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How uranium enrichment works

Discussion in 'Pakistan Strategic Forces' started by Neo, Feb 21, 2007.

  1. Neo

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    How uranium enrichment works
    By Marshall Brain

    HowStuffWorks.com

    North Korean and Iran are in the news constantly these days because both are making "enriched uranium." North Korea has been working toward the creation of nuclear bombs, while Iran says that it is peacefully enriching uranium for nuclear power plants.

    This leads to an obvious question: What, exactly, is enriched uranium? And why is it so hard to make it? Only a few countries can make enriched uranium right now: the United States, Britain, France, Russia, China, India and Pakistan. Iran and North Korea seek membership in this exclusive "nuclear club."

    Enrichment is a fascinating process, and the process starts at a uranium mine. You dig uranium ore out of the ground and pulverize the rock. Then you soak this crushed rock in concentrated sulfuric acid. The uranium leaches out of the rock with the acid. Once you neutralize the acid, take out the impurities, and dry it, you get a crumbly yellowish powder called "yellow cake." Yellow cake is uranium oxide. This is the raw material for making a bomb or powering a nuclear reactor.

    Yellow cake straight from the mine is useless in its natural form. Uranium oxide contains two kinds of uranium: U-235 and U-238. U-235 is what you need if you want to make a bomb or fuel a nuclear power plant. But yellow cake is about 99 percent U-238. So you need to somehow separate the U-235 from the U-238 and increase the amount of U-235. That is all that "enrichment" is - you are trying to increase the amount of U-235. To make a bomb, what you are trying to create is highly enriched uranium, which is 90 percent U-235. To fuel a nuclear power plant, the level of enrichment is much lower - only about 5 percent U-235.

    The key to enrichment is this simple fact: U-235 weighs slightly less than U-238. By exploiting this weight difference, you can separate the U-235 from the U-238. The first step is to combine the uranium with a powerful acid called hydrofluoric acid. After several steps, you create uranium hexafluoride, which is a gas.

    Now that the uranium is in a gaseous form, it is easier to work with. You can put the gas into a centrifuge and spin it up. The centrifuge creates a force thousands of times more powerful than gravity. The U-238 atoms, being just slightly heavier than the U-235 atoms, tend to move outward toward the walls of the centrifuge, while the U-235 atoms tend to stay more toward the center of the centrifuge.

    It is only a slight difference in concentrations, but by extracting the gas from the center of the centrifuge, it has slightly more U-235 than it did before. You place this slightly concentrated gas in another centrifuge and do the same thing. If you repeat this process thousands of times, you can create a gas that is highly enriched in U-235.

    At a uranium enrichment plant, thousands of centrifuges are chained together in long cascades. The output of one centrifuge feeds the input of the next one, and the concentration of U-235 rises just a little bit at each stage in the cascade. At the end of a long chain of centrifuges, what you have is uranium hexafluoride gas containing a high concentration of U-235 atoms.

    Although this sounds relatively simple, the creation of the centrifuges is a huge technological challenge. The centrifuges must spin very quickly - in the range of 100,000 RPM. To spin this fast, the centrifuges must have very light yet strong rotors, perfect balance and high-speed bearings, which are usually magnetic to reduce friction.

    Until recently, all three of these requirements have been out of range for all but the most technologically advanced countries. The recent development of inexpensive, high-precision computer controlled machining equipment has made things somewhat easier. This is why we have seen more countries learning to enrich uranium in recent years.

    At the end of the chain of centrifuges, you have uranium hexafluoride gas with a high concentration of U-235 atoms in it. Now you need to turn the uranium hexafluoride gas back into uranium metal. You do this by adding calcium. The calcium reacts with the fluoride to create a salt, and the pure uranium metal is left behind.

    With this highly concentrated U-235 metal, you can either make a nuclear bomb or power a nuclear reactor. Compared to the difficulty experienced in enriching the uranium, making the bomb is relatively simple.

    You need about 140 pounds of highly enriched uranium. You split the uranium into two parts. The two parts, by themselves, will not explode. But when you put the two parts together, you achieve a critical mass of uranium that explodes with amazing power. You use a large gun, similar to an artillery gun, to shoot one of the masses at the other. When they combine, you get the nuclear explosion. This is called a gun-assembled bomb.

    The first bomb dropped at Hiroshima was a gun assembled U-235 bomb. When it exploded, it had the power of about 21,000 tons of TNT - enough power to destroy an entire city.

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