1. Chapter 40 Nuclear Fission & Fusion
Conceptual Physics
Hewitt, 1999
Bloom High School

2. 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

3. 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

4. Nuclear Bomb Design Subcritical pieces separated by a safe gap
High explosive is used to push them together

5. 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

6. Fission Reactor

7. 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

8. 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

9. 40.5 Mass-Energy Equivalence
Mass lost due to binding energy in nucleus
Mass of 1p+= amu
Mass of 1n0= amu
Mass of 2H+= 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

10. Mass Spectrometer

11. 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

12. 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

13. Tomahawk Fusion Reactor

14. Inertial Fusion Reactor