Nuclear fission and nuclear fusion

Nuclear fission, chain reaction and the atom bomb

Discovered by **Otto Hahn and Fritz Strassmann** (1939). **Nuclear fission** is the splitting of a heavy nucleus into two lighter nuclei, with the release of energy and neutrons.

1. 1
   A slow neutron hits a uranium-235 nucleus, which splits into two lighter nuclei.
2. 2★
   ₉₂U²³⁵ + ₀n¹ → ₅₆Ba¹⁴¹ + ₃₆Kr⁹² + 3 ₀n¹ + energy

   Check: 235 + 1 = 141 + 92 + 3 = 236, and 92 = 56 + 36. About 200 MeV is released.
3. 3
   The 2 or 3 new neutrons hit more uranium-235 nuclei, which also split and give out more neutrons.
4. 4★
   The number of fissions multiplies: 1, 3, 9, 27 and so on. This self-sustaining process is a chain reaction.
5. 5
   Controlled chain reaction: only one neutron from each fission causes a new fission. Used in a nuclear reactor. Uncontrolled: all the neutrons cause fission. Used in an atom bomb.
6. 6
   Critical mass: the minimum mass of a fissile material (one that can undergo fission, like U-235 or Pu-239) needed to keep a chain reaction going. Below it, too many neutrons escape.
7. 7
   Atom bomb: pieces of U-235 or Pu-239, each below critical mass, are kept apart. An ordinary explosive pushes them together above critical mass, and an uncontrolled chain reaction explodes in a fraction of a second.

   Atom bombs destroyed Hiroshima and Nagasaki in Japan in 1945.

★ key step

How to draw a chain reaction

• At the left, draw a small dot labelled 'n' (neutron) hitting a large circle labelled 'U-235'.
• Show it splitting into two medium circles labelled 'Ba' and 'Kr', with 3 small dots (neutrons) flying out. Write 'energy'.
• Draw each of the 3 neutrons hitting a new U-235 nucleus, which again splits and gives out 3 neutrons.
• Continue for 3 generations, so the picture spreads like a tree: 1, 3, 9 nuclei.
• Label 'neutron', 'U-235', 'fission fragments' and 'energy'. Title it 'Uncontrolled chain reaction'.

Nuclear fusion and the energy of stars

Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, with the release of energy. Example: ₁H² + ₁H² → ₂He⁴ + energy. Here ₁H² is deuterium, a heavy isotope of hydrogen. Nuclei are positive and repel each other strongly. They fuse only at extremely high temperatures, about 10⁷ K to 10⁹ K, so fusion is also called a thermonuclear reaction. The Sun and other stars get their energy from fusion. In their cores, hydrogen nuclei fuse to form helium: 4 ₁H¹ → ₂He⁴ + 2 ₊₁e⁰ + energy (about 26.7 MeV). The hydrogen bomb works on fusion. An atom bomb (fission) inside it gives the very high temperature needed to start the fusion. It is far more powerful than an atom bomb.

Fission vs fusion

In both, the products weigh slightly less than what went in. This lost mass (the mass defect) appears as energy: E = mc², where c = 3 × 10⁸ m/s.

Worked example: mass into energy