Did You Know?
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The isotope which is produced from radioactive decay is called a progeny or daughter, while the original isotope is called a parent. For more information, see: For a lesson about this topic, see: |
HOW IT WORKS
Nuclear Power
Nuclear power plants use URANIUM to generate heat and boil water into steam. Uranium has the largest atoms of the 92 naturally occurring elements on earth, making them more likely than other atoms to split.
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One of these half-metre nuclear fuel bundles can provide enough electricity to power 100 homes for a year. |
When subatomic particles called NEUTRONS come into contact with uranium atoms, the atoms split, releasing heat energy. This occurs all the time in nature, but at a very slow rate. Nuclear reactors are able to greatly speed up this process by slowing down the neutrons and increasing the likelihood that they will hit and split the uranium atoms. When uranium atoms split they also release more neutrons which can then go on and split additional atoms ensuring a chain reaction of atom splitting. This is called NUCLEAR FISSION.
At the heart of every nuclear reactor are FUEL PELLETS no bigger than the tip of your finger. Despite their small size, these fuel pellets hold the potential to produce tremendous amounts of energy.
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A single nuclear fuel pellet can power an average home for 6 weeks |
Canada’s nuclear reactors use fuel pellets that are made from naturally occurring uranium that is mined in Canada. The pellets are inserted into tubes about half-a-metre in length made from zirconium alloy, a special type of metal that has a high resistance to corrosion. The tubes are welded shut and several are then assembled together into what is called a FUEL BUNDLE. One of these half-metre fuel bundles can provide enough electricity to power 100 homes for a year.
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Ontario’s Darlington Nucelar Station is one of the most efficient nuclear stations in the world and is capable of powering 15% of the entire province. |
Hundreds of fuel bundles are inserted into the core of a nuclear reactor where the uranium atoms split giving off vast amounts of heat. This heat is used to boil water to create steam, which then spins a turbine and generator producing electricity.
Nuclear power stations are able to produce tremendous amounts of electricity from a very small amount of fuel. A single 1.65 cm nuclear fuel pellet can produce the same amount of energy as 807 kilograms of coal, 677 litres of oil, or 476 cubic metres of natural gas.
As well, because nuclear power plants do not burn any fuels, they produce virtually no smog or greenhouse gas emissions (GHGs). They do however produce nuclear waste which needs to be handled and stored very carefully.
Source: Ontario Power Generation www.opg.com/education/program.asp
NUCLEAR GENERATING STATION
Source: Ontario Power Generation, www.opg.com/education/program.asp
Source: www.nuclearFAQ.ca
Basic Components of Nuclear Reactors
The basic technology used to harness the energy of nuclear fission is the nuclear reactor. Though there are many types of nuclear reactors, all have several components in common, such as fuel, moderator, coolant and control rods (see figure below).
Fuel
Uranium has two main isotopes: uranium-235 (235U) and uranium-238 (238U). The former, 235U, is the only fissile material found in nature, i.e., it can easily fission when hit by either thermal or fast neutrons. Thus, almost all reactors use uranium as fuel. Most fuels for commercial reactors are processed so as to contain a higher concentration of 235U than occurs in nature, typically 2 to 5% compared with the 0.711% found in nature; therefore, the fuel is considered to be enriched in 235U.
The remainder of the fuel, typically 238U, can fission only when hit by fast neutrons of certain energies; but when neutron capture does occur, it transforms eventually into plutonium-239 (239Pu). This isotope of plutonium (one of many) is also able to fission under the impact of thermal or fast neutrons, and its contribution to the energy output of a reactor gradually grows until it represents almost 30% of the power generated in a light-water reactor, or 50% in a heavy-water reactor. Some reactors use fuel in which plutonium is incorporated at the outset, called mixed oxide fuel (or MOX). This is one way of using up stocks of plutonium extracted from spent fuels, and which could otherwise represent waste.
Moderator
A moderator is necessary to slow down the fast neutrons created during fission to the thermal energy range so as to increase their efficiency in causing further fission. The moderator must be a light material that will allow the neutrons to slow down without being captured. Usually, ordinary water is used; alternatives in use are graphite, a form of carbon, and heavy water, which is formed with the heavier deuterium isotope of hydrogen.
Coolant
A coolant is necessary to absorb and remove the heat produced by nuclear fission and maintain the temperature of the fuel within acceptable limits. It can then transfer the heat to drive electricity-generating turbines. If water is used as the coolant, the steam produced can be fed directly to the turbines. Alternatively, it can be passed through a heat exchanger which will remove the heat and produce the necessary steam. Other possible coolants are heavy water, gases like carbon dioxide or helium, or molten metals such as sodium or lead and bismuth. A coolant can also be a moderator; water is used in this dual way in most modern reactors.
Control rods
Control rods are made of materials that absorb neutrons, for example, boron, silver, indium, cadmium and hafnium. They are introduced into the reactor to reduce the number of neutrons and thus stop the fission process when required; or, during operation, to control and regulate the level and spatial distribution of power in the reactor.
Other components
The fuel,along with the mechanical structure that holds it in place, forms the reactor core. Typically, a neutron reflector surrounds the core and serves to return as many of the neutrons as possible that have leaked out of the core and therefore maximize the efficiency of their use. Often, the coolant and/or moderator serve as the reflector. The core and reflector are often housed in a thick steel container called the reactor pressure vessel. Radiation shielding is provided to reduce the high levels of radiation produced by the fission process. Numerous instruments are inserted into the core and support systems to permit the monitoring and control of the reactor, for example the temperature, pressure, radiation and power levels.
- Reactor: fuel (green) heats pressurized water: control rods (grey) absorb neutrons to control or terminate fission
- Coolant and moderator: fuel and control rods are surrounded by water that serves as coolant and moderator
- Steam generator: hot water from the reactor is pumped through a heat exchanger to generate high-pressure steam
- Turbine generator: steam drives electricity generator to produce electricity
- Condenser: removes heat to convert steam back to water
- Cooling tower: removes heat to return cooling water to near-ambient temperature
Source: NEA Nuclear Energy Today, 2003, pp. 16–17, www.nea.fr/html/pub/nuclearenergytoday/nuclear-energy-today.html
Nuclear Fission and the Moderator
In order to control the generation of electricity from a nuclear reactor, it is necessary to have a critical, or self-sustained, fission reaction within the reactor. When the fission process occurs, fast moving neutrons (about 10% of the speed of light) are one of the products. These neutrons can be absorbed or slowed by other materials or absorbed by a fuel atom (e.g., 235U) which in turn may cause that atom to fission. Since the fission process releases several neutrons (either two or three for 235U), it is possible for the fission process to occur at an increasing rate with each new “generation” of neutrons. This is known as a super-critical reaction. If too many neutrons are absorbed or if there is insufficient fissile material to fission, a sub-critical state occurs, in which case the chain reaction is not sustainable. A critical reaction, then, is one in which the chain reaction is stable and there is no increase in the population of free neutrons within the reactor.
As a “quirk” of physics, some fissile material tends to better absorb slow or thermal neutrons rather than the fast ones produced in the fission process. In fact with 235U a fast neutron has about 1000 times smaller fission probability than one travelling at 1/100,000,000 the speed. The moderator’s function is to slow down the fast neutrons to increase the efficiency of the fission reaction. It turns out that the best moderators are graphite (a form of carbon) and heavy water (D2O – the hydrogen atoms each have a proton and a neutron, and are known as deuterium, D). These have the advantage of not absorbing many neutrons. Ordinary or light water, H2O is also a good moderator, however, it has the tendency to absorb neutrons thereby dampening the reaction. Reactors that use light water as a moderator require enriched fuel to overcome this loss of neutrons. Enrichment concentrates the 235U isotope in the fuel; increasing it from the natural value of 0.711% to 2–5% (the other 99.289% in natural uranium is the 238U isotope which is more difficult to fission).
The energy from this controlled reaction is absorbed into a material (a coolant, which is often the moderator) and its temperature increases. In some plants, the steam is produced directly: in others, the heated material is pumped to a steam generator, and heat is transferred to water, converting it to steam. This steam is then used to turn a steam turbine creating electricity.
- NEA, Nuclear Energy Today, 2003, pp. 16-17,
www.nea.fr/html/pub/nuclearenergytoday/nuclear-energy-today.html - www.cna.ca/english/pdf/NuclearFacts/08-NuclearFacts-howreactorworks.pdf
- www.cleansafeenergy.org/
- Ontario Power Generation: www.opg.com/education/program.asp