Chapter 40Nuclear Fission & Fusion

Conceptual PhysicsHewitt, 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”

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

Tokamak Fusion Reactor

Inertial Fusion Reactor