Fuel element failure

Transcription

There's a lot of talk about nuclear technology, what with Iran and Fukushima and Green Energy being thrown around every day. But how do we even MAKE nuclear fuel? Howdy atomic children, Trace here for DNews… Despite the controversy they often raise, nuclear power plants are a huge source of energy. The Environmental Protection Agency says nuclear power accounts for about 20% of electricity production in the U.S. One of the reasons why is because it’s the most efficient means of extracting energy from a fuel source - about 8,000 times more efficient than coal or oil. According to the Nuclear Energy Institute, a fingertip-sized pellet of nuclear fuel contains as much energy as "17,000 cubic feet of natural gas, 1,780 pounds of coal, or 149 gallons of oil." Nuclear energy comes in two flavors, fusion or fission. Fusion is when two hydrogen atoms fuse -- this happens in stars; and fission is when large "heavy" atoms are broken apart. Both release energy, and both have pros and cons, but so far, we've only figured out nuclear fission; so when I say fuel, I'm talking about fuel for nuclear fission. Nuclear fuel is commonly referred to in the news, as "highly-enriched Uranium," but getting it to that point requires a LOT of effort. In 1941, Enrico Fermi, created the first controlled nuclear chain reaction using a small amount of uranium-235; and since then we've gotten much better at taking uranium and creating usable fuel from it. Uranium ore is most commonly mined in Canada, Australia, Niger, Kazakhstan, Russia, and Namibia; though it's not THAT rare -- it's 40 times more prevalent than silver in the Earth's crust. Once drilled or dug out of the ground, the uranium atoms are mixed in with the surrounding minerals -- so it has to be processed -- this involves some pretty intense chemistry. First, the ore is crushed, and then heated, to dry out carbon content (like clay) so it can be washed away. That slurry of ore and water is leached with sulfuric acid. These processes cause the uranium atoms to bond with the sulfur and oxygen forming uranium oxide liquid. To get it to that yellow powder we recognize from movies, the uranium is pulled out of solution using ammonia. This "yellow cake" uranium is put in barrels and shipped off to be purified even MORE. At this point the uranium isn't super radioactive, yet… If you stood one meter from a barrel full of U three O eight, you'd get no more radiation than from the cosmic rays hitting passengers on a commercial airplane. This uranium still needs to then be enriched before it can be used in power generation. That yellow cake uranium is 99.3 percent Uranium 238 and only 0.7 percent of uranium-235. To make the fuel, scientists need that U235 isotope -- this is where the now-famous nuclear centrifuges come in. If you watch the news, you know Iran is developing a nuclear program -- whether for energy or weaponry, I'll leave that to the experts; but they use centrifuges to enrich that uranium. As things go forward from here, it gets more dangerous, and more radioactive, so the engineering has to be VERY precise or people can die. First, they take the yellowcake uranium and they turn it into a gas by creating a reaction with fluorine -- the resulting uranium hexafluoride gas is even MORE pure than yellowcake and ready to go in a centrifuge. A centrifuge is a giant spinning container designed to use physics in order to separate materials. When you donate plasma, doctors draw blood and spin it in a centrifuge. During the spinning, centrifugal -- or center fleeing -- forces cause the heavier red blood cells to come out of solution and collect as far from the center as possible; lighter plasma stays nearer the inside! In the case of uranium, it's the same. The heavier U238 isotopes get thrown outward, allowing the lighter U235 to stay closer to the middle. It's not as good as blood, because there's only a 1 percent difference in mass; so it has to be spun again and again in centrifuge after centrifuge THOUSANDS of times. Eventually, the gas in the middle of the centrifuge gets more and more concentrated -- or ENRICHED! The gas is MORE U235! Once the fuel is 5 percent U235 (95 percent U238) it's suitable for some nuclear reactors. Others require as high as 20 percent. But that's nowhere NEAR enriched enough for nuclear weapons, which can require as high at 90 percent U235. Once it's reached the desired enrichment for the type of power plant you want to run, the enriched uranium hexafluoride has to be turned into a solid by adding calcium. The calcium and fluoride react, creating a salt, leaving behind only uranium oxide, which is heated to 1400C and extruded into tiny ceramic pellets. Those uranium pellets are, in turn, put into rods, and then hundreds or thousands of those rods can be placed in various configurations inside a nuclear power plant. When we talk about nuclear energy programs in other countries, world leaders get nervous. And now that you know the process, can you see why? The massive centrifuges used make nuclear fuel, are the same ones that could create weapons grade uranium. It requires a lot of technical and chemical knowledge to GET to that point, but in the end it's dig uranium out, clean it up, and then spin it! Nuclear energy continues to be a controversial choice for powering the future, and it's connection to nuclear weapons is clear, but how do you feel about nuclear energy?