Section 1 of 5What Is Fission?
Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei.
Spontaneous fission
- A nucleus can occasionally split on its own, but this is rare and cannot be relied upon as an energy source.
Induced fission
- A slow-moving neutron is absorbed by a fissile nucleus such as uranium-235, making it very unstable so it splits.
Fission products
- The nucleus splits into two daughter nuclei, which are smaller nuclei, and two or three fast neutrons.
Fissile fuels
- Uranium-235 and plutonium-239 are fissile materials, meaning their nuclei can be split by neutrons.
Section 2 of 5Energy Released
Energy Released
Source of energy
- Energy stored in the forces within the nucleus is released when the nucleus splits.
Kinetic energy
- The daughter nuclei and the neutrons move apart at high speed, so nuclear potential energy becomes kinetic energy.
Thermal energy
- When the fast particles collide with surrounding atoms, their kinetic energy becomes thermal energy.
Gamma radiation
- Some energy leaves as gamma rays, which are very penetrating and must be shielded.
Section 3 of 5The Chain Reaction
1
A slow neutron is absorbed by a uranium-235 nucleus, which splits into two daughter nuclei and releases two or three neutrons.
2
Each of the released neutrons can be absorbed by another uranium-235 nucleus and trigger another fission.
3
Every fission releases more neutrons, so the number of fissions grows at a faster and faster rate.
4
In an uncontrolled chain reaction, the rapid release of energy causes an explosion, as in a nuclear weapon.
5
In a reactor the chain reaction is controlled so that on average one neutron from each fission goes on to cause another fission.
Section 4 of 5Worked Example
Question
- A chain reaction starts with a single fission in generation 1.
- Each fission causes two further fissions in the next generation.
- How many fissions occur in generation 11?
- The number of fissions doubles each generation.
- Generation 1 has 1 fission, so generation n has 2 to the power (n − 1) fissions.
- For generation 11: 2¹⁰ = 1024.
Answer 1024 fissions in generation 11, showing how quickly an uncontrolled reaction grows
Section 5 of 5Exam Checklist & Tips
2.9.1 – Key Points for the Exam
- Nuclear fission The splitting of a large, unstable nucleus into two smaller nuclei.
- Fissile A nucleus that can be split by absorbing a neutron.
- Chain reaction A series of fissions in which neutrons from one fission cause further fissions.
- Daughter nuclei The smaller nuclei produced when a larger nucleus splits.
Common Mistakes
- Saying that fission is the joining of nuclei; that is fusion.
- Believing fission is triggered by fast neutrons in a reactor;
- slow neutrons are the most effective at causing it.
- Thinking all the energy comes from burning the uranium, when it comes from the nucleus splitting.
- Forgetting that more than one neutron is released, which is why a chain reaction is possible.