Nuclear Fission and Nuclear Fusion

Section 1 of 7Nuclear fission

What happens

• What happens A large, unstable nucleus, such as uranium, splits into two smaller nuclei.
• The splitting releases a large amount of energy.

Where it is used

• Where it is used Fission is the energy source in nuclear power stations, where the fuel is held in a reactor.
• The energy is released in a controlled way and used to heat water and generate electricity.

Section 2 of 7Nuclear fusion

What happens

• What happens Two small nuclei, such as hydrogen nuclei, join together to form a larger nucleus.
• A huge amount of energy is released.

Where it occurs

• Where it occurs Fusion is the process that releases energy in the Sun and other stars.
• It needs extremely high temperature and pressure, which are found in the core of a star.

Section 3 of 7Comparing the two processes

Feature	Fission	Fusion
Change to nuclei	Large nucleus splits into smaller ones	Small nuclei join into a larger one
Starting nuclei	Large nuclei, such as uranium	Small nuclei, such as hydrogen
Energy released	Large amount	Huge amount
Conditions	Occurs in a nuclear reactor	Extremely high temperature and pressure
Example	Nuclear power station	The Sun

Section 4 of 7Energy from the nucleus

1Nuclear change

• Nuclear change Nuclei are rearranged by splitting or joining.

2Energy released

• Energy released The products have less stored nuclear energy than the starting nuclei, so energy is transferred out.

3Useful transfer

• Useful transfer In a power station the energy heats water to steam, which drives turbines and generators.

Section 5 of 7Fission and fusion in the real world

Fission fuel

• Fission fuel Nuclear power stations use large nuclei such as uranium.
  • The energy released by splitting them heats water to make steam, and the steam turns turbines connected to generators.

Fusion in stars

• Fusion in stars In the Sun, hydrogen nuclei fuse together to make larger nuclei.
  • The huge energy output of this process is why the Sun shines and keeps the Earth warm.

Why fusion is hard on Earth

• Why fusion is hard on Earth Nuclei repel one another because they are all positively charged.
  • Only at extremely high temperature and pressure do they move fast enough and get close enough to fuse,
  • which makes fusion difficult to achieve in a power station.

What you must be able to do

• What you must be able to do You are not asked for the detailed mechanism,
  • but you must be able to state clearly how fission and fusion differ in what happens to the nuclei.

Section 6 of 7Key terms

Term	Meaning
Nuclear fission	The splitting of a large nucleus into smaller nuclei, releasing energy.
Nuclear fusion	The joining of small nuclei to form a larger nucleus, releasing energy.
Nuclear reactor	A device in which fission is controlled to release energy steadily.
Nucleus	The dense central part of an atom that contains protons and neutrons.

Section 7 of 7Exam technique

1

Swapping the two processes. Fission splits, and fusion fuses.

2

Saying the Sun uses fission. It releases energy by fusion.

3

Forgetting the conditions for fusionextremely high temperature and pressure, as in a star's core.

4

Describing the energy released as chemical energy. It comes from changes in the nuclei, and not from changes in bonds between atoms.

5

Saying fission joins atoms of uranium together. It splits a uranium nucleus into smaller nuclei.