Section 1 of 5What Fission Is
Nuclear fission is the splitting of a large unstable nucleus into two smaller nuclei, releasing energy.
Fuel
- Nuclear power stations use isotopes of uranium or plutonium, such as uranium-235, as fuel.
Induced fission
- A uranium-235 nucleus absorbs a slow-moving neutron, becomes unstable and then splits.
Products
- Two smaller daughter nuclei, two or three neutrons and gamma radiation are produced.
Energy
- The fission fragments and neutrons move off at very high speed, so nuclear energy becomes kinetic energy and then thermal energy.
Section 2 of 5Chain Reaction
The neutrons released in each fission can cause further fissions.
1
A neutron strikes a uranium-235 nucleus, which splits and releases two or three more neutrons.
2
Each of these neutrons may strike another uranium-235 nucleus and cause it to split in turn.
3
The number of fissions grows rapidly if every neutron produces more fissions, which is a chain reaction.
4
In a reactor, control rods absorb some neutrons so that on average each fission leads to exactly one more fission.
5
In a nuclear weapon the reaction is uncontrolled, so the energy is released in a very short time as an explosion.
Section 3 of 5Reactor and Equations
Reactor and Equations
Moderator
- Slows down neutrons so that they are more likely to cause fission when they hit a uranium-235 nucleus.
Control rods
- Made of a material that absorbs neutrons, and they are raised or lowered to change the rate of fission.
Heat transfer
- The energy released heats a coolant that boils water to make steam and drive a turbine.
Nuclear equation
- Mass numbers and atomic numbers must each add up to the same totals on both sides.
| Quantity | Left side | Right side |
|---|---|---|
| Mass number total | 235 + 1 = 236 | 141 + 92 + 3 = 236 |
| Atomic number total | 92 + 0 = 92 | 56 + 36 + 0 = 92 |
Section 4 of 5Worked Example
Question
- A neutron is absorbed by uranium-235 (mass number 235, atomic number 92).
- The nucleus splits into barium-141 (atomic number 56), krypton-92 (atomic number 36) and some neutrons.
- How many neutrons are released?
- Total mass number before235 + 1 = 236.
- Mass number of the two daughter nuclei: 141 + 92 = 233.
- Mass number of the neutrons released236 − 233 = 3, and each neutron has mass number 1.
- Check the atomic numbers92 before and 56 + 36 = 92 after, so neutrons (atomic number 0) fit.
Answer 3 neutrons are released
Section 5 of 5Exam Checklist & Tips
6.2.4 – Key Points for the Exam
- Nuclear fission The splitting of a large unstable nucleus into two smaller nuclei.
- Chain reaction A series of fissions in which neutrons from one fission cause further fissions.
- Control rod A rod that absorbs neutrons to control the rate of fission.
- Daughter nuclei The smaller nuclei produced when a nucleus splits.
- Moderator A material that slows down neutrons in a reactor.
Common Mistakes
- Confusing fission (splitting a large nucleus) with fusion (joining small nuclei).
- Saying the control rods fuel the reaction, when they absorb neutrons to slow it.
- Forgetting the incoming neutron when balancing the mass numbers of a fission equation.
- Writing that fission needs no neutrons; most uranium-235 fission is induced by absorbing a neutron.