09.12 - 9/11/2007 Alternative Energy Continued Nuclear...

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9/11/2007 1 Alternative Energy Continued • Nuclear • Wind Nuclear Power (noo klee ur) Nuclear reactors produce 15% of US electricity and about 20% worldwide. The dome-shaped containment building at the Shearon Harris Nuclear Power Plant near Raleigh, NC Nuclear Fission All Uranium atoms have 92 protons in the nucleus, and >99% of them have 146 neutrons for a total atomic mass of 238, uranium-238. A small fraction of naturally found uranium atoms have only 143 neutrons, uranium-235. This isotope can undergo induced fission when bombarded with a neutron. Original mass is 235 + 1 = 236 amu Final mass is 235.8 amu 0.2 amu of mass is converted to energy by E=mc 2 Chain Reaction The three neutrons emitted from the fission of uranium- 235 may collide with other nearby uranium-235 atoms and cause fission. R i iti l f Requires a critical mass of uranium-235. If a critical mass is present, the reaction will snowball and the fuel will explode or melt down. Nuclear fission subcritical, critcial, supercritical
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9/11/2007 2 Enriched Uranium Naturally-occurring uranium is 0.7% uranium-235, not critical. Low enriched uranium is 3-4% uranium-235, used in reactors. High enriched uranium can be >80% uranium-235, used in weapons. Enrichment involves separating uranium-238 due to its slightly higher mass by spinning uranium- containing gas in a centrifuge. Nuclear Fuel Uranium is formed into pellets Pellets are inserted into long stainless steel rods to make a fuel assembly . The fuel assembly is super critical. Controlling the Reaction A material that absorbs neutrons can be inserted between the fuel rods to control the reaction. With no moderator, the fuel element would melt down, but with the moderator fully in place, the reaction is shut down entirely.
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09.12 - 9/11/2007 Alternative Energy Continued Nuclear...

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