Section 1 of 5What Happens in a Fission Event
Fission is usually started by a neutron colliding with a large nucleus.
Neutron absorbed
- A slow neutron enters a uranium-235 nucleus, making it unstable.
Splitting
- The unstable nucleus splits into two smaller daughter nuclei, which are usually radioactive.
Extra neutrons
- Two or three neutrons are released in each fission, and they can go on to hit other nuclei.
Energy
- The kinetic energy of the daughter nuclei and neutrons is large, and it is transferred to thermal energy of the surroundings.
Gamma rays
- Gamma radiation is also emitted, adding to the energy released and to the need for shielding.
Section 2 of 5The Chain Reaction
1
A neutron splits a first uranium-235 nucleus, releasing more neutrons and energy.
2
Each released neutron can be absorbed by another uranium-235 nucleus, causing that nucleus to split.
3
The number of fissions can grow rapidly, because each fission releases more than one neutron.
4
In an uncontrolled chain reaction a huge amount of energy is released in a very short time, as in a nuclear weapon.
5
In a reactor the chain reaction is controlled so that on average one neutron from each fission causes another fission.
Section 3 of 5Fuels and Their Properties
Fuels and Their Properties
Uranium-235
- This isotope makes up only about 0.7 percent of natural uranium, so reactor fuel is enriched to raise the proportion.
Plutonium-239
- This man-made isotope is also fissile and is made in reactors from uranium-238.
Critical mass
- A minimum mass of fissile material is needed so that enough neutrons stay inside to keep the chain reaction going.
Waste products
- The daughter nuclei are radioactive, which is why spent fuel must be stored carefully.
| Chain reaction type | Average fissions caused per fission | Result |
|---|---|---|
| Controlled | 1 | Steady energy output |
| Uncontrolled | more than 1 | Rapid growth, explosion |
Section 4 of 5Control and Safety
Control rods
- Rods that absorb neutrons are lowered between fuel rods to slow the reaction or raised to speed it up.
Moderator
- A material surrounding the fuel slows neutrons, because slow neutrons are more likely to cause fission.
Shutdown
- Fully inserting the control rods absorbs enough neutrons to stop the chain reaction.
Shielding
- Thick concrete and steel around the reactor absorb neutrons and gamma rays to protect workers.
Section 5 of 5Exam Checklist & Tips
6.4.2 – Key Points for the Exam
- Chain reaction A series of fissions in which neutrons from one fission cause further fissions.
- Fissile material A substance whose nuclei can be split by neutrons, such as uranium-235 or plutonium-239.
- Daughter nuclei The smaller nuclei produced when a nucleus splits.
- Critical mass The smallest mass of fissile material that can sustain a chain reaction.
- Enrichment Increasing the proportion of uranium-235 in uranium fuel.
Common Mistakes
- Saying that the atom splits in two without a neutron being involved;
- fission is normally started by neutron absorption.
- Forgetting that more than one neutron is released, which is why the reaction can multiply.
- Confusing fission products with the original fuel;
- the daughter nuclei are different, often radioactive, elements.
- Saying that control rods slow neutrons; they absorb neutrons, and the moderator slows them.