Nuclear fission · Grade 12
In Grade 12 you get fluent with: fission; chain reactions and reactors; balancing fission equations.
In fission, a heavy nucleus such as uranium-235 absorbs a slow neutron, becomes unstable and splits into two smaller nuclei, releasing energy and two or three more neutrons. Those neutrons can split more nuclei in a chain reaction. A nuclear reactor keeps the chain reaction steady and uses the energy to make steam for its generators.
Key facts
- Fission: a heavy nucleus splits into two smaller nuclei plus 2–3 neutrons, releasing energy.
- A chain reaction happens when released neutrons cause further fissions; control rods absorb neutrons, the moderator slows them.
- In a reactor, the energy from fission heats water into steam, which turns the turbines and generators.
- Mass numbers and atomic numbers must balance on both sides of a nuclear equation.
Worked examples
Example 1
What starts a fission reaction in a nuclear reactor?
Answer: a uranium-235 nucleus absorbing a neutron
Example 2
What is the job of the moderator in a nuclear reactor?
Answer: to slow down neutrons so they can cause more fission
Example 3
What is the job of the control rods in a nuclear reactor?
Answer: to absorb neutrons and control the rate of fission
Example 4
²³⁵U + ¹n → ¹⁴¹Ba + ⁹²Kr + x ¹n. Find x.
Answer: 3
Example 5
Uranium-235 absorbs a neutron and splits: ²³⁵U + ¹n → ¹⁴¹Ba + ⁹²Kr + neutrons. How many neutrons are released?
Answer: 3 neutrons
Example 6
In an uncontrolled chain reaction, each fission releases neutrons that cause 3 more fissions. Starting from 1 fission, how many fissions happen in generation 3 (the first fission is generation 1)?
Answer: 9 fissions