NUCLEAR ENERGYNuclear energyis the energy in thenucleus, or core, of an atom.Atoms are tiny units that make up all matter in theuniverse. Energy is what holds the nucleus together. There is a huge amount of power in an atoms dense nucleus. In fact, the power that holds the nucleus together is officially called the strong force.
Nuclear energy can be used to createelectricity, but it must first be released from the atom. Innuclear fission, atoms are split to release the energy.
Anuclear reactor, orpower plant, is a series of machines that can control nuclear fission to produce electricity. The fuel that nuclear reactors use to produce nuclear fission ispellets of the elementuranium. In a nuclear reactor, atoms of uranium are forced to break apart. As they split, the atoms release tiny particles called fission products. Fission products cause other uranium atoms to split, starting achain reaction. The energy released from this chain reaction creates heat.
The heat created by nuclear fission warms the reactors cooling agent. A cooling agent is usually water, but some nuclear reactors use liquid metal ormoltensalt. The cooling agent, heated by nuclear fission, produces steam. The steam turnsturbines, or wheels turned by a flowingcurrent. The turbines drivegenerators, or engines that create electricity.
Rods of material callednuclear poisoncan adjust how much electricity is produced. Nuclear poisons are materials, such as a type of the elementxenon, that absorbsome of the fission products created by nuclear fission. The more rods of nuclear poison that are present during the chain reaction, the slower and more controlled the reaction will be. Removing the rods will allow a stronger chain reaction and create more electricity.
About 15 percent of the worlds electricity is generated by nuclear power plants. Nations such as Lithuania, France, and Slovakia create almost all of their electricity from nuclear power plants.Nuclear Food: Uranium
Uranium is thefuelmost widely used to produce nuclear energy. Thats because uranium atoms split apart relatively easily. Its also a very common element, found in rocks all over the world. However, the specific type of uranium used to produce nuclear energy, calledU-235, is rare. U-235 makes up less than one percent of the uranium in the world.Although some of the uranium the United States uses is mined in this country, most isimported. The U.S. gets uranium from Australia, Canada, Kazakhstan, Russia, and Uzbekistan. Once uranium is mined, it must be extracted from otherminerals. It must also be processed before it can be used.
Because nuclear fuel can be used to createnuclear weapons as well as nuclear reactors, only nations that are part of theNuclear Non-Proliferation Treaty (NPT) are allowed to import uranium orplutonium, another nuclear fuel. The treaty promotes the peaceful use of nuclear fuel, as well as limiting the spread of nuclear weapons.
A typical nuclear reactor uses about 59,000 metric tons (65,000 tons) of uranium every year. Complex processes allow some uranium and plutonium to be re-enriched or recycled. This reduces the amount ofmining, extracting, and processing that needs to be done.
Nuclear Energy and People
Nuclear energy produces electricity that can be used to power homes, schools, businesses, and hospitals. The first nuclear reactor to produce electricity was located near Arco, Idaho, in the U.S. The Experimental Breeder Reactor began powering itself in 1951. The first nuclear power plant designed to provide energy to a community was established in Obninsk, Russia, in 1954.
Building nuclear reactors requires a high level of technology, and only the countries that have signed the Nuclear Non-Proliferation Treaty can get the uranium or plutonium that is required. For these reasons, most nuclear power plants are located in the developed world.
Nuclear power plants produce renewable,clean energy. They do notpollutethe air or producegreenhouse gases. They can be built in urban orrural areas, and do notradicallyalter the environment around them.
The steam powering the turbines and generators is ultimatelyrecycled. It is cooled down in a separate structure called acooling tower. The steam turns back into water and can be used again to produce more electricity. Excess steam is simply recycled into the atmosphere, where it does no harm as clean water vapor.
However, thebyproductof nuclear energy isradioactive material. Radioactive material is a collection ofunstable atomic nuclei. These nuclei lose their energy and can affect many materials around them, including organisms and the environment. Radioactive material can be extremelytoxic, causingburns and increasing the risk for cancers, blood diseases, and bonedecay.Radioactive wasteis what is left over from the operation of a nuclear reactor. Radioactive waste is mostly protective clothing worn by workers, tools, and cloths that have been in contact with radioactive dust. Radioactive waste is long-lasting. Materials like clothes and tools can stay radioactive for thousands of years. The government regulates how these materials are disposed of so they dontcontaminateanything else.
Used fuel and rods of nuclear poison are extremely radioactive. The used uranium pellets must be stored in special containers that look like large swimming pools. Water cools the fuel andinsulates the outside from contact with the radioactivity. Some nuclear plants store their used fuel in dry storage tanks above ground.
Chernobyl
Critics of nuclear energy worry that the storage facilities for radioactive waste will leak, crack, orerode. Radioactive material could then contaminate the soil and ground waternear the facility. This could lead to serious health problems for the people and organisms in the area. All communities would have to beevacuated.
This is what happened in Chernobyl, Ukraine, in 1986. A steam explosion at one of the power plants four nuclear reactors caused a fire, called aplume. This plume was highly radioactive, creating a cloud of radioactive particles that fell to the ground, calledfallout. The fallout spread over the Chernobyl facility, as well as the surrounding area. The fallout drifted with the wind, and the particles entered thewater cycleas rain. Radioactivity traced to Chernobyl fell as rain over Scotland and Ireland. Most of the radioactive fallout fell in Belarus.The environmental impactof the Chernobyl disaster was immediate. For kilometers around the facility, the pine forestdried up and died. The red color of the deadpines earned this area the nickname theRed Forest. Fish from the nearby Pripyat River had so many radioactivities that people could no longer eat them.Cattleand horses in the area died.
More than 100,000 people wererelocated after the disaster, but the number of humanvictims of Chernobyl is difficult todetermine. The effects ofradiation poisoning only appear after many years. Cancers and other diseases can be very difficult to trace to a single source.
Future of Nuclear Energy
Nuclear reactors use fission, or the splitting of atoms, to produce energy. Nuclear energy can also be produced through fusion or joining (fusing) atoms together. The sun, for instance, is constantly undergoingnuclear fusion ashydrogenatoms fuse to formhelium. Because all life on our planet depends on the sun, you could say that nuclear fusion makes life on Earth possible.
Nuclear power plants do not have thecapabilityto safely and reliably produce energy from nuclear fusion. Its not clear whether the process will ever be an option for producing electricity. Nuclear engineers are researching nuclear fusion, however, because the process will likely be safe and cost-effective.Source: http://education.nationalgeographic.com/encyclopedia/nuclear-energy/
Imagine following a volt of electricity back through the wall socket, all the way through miles of power lines to thenuclear reactorthat generated it. You'd encounter the generator that produces the spark and the turbine that turns it. Next, you'd find the jet of steam that turns the turbine and finally the radioactive uranium bundle that heats water into steam. Welcome to the nuclear reactor core.The water in the reactor also serves as a coolant for the radioactive material, preventing it from overheating and melting down.As of March 1, 2011, there were 443 operating nuclear power reactors spread across the planet in 47 different countries [source:WNA].Uranium-235 isn't the only possible fuel for a power plant. Another fissionable material is plutonium-239. Plutonium-239is created by bombarding U-238 with neutrons, a common occurrence in a nuclear reactor.Despite all the cosmic energy that the word "nuclear" invokes, power plants that depend on atomic energy don't operate that differently from a typical coal-burning power plant. Both heat water into pressurized steam, which drives a turbine generator. The key difference between the two plants is the method of heating the water.While older plants burn fossil fuels, nuclear plants depend on the heat that occurs duringnuclear fission, when oneatomsplits into two and releases energy. Nuclear fission happens naturally every day.Uranium, for example, constantly undergoes spontaneous fission at a very slow rate. This is why the element emits radiation, and why it's a natural choice for theinduced fissionthat nuclear power plants require.Uranium is a common element on Earth and has existed since the planet formed. While there are several varieties of uranium,uranium-235(U-235) is the one most important to the production of both nuclear power andnuclear bombs.U-235 decays naturally by alpha radiation: It throws off an alpha particle, or two neutrons and two protons bound together. It's also one of the few elements that can undergo induced fission. Fire a free neutron into a U-235 nucleus and the nucleus will absorb the neutron, become unstable and split immediately.As soon as the nucleus captures the neutron, it splits into two lighter atoms and throws off two or three new neutrons (the number of ejected neutrons depends on how the U-235 atom splits). The process of capturing the neutron and splitting happens very quickly.The decay of a single U-235 atom releases approximately 200 MeV (million electron volts). That may not seem like much, but there are lots of uranium atoms in a pound (0.45 kilograms) of uranium. So many, in fact, that a pound of highly enriched uranium as used to power a nuclear submarine is equal to about a million gallons of gasoline.The splitting of an atom releases an incredible amount of heat andgamma radiation, or radiation made of high-energy photons. The two atoms that result from the fission later releasebeta radiation(superfast electrons) and gamma radiation of their own, too.But for all of this to work, scientists have to first enrich a sample of uranium so that it contains 2 to 3 percent more U-235. Three-percent enrichment is sufficient for nuclear power plants, but weapons-grade uranium is composed of at least 90 percent U-235.
Inside a Nuclear Power PlantIn order to turn nuclear fission into electricalenergy, nuclear power plant operators have to control the energy given off by the enriched uranium and allow it to heat water into steam.Enriched uranium typically is formed into inch-long (2.5-centimeter-long) pellets, each with approximately the same diameter as a dime. Next, the pellets are arranged into longrods, and the rods are collected together intobundles. The bundles are submerged in water inside a pressure vessel. The water acts as a coolant. Left to its own devices, the uranium would eventually overheat and melt.To prevent overheating,control rods made of a material that absorbs neutrons are inserted into the uranium bundle using a mechanism that can raise or lower them. Raising and lowering the control rods allow operators to control the rate of the nuclear reaction. When an operator wants the uranium core to produce more heat, the control rods are lifted out of the uranium bundle (thus absorbing fewer neutrons). To reduce heat, they are lowered into the uranium bundle. The rods can also be lowered completely into the uranium bundle to shut the reactor down in the event of an accident or to change the fuel.The uranium bundle acts as an extremely high-energy source of heat. It heats the water and turns it to steam. The steam drives a turbine, which spins a generator to produce power. Humans have been harnessing the expansion of water into steam for hundreds of years.In some nuclear power plants, the steam from the reactor goes through a secondary, intermediate heat exchanger to convert another loop of water to steam, which drives the turbine. The advantage to this design is that the radioactive water/steam never contacts the turbine. Also, in some reactors, the coolant fluid in contact with the reactor core is gas (carbon dioxide) or liquid metal (sodium, potassium); these types of reactors allow the core to be operated at higher temperatures.Once you get past the reactor itself, there's very little difference between a nuclear power plant and a coal-fired or oil-fired power plant, except for the source of the heat used to createsteam. But as that source can emit harmful levels of radiation, extra precautions are required.A concrete liner typically houses the reactor's pressure vessel and acts as a radiation shield. That liner, in turn, is housed within a much larger steel containment vessel. This vessel contains the reactor core, as well as the equipment plant workers use to refuel and maintain the reactor. The steel containment vessel serves as a barrier to prevent leakage of any radioactive gases or fluids from the plant.An outer concrete building serves as the final layer, protecting the steel containment vessel. This concrete structure is designed to be strong enough to survive the kind of massive damage that might result from earthquakes or a crashing jet airliner. These secondary containment structures are necessary to prevent the escape of radiation/radioactive steam in the event of an accident.
Pros and Cons of Nuclear PowerWhat's nuclear power's biggest advantage? It doesn't depend on fossil fuelsand isn't affected by fluctuating oil and gas prices. Coal andnatural gaspower plants emit carbon dioxide into the atmosphere, which contributes to climate change. With nuclear power plants, CO2emissions are minimal.According to the Nuclear Energy Institute, the power produced by the world's nuclear plants would normally produce 2 billion metric tons of CO2per year if they depended on fossil fuels. In fact, a properly functioning nuclear power plant actually releases fewer radioactivities into the atmosphere than a coal-fired power plant [source:Hvistendahl]. Plus, all this comes with a far lighter fuel requirement. Nuclear fission produces roughly a million times more energy per unit weight than fossil fuel alternatives [source:Helman].And then there are the negatives. Historically, mining and purifying uranium hasn't been a veryclean process. Even transporting nuclear fuel to and from plants poses a contamination risk. And once the fuel is spent, you can't just throw it in the city dump. It's still radioactive and potentially deadly.On average, a nuclear power plant annually generates 20 metric tons of used nuclear fuel, classified as high-level radioactive waste. When you take into account every nuclear plant onEarth, the combined total climbs to roughly 2,000 metric tons a year [source:NEI]. All of this waste emitsradiationand heat, meaning that it will eventually corrode any container that holds it. It can also prove lethal to nearby life forms. As if this weren't bad enough, nuclear power plants produce a great deal oflow-level radioactive wastein the form of radiated parts and equipment.Over time, spent nuclear fuel decays to safe radioactive levels, but this process takes tens of thousands of years. Even low-level radioactive waste requires centuries to reach acceptable levels. Currently, the nuclear industry lets waste cool for years before mixing it with glass and storing it in massive cooled, concrete structures. This waste has to be maintained, monitored and guarded to prevent the materials from falling into the wrong hands. All of these services and added materials costmoney-- on top of the high costs required to build a plant.
Nuclear FissionIf a massive nucleus like uranium-235 breaks apart (fissions), then there will be a net yield of energy because the sum of the masses of the fragments will be less than the mass of the uranium nucleus. If the mass of the fragments is equal to or greater than that of iron at the peak of the binding energy curve, then the nuclear particles will be more tightly bound than they were in the uranium nucleus, and that decrease in mass comes off in the form of energy according to the Einstein equation. For elements lighter than iron, fusion will yield energy.The fission of U-235 in reactors is triggered by the absorption of a low energy neutron, often termed a "slow neutron" or a "thermal neutron". Other fissionable isotopes which can be induced to fission by slow neutrons are plutonium-239, uranium-233, and thorium-232.
Uranium-235 FissionIn one of the most remarkable phenomena in nature, a slow neutron can be captured by a uranium-235 nucleus, rendering it unstable toward nuclear fission. A fast neutron will not be captured, so neutrons must be slowed down by moderation to increase their capture probability in fission reactors. A single fission event can yield over 200 million times the energy of the neutron which triggered it!
Uranium Fuel Natural uranium is composed of 0.72% U-235 (the fissionable isotope), 99.27% U-238, and a trace quantity 0.0055% U-234. The 0.72% U-235 is not sufficient to produce a self-sustaining critical chain reaction in U.S. style light-water reactors, although it is used in Canadian CANDU reactors. For light-water reactors, the fuel must be enriched to 2.5-3.5% U-235. Uranium is found as uranium oxide which when purified has a rich yellow color and is called "yellowcake". After reduction, the uranium must go through an isotope enrichment process. Even with the necessity of enrichment, it still takes only about 3 kg of natural uranium to supply the energy needs of one American for a year.
Fissionable Isotopes While uranium-235 is the naturally occurring fissionable isotope, there are other isotopes which can be induced to fission by neutron bombardment. Plutonium-239 is also fissionable by bombardment with slow neutrons, and both it and uranium-235 have been used to make nuclear fission bombs. Plutonium-239 can be produced by "breeding" it from uranium-238. Uranium-238, which makes up 99.3% of natural uranium, is not fissionable by slow neutrons. U-238 has a small probability for spontaneous fission and also a small probability of fission when bombarded with fast neutrons, but it is not useful as a nuclear fuel source. Some of the nuclear reactors at Hanford, Washington and the Savannah-River Plant (SC) are designed for the production of bomb-grade plutonium-239. Thorium-232 is fissionable, so could conceivably be used as a nuclear fuel. The only other isotope which is known to undergo fission upon slow-neutron bombardment is uranium-233.
Nuclear Binding EnergyNuclei are made up of protons and neutron, but the mass of a nucleus is always less than the sum of the individual masses of the protons and neutrons which constitute it. The difference is a measure of the nuclear binding energy which holds the nucleus together. This binding energy can be calculated from the Einstein relationship:Nuclear binding energy = mc2
For the alpha particle m= 0.0304 u which gives a binding energy of 28.3 MeV.
The enormity of the nuclear binding energy can perhaps be better appreciated by comparing it to the binding energy of an electron in an atom. The comparison of the alpha particle binding energy with the binding energy of the electron in a hydrogen atom is shown below. The nuclear binding energies are on the order of a million times greater than the electron binding energies of atoms.
Fission and fusion can yield energy
Nuclear Binding Energy CurveThe binding energy curve is obtained by dividing the total nuclear binding energy by the number of nucleons. The fact that there is a peak in the binding energy curve in the region of stability near iron means that either the breakup of heavier nuclei (fission) or the combining of lighter nuclei (fusion) will yield nuclei which are more tightly bound (less mass per nucleon). The binding energies of nucleons are in the range of millions of electron volts compared to tens of eV for atomic electrons. Whereas an atomic transition might emit a photon in the range of a few electron volts, perhaps in the visible light region, nuclear transitions can emit gamma-rays with quantum energies in the MeV range. The iron limit
The buildup of heavier elements in the nuclear fusion processes in stars is limited to elements below iron, since the fusion of iron would subtract energy rather than provide it. Iron-56 is abundant in stellar processes, and with a binding energy per nucleon of 8.8 MeV, it is the third most tightly bound of the nuclides. Its average binding energy per nucleon is exceeded only by 58Fe and 62Ni, the nickel isotope being the most tightly bound of the nuclides.
Fission and Fusion Yields
Deuterium-tritium fusion and uranium-235 fission are compared in terms of energy yield. Both the single event energy and the energy per kilogram of fuel are compared. Then they expressed in terms of a nominal per capita U.S. energy use: 5 x 1011 joules. This figure is dated and probably high, but it gives a basis for comparison. The values above are the total energy yield, not the energy delivered to a consumer.
Light Water ReactorsThe nuclear fission reactors used in the United States for electric power production are classified as "light water reactors" in contrast to the "heavy water reactors" used in Canada. Light water (ordinary water) is used as the moderator in U.S. reactors as well as the cooling agent and the means by which heat is removed to produce steam for turning the turbines of the electric generators. The use of ordinary water makes it necessary to do a certain amount of enrichment of the uranium fuel before the necessary criticality of the reactor can be maintained. The fission of a U-235 nucleus in one fuel rod releases an average of 2.4 fast neutrons per fission. These neutrons are slowed down or "moderated" by the water between fuel rods, increasing the cross-section for neutron capture and fission by a U-235 nucleus in a neighboring fuel rod. The two varieties of the light water reactor are the pressurized water reactor (PWR) and boiling water reactor (BWR).
Uranium EnrichmentNatural uranium is only 0.7% U-235, the fissionable isotope. The other 99.3% is U-238 which is not fissionable. The uranium is usually enriched to 2.5-3.5% U-235 for use in U.S. light water reactors, while the heavy water Canadian reactors typically use natural uranium. Even with the necessity of enrichment, it still takes only about 3 kg of natural uranium to supply the energy needs of one American for a year. Uranium enrichment has historically been accomplished by making the compound uranium hexaflouride and diffusing it through a long pathway of porous material (like kilometers!) and making use of the slightly higher diffusion rate of the lighter U-235 compound. There have been tests of centrifugal separators, but modern efforts are directed toward laser enrichment procedures. The uranium fuel for fission reactors will not make a bomb; it takes enrichment to over 90% to obtain the fast chain reaction necessary for weapons applications. Enrichment to 15-30% is typical for breeder reactors. Uranium Diffusion EnrichmentTo produce the highly enriched uranium-235 needed for the development of nuclear weapons, a huge diffusion plant was built during World War II at Oak Ridge, Tennessee. Two other massive plants for uranium enrichment were built at Paducah, KY and Portsmouth, OH after the war. The compound uranium hexafluoride was produced and allowed to diffuse through thousands of stages of porous material, making use of the fact that the slightly lighter U-235 compound would diffuse faster than the U-238 compound. While electric power reactors require only enrichment from the 0.7% of natural uranium ore to about 3% U-235, the weapons applications required enrichment to over 90% U-235. Part of the enriched uranium was used to breed plutonium-239 for the more widely used plutonium devices. Heavy Water ReactorsNuclear fission reactors used in Canada use heavy water as the moderator in their reactors. Since the deuterium in heavy water is slightly more effective in slowing down the neutrons from the fission reactions, the uranium fuel needs no enrichment and can be used as mined. The Canadian style reactors are commonly called CANDU reactors. Heavy water (D2O) is 10% heavier than ordinary water and has a neutron moderating ratio 80 times higher than ordinary water. As of January 2002, 32 of the 438 nuclear reactors in operation around the world were of CANDU type.
