Spec links: 4.4.4.1 Nuclear fission (Physics only); 4.4.4.2 Nuclear fusion (Physics only); 4.1.1.4 Power; 4.1.3 National and global energy resources · Reading time: about 4 minutes
Five seconds that made history
In late 2023, near Oxford, a giant ring-shaped machine called JET ran its final experiments. For about five seconds, it released 69 megajoules of energy from nuclear fusion. That set a world record. The fuel used was only about 0.2 milligrams, which is less than a thousandth of a gram. Then, after around 40 years of research, JET was switched off for good.
Could fusion one day power our homes? To answer that, we need to compare it with the nuclear power we already use.
Splitting atoms: fission
Today’s nuclear power stations use nuclear fission. This is the splitting of a large, unstable nucleus, such as uranium-235. For fission to happen, the nucleus usually has to absorb a neutron first.
The nucleus then splits into two smaller nuclei of roughly equal size. It also releases two or three neutrons and some gamma rays. Energy is released by the fission. All of these products have kinetic energy.
The new neutrons can hit other uranium nuclei and cause more fissions. This is a chain reaction. In a reactor, the chain reaction is controlled. Control rods absorb neutrons, so fewer fissions happen. Lowering the rods slows the reaction down. A moderator, such as graphite or water, slows the neutrons down so that they are more likely to be absorbed by uranium nuclei. In a nuclear weapon, the chain reaction is not controlled, and it causes an explosion.
Joining atoms: fusion
Fusion is the opposite. Two light nuclei join together to form a heavier nucleus. In the process, some of the mass may be converted into energy of radiation. This is how the Sun and other stars release energy. In the Sun’s core, hydrogen nuclei fuse to make helium.
Fusion is hard to achieve. Nuclei are positively charged, so they repel each other. To make them collide hard enough to fuse, the fuel must be at an extremely high temperature, many millions of degrees Celsius, and at a high pressure or density. No material container can hold something that hot, so machines like JET use strong magnetic fields to keep the hot fuel away from the walls.
How powerful was JET?
We can find the average power during JET’s record pulse using energy transferred = power × time, or E = P t. Rearranging gives P = E ÷ t. So P = 69 000 000 J ÷ 5 s = 13 800 000 W, or about 14 MW. That sounds impressive. But JET used much more energy to heat the fuel and run its magnets than the fusion released. A useful power station must give out more energy than it takes in.
The road ahead
A larger international machine called ITER is being built in southern France. Its aim is to show that fusion can release much more energy than is used to heat the fuel. It will not supply electricity to homes, but it should help scientists design future power stations.
Fusion has big possible advantages. It does not produce carbon dioxide, and it creates far less long-lived radioactive waste than fission. But it is not ready yet. For now, fusion remains a hope for the future.
Key words
- Nuclear fission: the splitting of a large and unstable nucleus, such as uranium or plutonium, into two smaller nuclei.
- Nuclear fusion: the joining of two light nuclei to form a heavier nucleus.
- Chain reaction: when neutrons released by fission go on to cause further fission reactions.
- Control rods: rods that absorb neutrons to control the rate of fission in a reactor.
- Moderator: a material that slows down neutrons in a reactor so they are more likely to cause fission.
- Neutron: an uncharged particle found in the nucleus of an atom.
Questions
- How much energy did JET release in its record experiment? [1 mark]
- Name the process that releases energy in the Sun. [1 mark]
- Give two things released, as well as two smaller nuclei, when a uranium-235 nucleus undergoes fission. [2 marks]
- Explain how control rods and a moderator are used in a nuclear reactor. [3 marks]
- Explain why fusion only happens at extremely high temperatures. [2 marks]
- A fusion experiment releases 12 MJ of energy at an average power of 4.0 MW. Calculate how long the experiment lasts. Give your answer in seconds. [3 marks]
- A nuclear power station has an output power of 1200 MW. Calculate the energy it transfers in one hour. Give your answer in joules, in standard form. [3 marks]
- Compare nuclear fission and nuclear fusion as ways of generating electricity in the future. Describe how each process releases energy and evaluate the advantages and disadvantages of each. [6 marks]
Show answers
- 69 MJ (69 megajoules) (1)
- Nuclear fusion (1)
- Any two from: two or three neutrons (1); gamma rays (1); energy (1)
- Control rods absorb neutrons (1); lowering them reduces the number of fissions / slows the chain reaction (1); the moderator slows down neutrons so they are more likely to be absorbed by uranium nuclei and cause fission (1)
- Nuclei are positively charged so they repel each other (1); at high temperatures they move fast enough to overcome the repulsion and fuse (1)
- t = E ÷ P (1); t = 12 000 000 ÷ 4 000 000 (1); t = 3.0 s (1)
- E = P t with t = 3600 s (1); E = 1 200 000 000 × 3600 (1); E = 4.32 × 1012 J (1)
- Indicative content:
• Fission: a large unstable nucleus (e.g. uranium-235) absorbs a neutron and splits into two smaller nuclei, releasing neutrons, gamma rays and energy
• Neutrons cause a chain reaction, controlled by control rods and a moderator
• Fusion: two light nuclei join to form a heavier nucleus; some mass is converted into energy
• Fusion needs extremely high temperature and pressure to overcome repulsion between nuclei
• Both: no carbon dioxide released while generating electricity; reliable, not dependent on weather
• Fission advantages: proven technology, already used in power stations
• Fission disadvantages: long-lived radioactive waste; risk of accidents; decommissioning costs
• Fusion advantages: far less long-lived radioactive waste; uses a tiny mass of fuel
• Fusion disadvantages: not yet working as a power station; no fusion power station has yet given out more energy than it takes in overall; very expensive
• Judgement: fission is available now; fusion may be better long term but is not ready
Level 3 (5–6 marks): accurate descriptions of both processes and a balanced evaluation with a reasoned conclusion; Level 2 (3–4 marks): describes both processes with some advantages or disadvantages; Level 1 (1–2 marks): simple statements about one process or one advantage.
More reading: See all 30 GCSE Physics comprehension articles