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Extraction of U235


Chemically, U235 and U238 are identical so there is no chemical process that can be used to separate them. The only difference is physical in that an atom of U238 has a mass slighly larger than an atom of U235 so this is the only practicable method of isolating the lighter isotope.

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Let us go back to a stage were at earlier when we created uranium tetrafluoride. this can be treated with fluorine at 350 degrees centigrade to form volatile uranium hexafluoride which can be fractionally distilled to separate off a higher concentrate of 235. Three main techniques are then used to further refine the product; gas diffuseon, magnetic separation and gas centrifuging. Firstly, the gaseous hexafluoride gas, pushed by low powered pumps, is allowed to seep through a succession of fine porous filters and since the atoms of the U235F6 are slightly smaller and lighter than those of the U238F6 atoms they pass through slightly more quickly but since the proportion of 235 that is collected only increases slowly this process has to be repeated over and over again before a worthwhile concentration is achieved. A different, or possibly complementary, procedure involves treating uranium dioxide with carbon tetrachloride to form uranium tetrachloride; the gas is electricall charged and then passed through a magnetic field. Since the U235 atoms are lighter they are less attracted by the magnetism and can be filtered off. A high speed gas centrifuge is used in the final stage; uranium hexafluoride gas is introduced and the heavier molecules of U238F6 are thrown out to accumulate against the wall of the centrifuge whilst the lighter U235F6 molecules collect away from it. Again this has to be repeated over many stages in order to give a high degree of separation.

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