995-12) ‑ Characteristics of string insulators of the cap and pin type

IEC 60372 (1984-01) ‑ Locking devices for ball and socket couplings of string insulator units:
dimensions and tests

IEC 60383-1(1993-04) ‑ Insulators for overhead lines with a nominal voltage above 1000V: Ceramic or
Glass Insulator units for A.C system

IEC 60383-2(1993-04) - Insulators for overhead lines with a nominal voltage above 1000V: Insulator
String and Insulator for A.C System

IEC CISPR 18-2 ‑ Radio Interference characteristics of overhead power lines and high voltage
equipment
IEEE

IEEE 987 (2001) - IEEE Guide for Application of Composite Insulators
(includes sample and routine tests for tension loading)

IEEE C135.61 (1997) - IEE Standard for the Testing of Overhead Transmission and
Distribution Line Hardware
(includes batch testing guidelines)


2.0 Customer requirements, selection, installation, wearing
Line insulator manufacturers supply their insulators to customers such as utilities or utility contractors based on customer and application requirements, which in good practice should exceed application requirements. Prior to placing an order for line insulators, the customer, contractor or design consultant would specify line insulator designs based on load bearing requirement calculations, electrical withstand requirements and creepage distances (which are typically based on IEC 815 pollution indices).
Longrod and disc types insulators are widey applied as either strain (tension) type insulators or suspension type insulators (Figures 3,4) [6].


Figure 3
Line insulator assembly for strain (tension) applications.
(Source: Yusof, 2006a)



Figure 4
Line insulator assembly for suspension applications.
(Source: Yusof, 2006a)
Strain type insulator assemblies are commonly applied at long river and road crossings and changes of transmission line directions (Figure 5).


Figure 5
Line insulators taking strain (tension) at change of transmission line direction
(Source: Yusof, 2006a)

2.1 Load bearing requirements
To meet mechanical service requirements, line insulator applications are normally ordered and supplied based on specifications for holding tension1, failing load2 and nominated conductor tension3 [31] based on load-bearing calculations. An example of a line insulator load bearing calculation is given Appendix A. The transmission line designer must be aware of such load specifications, dynamic tension and safety margins to apply the insulator correctly. In practice, overhead line system designers will limit working loads for insulators to 50% of the manufacturer’s nominated conductor tension (SML) [10].
Sometimes, porcelain post-type insulators tested to ANSI standards may only have a single mechanical rating given as an average of failing load (Ultimate Strength) test results. In such circumstances, IEEE Std 987 recommends taking the MDWL as 40% of failing load [10]. It is important for the line designer or drafter to be aware of the insulator rating systems being used.
To prevent galvanic corrosion, electrical contact (mating) surfaces of the insulator are normally specified to be of similar material to the adjacent connections [15].
Notes: 1. Also referred to as Maximum Design Withstand Load (MDWL) [10]
2. Also referred to as Ultimate Strength, which Is usually twice the MDWL [10]
3. Also referred to as Specified Mechanical Load (SML) [10]

2.2 Electrical withstand requirements
The transmission line designer must also specify the desired electrical withstand of the insulators, including switching impulse voltage magnitudes [29]. For voltages less then 220kV lightning impulses may have more effect on voltage transients than switching, so the designer may decide that insulator lightning withstand spec is the critical spec for insulators applied at less then 220kV.

2.3 Creepage distance requirements
Creepage distance calculations are based on pollution indices at the area of installation. An sample creepage distance calculation is given in Appendix A and is based on IEC 815 pollution level classifications. Minimum distances for porcelain and composite synthetic insulators may also be specified (typically 20-25 mm/kV).

2.4 Grading requirements and string efficiency
The transmission line designer must be aware that voltage stress falls off linearly from the line end disc with number of insulator discs used. There is also capacitance coupling of each disc with the pylon and neighbouring structures. This has an effect on string efficiency, defined as {total voltage insulated} divided by {number of discs x voltage on line end unit} [24].
Grading devices (Figure 12, section 1.1, Figure 28, section 2.10) are often used to even out voltage stress along the string by spreading the voltage stress densities away from line-end units, increasing string efficiency, [24]. Grading devices also channel external flashovers away from insulator surfaces, preventing surface damage. They are specified for insulators 230kV and above in [10].
Capacitance grading may also be used to increase string efficiency but is rarely implemented because discs of different capacitance are tedious to stock and replace [32].
Typically, 132kV lines have 10-14 11kV insulator discs and 275kV lines 20-25 11kV discs, depending on grading ring design and pylon material [32].

2.5 Handling requirements
Synthetic composite insulators are light and can cut and scratch easily. The following handling practice is recommended in IEEE Std 987:
1) Storage away from rodents.2) Careful cutting and unpacking of the packaging they are delivered in.3) Insulators should not be stepped on or stacked directly on one another because their sheds can cut into each other. 4) They should be handled by their end fittings and inspected for cuts and abrasions prior to mounting and secured to pre-erected pylons to prevent swinging and compression. 5) Lift slings should never be placed over the sheds.

An inspection of type test certificates and data sheets supplied with the insulators is usually done by nominated utility inspectors prior to actual installation. On-site certificate checking prior to installation may include checks of sample and routine test certificates.

2.6 Installation practice
A summary of insulator and line installation is as follows.
First, electrical pylons are designed and constructed according to function: suspension, strain or dead-end. Line insulator specification selection is also made and is thus part of the pylon’s design and function [33]. For construction efficiency, line stringers (travelers) and line insulator assemblies are mounted while the pylon is assembled on the ground [34].
However, if pylon assembly is done from the air via the “pole stack” method, line insulator mounting is only done after the entire pole stack is completed to prevent “impact loading” of the line insulators.
IEEE Std 1025 emphasises grounding of structures under erection when erecting in the vicinity of energized lines.
After complete assembly and erection of pylons, all pylon bolts should be securely tightened. Line insulator bolts and nuts must be torqued to design drawing specifications.
Overhead lines are then strung along travellers at 5-8 km/h, avoiding unnecessary torsion. Cable static seating for more than 24 hours is avoided to prevent cable creepage [34].


Figure 6
Overhead lines strung along travellers after pole erection [38].
After cables are tensioned and sagged to design drawings, suspension insulator connection locations are then attached (“clipped”) to their appropriate locations on the lines [35]. All lines are grounded during clipping.
After clipping, line conductors are lifted, stringers (travelers) removed, and suspension clamps are placed on conductors for permanent line tension.
2.7 Voltage stress effects
For line insulators in general, changes in surface resistance due to chemical changes and variations on the surface or pollutive films covering the surface have an effect on surface resistance, leakage currents and withstand voltage of the insulator [10].
Hence voltage discharge external to the insulator may occur when the insulation material is too polluted, wet, and has a reasonably low resistance path allowing for the discharge during lightning, switching or transient overvoltages. Every time a discharge (external flashover) occurs, the insulator is at risk of “tracking”, a phenomenon where a physical indentation or scar appears as a semiconductive “track” caused by an electrical arc over the insulator surface [22].
Over time, with more and more discharges along the surface, the track may worsen and weaken the insulator further. Arcing horns [36] installed on line insulators may reduce the risk of surface tracking by providing a discharge path further away from the insulator material.
For the specific case of synthetic composites, studies have shown that the surfaces synthetic composite polymer housings are relatively mobile compared to porcelain and glass, and “have much greater freedom for rearranging in the bulk or at the surface” [10]. Polymer surfaces also have the interesting ability to interact with pollutants to reduce the conductance of the pollution layer, thereby improving insulator performance [10].

2.8 Mechanical stress effects
Mechanical stresses are caused by tension, bending, compression or torsion loads, which may be static or dynamic [10]. Repeated mechanical stresses can result in a unique creep1 phenomenon for composite synthetic insulators, where the residual strength of the composite material remains very high until the instant of failure [10]. Mechanical stress effects are minimised by stringent mechanical testing of insulators.
Long term mechanical and insulative performance of composite synthetic insulators are critically dependent on the continued protection provided by the housing [10]. Housing deterioration or aging must never result in exposure of the rod to the environment because this will rapidly change the mechanical characteristics of the rod [10].

2.9 Environmental effects
Environmental effects on line insulators such as end-fitting moisture ingress, surface area contamination and extreme wind-loading (such as in typhoons) contribute to insulator failure [37], [21]. Saltwater ingress can also corrode disc insulator pin material and weaken its strain (tension) rating (Figure 7).


Figure 7
Disc insulator pin corroded by airborne saltwater.
(Source: Yusof, 2006a)

Periodic testing of insulators using the “Hi-test insulator tester” or “Buzzer” method (section 3.1) minimises risk of insulator failure due to moisture ingress [37], [15].
Insulators installed in areas with heavy airborne pollutants such as phosphate, cement, pulp and lime processing plants have experienced external flashovers due severe insulator pollution that sometimes engulfs the entire insulator. In South Africa, line insulator reliability was found to suffer due to bird droppings and bird electrocutions. In South Africa, birds perched above line insulators that do not bridge an electrical circuit still get electrocuted when they excrete continuous “streamer” droppings or urine that bridge a circuit across the below insulators [39].
Periodic insulator washing (section 3.2 and Figure 9) reduces the risk of insulator failure due to surface area contamination. Maintenance techniques to prevent bird perching are discussed in section 3.3.
Bird electrocution occurs when a bird’s wings or “other appendages” complete an electrical circuit by bridging the gap between two live wires, or a live wire and a grounded wire or structure [40]. In addition to insulator failure, line outages and endangered bird deaths, hazardous bushfires and wildfires sometimes start when the electrocuted bird catches fire and falls to the ground. During electrocution, the insulator is shorted out, the bird is killed or severely burned, and power outages occur.
Studies by The German Society for Nature Conservation [41] have shown that:
1) Birds as small as 25 cm can short out pin-type or pot-type “upright” insulators between the metallic cross-bar and the live line [42].2) Insulator arcing horns with gaps 60 or less are particularly at risk of shorting due to bird electrocution.3) Insulators that most at risk to shorting via bird electrocution are those designed between 1 - 60kV transmission.

A bird-safe arcing horn has been developed in Japan by IERE [43]. In effect, the device utilises a subhorn and “mini-insulator” that prevents bird electrocution through the main horns under normal operating conditions. Abnormal operating currents from lightning strikes and voltage surges are passed through the mini-insulator to the main horns. The subhorn picks up flashovers due to surface area contamination.
Although vultures in South Africa have been known to eat fibre-optic cable insulation [42], incidents of synthetic composite line insulator housing being eaten by vultures or other wildlife are rare.
Crows’ nests that contain abandoned wiring material have also been known to short out insulators when they span a circuit bridge [39]. Maintenance techniques to reduce such risks are discussed in section 3.3.

2.10 Vandalism
Glass and porcelain insulators are susceptible to shattering by thrown or shot projectiles (Figure 8). This is a major reason for recent trends to replace older glass and porcelain insulators with composite synthetics.
Figure 8
A vandalised disc insulator suspension string.
Note the cement integrity and adhesion to remaining glass despite disc destruction.
Note also the grading device rings below the suspension string to help distribute voltage stress.
(Source: Yusof, 2006a)


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Overhead line insulators are used to electrically insulate pylons from live electrical cables. This Knol on overhead line insulators covers aspects such as insulator type, design, installation, environmental and voltage stress effects, relevant standards, condition monitoring techniques. AS standards mentioned below are based on IEC and ISO standards and thus may be applied broadly





1.0 What is an overhead line insulator?


Overhead line insulators, as the name suggests, are used to electrically insulate pylons from live electrical cables.

Overhead line insulators may consist of a string of insulator units (see Figure 1), depending on insulator type and application. The higher the line voltage insulated, the more insulator units used in the string.

Different types of line insulators are used, depending on voltage and mechanical strain (tension) requirements. The more widely used types are as follows [3]:
1) Disc type, where insulation discs (also called insulation units) are strung together depending on the insulation level desired. Each disc is typically rated at 10-12kV, with a capacitance of 30-40pF [24]. Discs are strung together via their caps and pins. Locking mechanisms may be ball-socket or clevis-tongue type. The cap is insulated form the pin via the porcelain (or glass) disc which adheres to the cap and pin via adhesive cement.

2) Longrod type. These may also be strung together for higher insulation and may have similar ball-socket and clevis-tongue locking mechanisms used among the disc types [3]. Their longer length makes them applicable for phase-to-phase insulation to reduce line galloping during strong winds [10]. Both disc and longrod-type insulators are commonly used in suspension or strain (tension) insulator applications [4].


3) Pin type. Pin types are screwed onto a bolt shank secured on the cross-arm of the transmission pole or pylon. The pin type does not take main transmission line strain (tension) [6], and functions as a jumper line insulator [5].

3.0 Condition monitoring and maintenance
Studies have shown that the highest incidence of insulator failure occurs at dead-end applications, where the mechanical and electrical stresses are higher [21]. Non-visible insulator failure is more common than visible insulator failure, and symptoms of non-visible insulator failure include [37]:
1) Radio frequency interference (RFI)
2) Blinking lights
3) Nuisance overcurrent relay and ground fault tripping
4) Blown fuses
5) Pole top fires
Non-visible insulator failure includes non-visible moisture ingress inside an insulation string. The moisture may get vapourised during a lightning strike or switching surge, resulting in sudden internal volume expansion and the insulator blown apart [37].
Condition monitoring of line insulators allows operation and maintenance crews to detect and pinpoint insulator failure. This may be done in real time via leakage current monitoring, RFI troubleshooting, during periodic inspections or as a safety procedure prior to live line work [37].

3.1 Condition monitoring techniques
A power utility that receives customer complaints about non-visible insulator failure symptoms will usually investigate sources of RFI interference along nearby overhead line insulators. Such investigations may utilise Radio Directional Finding (RDF) techniques.
Alternatively, or in conjunction with RDF, infrared or UV photography may be applied to further pinpoint the insulator responsible for the RFI noise (Figures 32, 33). An advantage of UV over IR photography is the UV camera’s ability to capture electrical corona discharges on cracked or punctured insulators at up to 150m range [44].
Since non-visible insulator failure may also pose a safety threat to line crews, standard practice for condition monitoring prior to live-line work involves line testing using a the “Hi-test insulator tester” [37] or “Buzzer” [15] method. This method has also been used to detect non-visible defects on insulators within hotstick range.
The “Hi-test insulator tester” or “Buzzer” method involves fitting a 10kVDC self-contained insulator tester at the end of a hotstick and placing the tester’s two probes such that the line insulator is in between them. The condition of the insulator is then indicated via a LED display or audible warning buzzer [37].

3.2 Maintenance
One way of reducing or eliminating radio frequency interference (RFI) caused by insulator failure or defects is to de-energise the line and replace the insulator. A temporary method for porcelain or glass insulators is to repair or patch the the insulator's arc voids using grease. Greasing is not recommended for synthetic composite [10], because the grease may worsen tracking along the composite’s surface.
Preventive maintenance of line insulators involves ensuring they are debris, salt and pollution-free. There have been studies and proposed techniques for assessing line insulator pollution rates by monitoring insulator leakage currents [45] but such real-time pollution monitoring techniques are uncommon.
Scheduled washing of line insulators are a more practical and economical practice for preventive maintenance. One method of washing is by helicopter, however, washing via live line approaches are also practiced.
IEEE Std 987 recommends consulting with insulator manufacturers prior to high pressure washing (3-7 MPa), because not all composite synthetic insulator designs can withstand such forces.

3.3 Replacement
Damaged and defective insulators usually require replacement. If a porcelain or glass insulator is to be replaced with its synthetic composite equivalent, care must be taken in selection of the new insulator because insulators with similar dry arc ratings may have different electrical characteristics due to the difference in end-fitting lengths. However, if grading rings are used, the dry arc distance is still the distance between rings [10].
Insulator replacement (Figure 9) is typically done via live-line methods (KTPower, 2006 and Smith & Mailey, 2003) to prevent grid power interruption.


Figure 9
Live line insulator replacement [46].
3.4 Reducing bird risk
A guide to reducing risks of bird electrocutions by making electrical pylon and design and insulator location more bird-friendly is provided in a copyrighted brochure by The German Society for Nature Conservation.
Current practice in South Africa and North America is to place safe perches and bird guard structures on pylons (Figure 10).


Figure 10
Application of safe bird perches and bird guards to protect line insulators [47].
To reduce the risk of birds nesting over power lines and their urine or dropping shorting out line insulators, bird nesting platforms are sometimes constructed in pylons safely below the level of the line insulators.