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Chapter 40 Nuclear Fission & Fusion. Conceptual Physics Hewitt, 1999 Bloom High School. 40.1 Nuclear Fission. Nuclear Strong force- keeps nuclei together Electric force- tears nuclei apart after Z=92 Nuclear fission- splitting of the nucleus Typically by neutron bombardment
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Chapter 40Nuclear Fission & Fusion Conceptual Physics Hewitt, 1999 Bloom High School
40.1 Nuclear Fission • Nuclear Strong force- keeps nuclei together • Electric force- tears nuclei apart after Z=92 • Nuclear fission- splitting of the nucleus • Typically by neutron bombardment • 1n + 235U 91Kr + 142Ba + 31n • Because 3 new neutrons are being released, 3 additional 235U’s can be split! • Causes a chain reaction
Ore & Critical Mass • In uranium ore (238U is most common), the nucleus absorbs the 1n, so no fission takes place • 233U and 235U are fissionable • Critical Mass • If a chain reaction occurs in a tiny piece (short path length) of 235U, no explosion occurs (subcritical) • If a chain reaction occurs in a large piece (longer path length) of 235U, an explosion occurs (supercritical) • Atomic bomb
Nuclear Bomb Design • Subcritical pieces separated by a safe gap • High explosive is used to push them together
40.2 The Nuclear Fission Reactor • All generators move a turbine • Coal- heating water to move steam past the blades • Hydroelectric- falling water over the blades • Wind- wind moves the blades • Nuclear- heating water to move steam past the blades • 1 kg of uranium has the same energy as 30 rail cars of coal • Fission controlled by rods that can absorb 1n without causing a chain reaction • Fission fragments (product of splitting) are radioactive because they have too many 1n now • PhET “Nuclear Fission”
40.3 Plutonium • 1n + 238U 239U 239Np + b- 239Pu + b- • 238U absorbs 1n and becomes 239U briefly • Decays to 239Np (Neptunium) • 239Np decays to 239Pu (Plutonium) by emitting b- • 239Np has a half-life of 2.3 days (decays quickly/easily) • 239Pu has a half-life of 24,000 years! • Decays slowly, but also rapidly forms compounds: PuO, PuO2, Pu2O3 • Emits a particles, which are easily blocked
40.4 The Breeder Reactor • Breeder reactor- uses fissionable material to make more fissionable material • Consumes non-fissionable material (238U) to make more fissionable fuel (239Pu) • Small amounts of 239Pu with large amounts of 238U cause more fission to form 239Pu
40.5 Mass-Energy Equivalence • Mass lost due to binding energy in nucleus • Mass of 1p+= 1.00728 amu • Mass of 1n0= 1.00866 amu • Mass of 2H+= 1.87482 amu (7% loss) • Mass can also be converted to energy when a nucleus splits (fission reaction) • Exception is 4He- would need to add energy to split, not give off energy • Mass spectrometer- used to measure the masses of isotopes
40.6 Nuclear Fusion • Fusion- to combine 2+ nuclei to form a new nuclei • 238U gains mass in fusion and doesn’t give off energy • Fe gains mass in fusion and fission and doesn’t give off energy either way • When products lose mass in fusion, the loss is converted to KE of the new particle (½mv2!) • Thermonuclear fusion- occurs at high temperatures (star interiors) • 657M tons of 1H 653M tons 2He + “4M tons” of E
40.7 Controlling Nuclear Fusion • Fusion reactions still take more energy than they make (not self sustaining) • 1. Needs strong magnetic fields to hold super-hot plasma and compress it to fuse it • 2. Can also use lasers to heat pellets of 2H (D) • No risk of chain reaction because nothing is radioactive • 30L of water can release the energy of 10kL of gasoline or 80 tons of TNT
