Most nuclear power plants (NPPs), utilise enriched Uranium Ore, however some reactors run on a mixture of Uranium and Plutonium.
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Uranium is an element in the Actanoid series of the periodic table. It has atomic number 92, is in the f-block of elements and has a relative atomic mass of 238.03. It is a solid at STP and a metal. |
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For a nuclear reaction to occur and energy released a high concentration of fissable (splittable) material is needed. Uranium exists in two different naturally occuring isotopic forms. The most common being Uranium 238, with 99.3% of all isotopes existing in this form. However only Uranium 235 is fissable. Uranium 235 has a relative abundance of 0.7%. In other words less than one in a hundered atoms are fissable. It is necessary to seperate these isotopes to obtain a relativly pure sample of unstable Uranium 235.
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Uranium is a relativly common element in the Earth. It's most common ore is known as pitchblende, U3O8. This ore is of course a mixture of Uranium-238 and Uranium-235, only one of which are useful. |
The seperation of these two isotopes can be done using an effusion technique. Heavier gases take longer to effuse through a gap in a closed system.
It is therefore possible by applying this procedure over and over to obtain a sample of Uranium-235 pure enough to undergo nuclear reactions. However, Uranium is metal with a boiling point of almost 4000 kelvin. It is not practical to simply boil Uranium and effuse it that way. Instead UO2 is converted into UF6, a fluride of Uranium using Hydro Fluoric acid, HF. Uranium Hexafluride is a volatile liquid at RTP with a melting point of around 333 kelvin. It is very easy to convert this into a gas made up of two constituent masses. UF6 with U-238 will have mass 352 and U-235 349.
The conversion of U3O8 to UF6 is achieved in several steps.
The effusion rates of two gases are related to their masses by the following rule:
(Rate of effusion of Gas 1/Rate of effusion of Gas 2) =
(Mass of Gas 2/Mass of Gas 1)½
Therefore, assigning Gas 1 to U-235 and Gas 2 to U-238, we can see that U-235 effuses 1.0043 time faster than U-238.
This isn't a huge difference, but when the process is applied over and over, a 99% pure sample can be obtained at the end.
