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 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

Fission Reactor

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

Mass Spectrometer

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

Tomahawk Fusion Reactor

Inertial Fusion Reactor