Section 1 of 5How a Chain Reaction Starts
A chain reaction is a sequence of fission events in which each event triggers the next.
1
A slow-moving neutron is absorbed by a uranium-235 nucleus, which becomes unstable and splits in a process called fission.
2
Fission releases energy as the kinetic energy of the products, and two or three fast-moving neutrons are also released.
3
Each released neutron can be absorbed by another uranium-235 nucleus, which then undergoes fission and releases more neutrons.
4
If, on average, exactly one neutron from each fission causes another fission, the reaction continues at a steady rate.
5
If more than one neutron from each fission causes further fission, the rate of reaction increases rapidly.
Section 2 of 5Controlling a Reactor
A reactor is designed so that, on average, exactly one neutron from each fission goes on to cause another fission.
Control rods
- Control rods Rods of boron or cadmium absorb neutrons, and pushing them further into the core reduces the number of fissions.
Moderator
- Moderator Graphite or water slows fast neutrons down, making them more likely to be absorbed by uranium-235 nuclei.
Coolant
- Coolant A coolant carries heat away from the fuel, and the heat is used to produce steam for turbines.
Shielding
- Shielding Thick concrete and lead shielding absorbs radiation so that people working near the reactor are protected.
| Situation | Average neutrons causing further fission | Result |
|---|---|---|
| Reactor, controlled | Exactly 1 | Steady release of energy |
| Control rods fully inserted | Less than 1 | Reaction slows and stops |
| Uncontrolled chain reaction | More than 1 | Rate increases rapidly, leading to an explosion |
Section 3 of 5Why the Chain Matters
Why the Chain Matters
Critical
- Critical A reactor is described as critical when the reaction is self-sustaining, with exactly one neutron on average causing each fission.
Supercritical
- Supercritical A supercritical arrangement has more than one neutron per fission causing further fission, so the reaction speeds up.
Subcritical
- Subcritical A subcritical arrangement loses too many neutrons from its surface, so the chain reaction dies out.
Section 4 of 5Worked Example
Question
- Assume each fission releases 2 neutrons and each neutron causes one further fission.
- Starting with 1 neutron, how many neutrons are produced in the fifth generation?
- Generation 0: 1 neutron
- Each fission releases 2 neutrons, so the number doubles every generation
- After 5 generations: 1 × 2⁵ = 32 neutrons
Answer 32 neutrons are produced in the fifth generation.
Section 5 of 5Exam Checklist & Tips
4.4.2 – Key Points for the Exam
- Nuclear fission The splitting of a heavy nucleus, such as uranium-235, into two smaller nuclei after absorbing a neutron.
- Chain reaction A reaction in which a product of one step triggers the next step, such as neutrons causing repeated fission.
- Control rod A rod that absorbs neutrons to regulate the rate of a nuclear chain reaction.
- Moderator A material that slows fast neutrons so that they are more likely to cause fission.
- Critical Describes a reactor in which the chain reaction is self-sustaining at a steady rate.
Common Mistakes
- Saying control rods slow neutrons down; control rods absorb neutrons, while the moderator slows them.
- Claiming that a chain reaction always runs away;
- reactors are designed so that exactly one neutron per fission continues the reaction.
- Confusing fission with fusion, which joins light nuclei rather than splitting heavy ones.
- Forgetting that fission releases energy and neutrons as well as two smaller nuclei.