Nuclear fissionIn nature Uranium occurs at two isotopes: U-235 0.7% and U-238 91.3%. The U-235 is fissile, meaning it splits (fissions) when it absorbs an extra neutron. When it fissions, it gives off 3 neutrons that can start other U-235 fissions. This a chain reaction. If it happens quickly, it is an atomic bomb, under control it is a power station. U-235 is also radioactive, meaning it spontaneously decays by giving off an alpha particle and can be detected by a radio. This has nothing to do with fission. There are many different designs, and these are looked at briefly on these 3 pages. |
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Nuclear Fission ReactorWikipedia description of fission Nuclear power stations use Uranium 235 or plutonium 239. When slow neutrons are captured, the nuclei become unstable and break apart forming daughter elements, these too are unstable and break down into a chain of other elements. Each stage gives off heat. Eventually some of these absorb neutrons and slow the reaction down. At this stage the fuel is removed. The fuels rods are stored under water until all the decay products have finished their own decays. It can be disposed of or purified to remove the U325 and P239 for re-use. Nuclear fission is very expensive. In every country in the world it has been heavily subsidised by the government. The price of uranium is 3-7% of the total cost of delivering electricity from a nuclear power plant. The economics only make sense if the fuel is reprocessed or used in breeder reactors. However despite decades of research no one has developed an economic fast breeder reactor. |
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Radioactive wasteAt present the waste is building up faster than it is being buried. The decisions are being postponed as too difficult. Most waste is cooling down at power stations prior to underground disposal. There is talk of "burning" (fissioning) the waste in a fast reactor. |
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Heating water to make steamAs the nuclei break up they move about and cause water molecules to move as well. This movement is heat. The neutrons given off collide with water. A kilogram of uranium-235 (U-235) releases three million times more energy than a kilogram of coal. Removing the heat - coolingThere are different reactor designs:
Controlling the fissionThe power output of the reactor is adjusted by controlling how many neutrons are able to create more fissions. Neutron moderatorIn some reactors, the coolant also acts as a neutron moderator. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal (slow) neutrons. Thermal neutrons are more likely than fast neutrons to cause fission.
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