What is nuclear fission? Explain how a chain reaction takes place. How is it controlled in a reactor?

1. 1 · Define fission. The splitting of a heavy nucleus, such as ²³⁵U, by a slow neutron into two medium nuclei with release of energy.
2. 2 · Write the equation. ¹₀n + ²³⁵₉₂U → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n + about 200 MeV.
3. 3 · Explain the energy. The products are lighter than the reactants; the mass lost, about 0.215 u, appears as about 200 MeV.
4. 4 · Explain the chain reaction. Each fission gives 2 or 3 neutrons, which split more nuclei, and so on.
5. 5 · Draw the chain. [Diagram: one neutron splitting a ²³⁵U nucleus into two fragments and three neutrons, each of which splits another nucleus]
6. 6 · Explain control. Control rods of cadmium or boron absorb extra neutrons, so only one neutron per fission causes another.
7. 7 · Real life. India's reactors at Tarapur and Kaiga use controlled chain reactions to make electricity.
8. 8 · Close in one line. So, a controlled chain reaction gives steady power; an uncontrolled one gives an explosion.

Scoring tip: Balance A and Z in your equation and draw at least two generations of the chain. Name cadmium or boron for control.