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Nuclear Energy: Background

Nuclear Energy

Enormous forces hold together the nuclei of atoms. When certain types of atoms are split they release large amounts of energy. The process of splitting atoms is called nuclear fission. The heat produced from the nuclear fission is used to generate electricity in power plants. The generation of electricity from nuclear fission does not involve the use of fossil fuels and can be sustainable and clean. However, while nuclear energy does not release greenhouse gases, the risk of environmental contamination from nuclear accidents can be very serious. Despite this, nuclear power is presently producing electricity, without incident, in more than 400 power plants worldwide.

Nuclear Power Plants
Thick concrete walls with rebar contain the plant. A nuclear power plant and electricity power lines in the United States.
Source from Left to Right NRC | Wikipedia

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Nuclear Fission

In nuclear fission, atoms are split by artificially bombarding them with neutrons. Only specific kinds of atoms are used in nuclear reactors. For example, uranium 235 atoms (U-235) are fissile, but atoms of uranium 238 (U-238) do not readily split apart. Different varieties of the same atom are called isotopes. Each isotope contains an unusual number of neutrons in its nucleus, but the same number of electrons and protons. Fissile elements are split by firing neutrons at them. Different elements split upon bombardment at different speeds. In nuclear reactors, when U-235 is hit by a neutron it is initially absorbed into the nucleus to form a new isotope U-236. However, U-236 is unstable and the extra neutron causes the atom to split apart into different atoms. There are over twenty different products that can be formed during this process, but the combined mass of all the products always adds up to 236, which is the atomic mass of the original atom plus the extra neutron. Fission results in the formation of new elements, the release of neutrons, and very importantly - the release of energy. The energy is released from the forces that hold the particles in the nuclei of the atoms together.

Diagram of a chain reaction
Source: NRC

By releasing additional neutrons, more collisions with source material U-235 can occur. Consequently, more energy is released, more byproducts are formed, and even more neutrons are released. This way, as long as there is a supply of fissile source material, free neutrons can keep splitting it down, creating more byproducts and more particles to keep splitting the source. This process is called a chain reaction or runaway nuclear reaction. Throughout this process, the energy in the nuclei of U-235 atoms is released. In a nuclear reactor, this process has to be controlled from running away, and releasing too much energy to cause a meltdown of the reactor. Thus, the numbers of neutrons that are released to generate the next phase of fission are carefully monitored and moderated.

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Enrichment of Nuclear Fuel

When uranium is mined from the Earth, it contains numerous isotopes, not just U-235. Many of them do not readily split upon bombardment with neutrons. In nature, less than one percent of mined uranium is U-235. As a result, the uranium must be enriched before it can be used as fuel in a nuclear reactor. To do this, uranium ore is added to fluorine to form uranium hexafluoride. This compound is heated to form a vapor. These uranium bearing molecules are quite light and pass through filters enabling them to be concentrated. After collection, the uranium is then stripped away from the fluorine to produce an enriched fuel with a concentration of about 4% U-235.

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Inside a Nuclear Power Plant

enriched uranium
Pellets enclosing enriched uranium.
Source: DOE

A nuclear reactor is the part of a nuclear plant where nuclear fission occurs. The reactor contains very few parts. Fuel rods, each about 3.5 meters in length, are bundled into assemblies of several hundred. Each fuel rod contains hundreds of pellets of uranium fuel, each about 1 cm long. Within the reactor core are also control rods. These are composed of materials, such as cadmium, which absorb neutrons. They can be lowered or raised into the core. When lowered, they absorb neutrons and slow down the nuclear reactions in the uranium fuel. Water is also used in the core as a moderator to slow down the speed of very fast moving neutrons so they can be captured by the nuclei of the uranium. If they move too quickly, they pass straight through the nuclei and splitting does not occur.

Control Room of a Nuclear power plant
The operation of the plant is constantly monitored by engineers and technicians from a central control room.
Source: NRC

Water is used in nuclear reactors as a coolant. Coolant absorbs the heat released by nuclear fission. The water is kept under very high pressures so that is does not simply boil away. This raises its boiling point to above 100° C. This is a key process in pressurized water reactors (PWRs). During this process the water becomes radioactive and contains radioactive byproducts from the fission process. The heated water must remain isolated and flows through a heat exchanger where the heat is passed on to non-radioactive water. This must be done in a closed cell so that the clean water does not become contaminated. The heat from the coolant flows through pipes that have other pipes inside them containing clean water for heating. The hot, clean water is then moved to a turbine to generate electricity. The process for generating electricity is much the same as with a coal burning power plant. However, about one kg of concentrated nuclear fuel releases the same energy as 17,000 kg of coal. Even after passing through the turbine the water is still very hot. It is then moved into very large cooling towers and air cooled into steam. Some plants do not have towers and the hot clean water flows into a reservoir and is replaced by pumping in cold water.

Diagram of a nuclear power plant
Nuclear power generation and the transport of electricity.
Source: NRC

There are several other types of nuclear power plants as well. PWR plants are the most common. In Boiling Water Reactors (BWR), radioactive water drives the turbine directly. This allows the core to be kept under lower pressures. Over time, the turbines become radioactive and must be carefully disposed of when the reactor is dismantled. In Heavy Water Reactors (HWR), deuterium, an isotope of hydrogen with one neutron, is used. This allows un-enriched uranium to be used as fuel because it is very efficient at controlling the bombarding neutrons that split the uranium atoms.

Nuclear Power Plant
Cooling towers from the Cofrentes (Spain)
nuclear power plant.
Source: Wikipedia

Nuclear power plants have several important safety features. The fuel remains bound in ceramic pellets until it is used. The fuel rods are placed within a steel pressure vessel with walls up to 30 cm thick. The walls of the plant are made of reinforced concrete at least 1 m thick to reduce the risk of an impact on the exterior affecting the core. Nuclear power plants have complex systems to ensure their safety from mechanical, computer, or user error. They have back-up water systems to ensure continuous cooling of the fuel as well as back-up generators in case of a loss of power.

Diagram of a reactor
A PWR reactor fuel assembly
Source: Wikipedia

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Nuclear Waste

Man looking at a pond for spent fuel rods
Spent fuel rods are kept underwater for months.
Source: Wikipedia

The time period for storing high level nuclear waste is incredibly long, in the order of thousands of years. This is because of the time it takes for the amount of radiation in the waste to decay to a level that is considered safe. For this reason, the methods for storing nuclear waste are considered temporary. Spent fuel rods are the most radioactive form of waste. They contain products from nuclear fission as well as unused uranium. Consequently, they emit a number of different kinds of particles that are hazardous to health. The amount of time it takes for half of the mass of nucleus of a radioactive atom to decay is called the half-life. Half-lives vary from one element to another. Some isotopes decay in a few minutes. Strontium-90 and cesium-137 have half-lives of about 30 years. Plutonium-239 has a half-life of 24,000 years. After being used, they are removed from the reactor core and cooled down in a pool of boric acid to remove some the radioactivity. Other neutron absorbing materials are placed between the rods. They remain in a pool for six months to several years. Many nuclear plants have to enlarge their pools to accommodate more spent fuel rods. Fuel rods must not be overcrowded or there is a risk of restarting a chain reaction. Therefore, the rods must be continuously monitored. After five years in a pool, some radioactive waste is encased in concrete casks and placed in concrete bunkers, called dry cask storage. Other forms of low level radioactive waste can be buried in the crust where it decays by breaking down in a few hundred years.

Storage of lowlevel nuclear waste in the ground
Low level radioactive waste is stored in the
ground in sealed units at government approved
sites. Source: NRC

No permanent solution has been found for high level nuclear waste. Several ideas have been proposed, including deep geological burial in the ground in sealed casks. Proposed sites must avoid contamination of groundwater. They must also have a very low risk of earthquakes. Locations must also be far from major population centers. In the U.S., a permanent storage site for high level nuclear waste was selected at Yucca Mountain, 660 meters below the surface, in a remote part of Nevada. Studies there began in 1978. Tunnels leading into the mountain were to be used to move radioactive waste underground. The site was due to open in 2017 but was cancelled in 2009, prior to completion, because of concerns for safety. The Environmental Protection Agency has imposed stringent regulations on any future use of the site during the next 10,000 to 1 million years in the future.

47-meter long storage chamber
Testing a nuclear storage facility: a 47-
meter long storage chamber deep inside
Yucca Mountain. Source: U.S.
Department of Energy

Politicians, environmentalists, scientists, and the public continue to debate the merits of storage in places as distant as Earth's mantle, the seafloor, and Antarctica. The fact that no solution has been found underscores the long term risk involved in decisions surrounding the management of high level nuclear waste.

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History of Nuclear Energy Production

In 1932, Enrico Fermi, an Italian physicist, discovered that upon bombarding uranium with neutrons the remaining mass was much lighter than expected. Other scientists performing similar experiments reasoned that during the splitting process, part of the uranium's mass had been converted to energy. Scientists around the world began to believe in the possibility of inducing a self-sustaining nuclear reaction. In 1942, the first nuclear reactor was built in a squash court at the University of Chicago by a group of scientists led by Fermi. The ingredients were uranium and graphite and they were arranged in a cube. This reactor became known as Chicago Pile 1. Soon after, the attention of scientists was diverted to military applications at the end of the World War Two. The Manhattan Project was the codename for the U.S. government's top secret nuclear bomb program.

Scientists continued to work on breeder reactors where chain reactions made more fissionable materials than they used to release energy. In 1946, the Atomic Energy Commission oversaw the building of Experimental Breeder Reactor I in Idaho. The reactor successfully generated the first electricity in 1951, powering four 200-Watt light bulbs. In 1955, Arco, Idaho, a small town of just 1000 people, was powered by a nuclear reactor called BORAX III. A few years later, in 1957, at Shippingport, Pennsylvania, the world's first large-scale commercial nuclear reactor generated electricity to power Pittsburgh. Water was used to cool the reactor core. In 1959, in Illinois, the first non-government funded plant produced electricity. By the 1960's nuclear power was seen as safe, clean, and an economical means to generate electricity. In 1963, the Oyster Creek plant in New Jersey was built as a viable alternative to coal power. By 1974, it was the first plant to achieve 1000 Megawatts of capacity. Over the next two decades, interest in nuclear power declined as concerns grew about the safety and environmental risks associated with malfunctions at nuclear plants and the disposal of waste. Despite this, by the 1980's the U.S. had built more than 100 nuclear power plants.

Borax III Reactor
The Borax III Reactor powered a
small town in Idaho, 1955.
Source: U.S. Department of Energy

Since about 2000, the term "nuclear renaissance" has been used to describe a renewed interest in nuclear power around the world. It is largely driven by fossil fuel prices and concerns about greenhouse gas emissions. New reactors are being built in France and Finland. China is adding 25 new reactors to the 14 already in service. South Korea, India, and Russia are also building reactors. Meanwhile, older reactors, often with smaller capacities, will be decommissioned over the next 10-15 years. However, whenever a nuclear accident occurs, such as in Japan in 2011, the development process is restricted.

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Nuclear Accidents

Fortunately, there have been few nuclear accidents. However, because of the threat from high-level radiation leaks, any accident involving nuclear power plants are considered very serious.

In 1979, at 3 Mile Island in Pennsylvania, a system error caused the worst nuclear accident in U.S. history when spent coolant was released into the environment. No one was injured. In 1986, two explosions in the Chernobyl No.4 nuclear power plant in the former U.S.S.R. released large amounts of radiation into the environment. The design of a reactor similar to the one from the Chernobyl disaster has never been approved in North America or Europe. According to figures from the United Nations, the Chernobyl death toll is 56 (31 workers at the time, more since and 9 from thyroid cancer). Most recently, in Japan in March 2011, a 15 meter tsunami disabled the cooling of three of eleven nuclear reactors in the Fukushima area. All three reactor cores melted in three days and four reactors were determined to be nonfunctional. Full "cold shut down" was not achieved for nine months. A recent oceanic study estimates 27,000 terabecquerels of radioactive cesium 137 leaked into the sea. Reactor leakage and at least one damaged spent fuel pool may have emitted 35,800 terabecquerels of cesium 137 into the atmosphere. A becquerel represents one radioactive decay per second and involves the release of atomic energy, which can damage human cells and DNA. A terabecquerel is 1 million times 1 million becquerels. The estimated amount from the Fukushima plant is about 42 percent of that released into the atmosphere during the Chernobyl explosion in 1986. Cesium 137 has a half-life of 30 years and is an ongoing source of concern for public health. Analysts have predicted that around 30 nuclear plants may be closed world-wide, with those located in seismic zones or close to national boundaries being the most likely to be shut down.

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Global Distribution of Nuclear Power

According to the European Nuclear Society, as of 2012, there are 435 nuclear power plant units worldwide. Each unit can contain several reactors. Their installed electric net capacity is about 368 Gigawatts. Between 1951 and 2010 a total of 67,240 billion kWh of electricity has been generated. The total operating experience amounted to 14,745 years by February 2012. The United States presently has 104 nuclear reactors which nearly exceeds the total of the next two countries combined, France (58) and Japan (50). Across 15 countries, a further 63 plants are under construction with an installed capacity of 61 Gigawatts. Of the 63 new plants under construction, 26 are in China and 10 are in the Russian Federation.

Following the Fukushima accident, China, Germany, Switzerland, Thailand, United Kingdom, Italy and the Philippines are reconsidering their nuclear power programs. The International Energy Agency halved its estimate of additional nuclear generating capacity to be built by 2035. Many countries remain opposed to nuclear power, including Australia, Austria, Denmark, Greece, Ireland, Latvia, Lichtenstein, Luxembourg, Portugal, Israel, Malaysia, New Zealand, and Norway.

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Nuclear Power in the United States

The United States has embraced nuclear power for more than 50 years. Approximately 20% of the nation's electricity is generated from nuclear fuel. The following map of the location of plants in the United States shows a clear concentration of plants in the east of country. Only three states in the west utilize nuclear power: California, Arizona and Washington. In the east, only Indiana, Kentucky, West Virginia, Delaware and Maine do not use nuclear power. Since 1974, U.S. nuclear power plants have been overseen by the Nuclear Regulatory Commission (NRC). Because of the risk of the mismanagement of nuclear materials, the NRC acts to ensure reactor safety and security, reactor licensing and renewal, radioactive material safety, security and licensing, and spent fuel management (storage, security, recycling, and disposal). Nuclear research facilities in the United States are also overseen by the NRC.

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The Risks from Nuclear Power

The major risks associated with nuclear power come from the health effects of radiation. The natural environment contains a background level of radiation. This means that sub-atomic particles moving at very high speeds are continuously bombarding our bodies. In fact, 15,000 particles strike our bodies from natural sources every second. During an X-ray, around 100 billion particles strike our bodies. The chance of natural radiation causing cellular damage or cancer is approximately 1 in 300 million billion. Natural radiation is estimated to cause about 1% of all cancers. Accidents involving the release of radioactive materials from nuclear power plants are very rare. It has been calculated that because of nuclear power plants, the average American is exposed to about an additional risk of 0.2% above their exposure from natural radiation (1%), which is a very small figure.

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Advantages of Nuclear Power

Safely produced nuclear power is a relatively clean way of generating energy. It does not use fossil fuels and does not release greenhouse gases or other pollutants such a sulfur dioxide or soot. The technology to produce nuclear power exists and is ready to be installed. A single nuclear facility can produce a very large amount of electricity from a relatively small amount of fuel.

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Despite the advantages of clean power, safety remains a major concern. Because of the radioactive nature of the fuel, the coolants, and waste from nuclear reactors, any leak has far reaching and potentially long term effects on the environment and humans. The waste material from nuclear power plants remains dangerously radioactive for thousands of years. It requires special processing and expensive storage in secure facilities to ensure that it does not leak. According to the Environmental Protection Agency, waste must be confined safely for 10,000 years. The byproducts of nuclear fuel can be used in nuclear weapons which have catastrophic capabilities. The proliferation of nuclear weapons increases the risk of a nuclear accident or wars.

The initial capital costs for building a nuclear power plant are also very high, between 3 and 5 billion dollars. The planning and building process for power plants can take several decades. The cost per Megawatt of electricity generated from a nuclear power plants is about the same as that for coal. This cost includes mining and processing fuel, transportation, management, safety compliance, and decommissioning a nuclear plant. Fuel efficiency is also a concern. Current nuclear power plants use one fiftieth of the total energy content in U-235. Nuclear power plants also have a finite lifespan and must be decommissioned after a couple of decades. It can take several decades to restore the site of a nuclear power plant to its original state.

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