Energy and particles · GCSE Physics
Fission and fusion
Teacher-written GCSE Physics revision on nuclear fission and fusion: chain reactions, control rods and moderators, hydrogen fusing to helium in stars, and why fusion on Earth is still a control problem.
Fission is a heavy nucleus splitting. Fusion is light nuclei joining. Both release energy because the products are more tightly bound. Stars fuse; reactors on Earth currently fission.
The important bits
What you need to know
- 1
Nuclear fission is the splitting of a large, unstable nucleus (uranium-235 or plutonium-239) into two smaller nuclei, plus neutrons, with a release of energy.
- 2
A neutron is absorbed, the nucleus becomes unstable and splits. Emitted neutrons can induce further fissions: a chain reaction.
- 3
In a nuclear reactor, a moderator (water or graphite) slows neutrons so they are more likely to cause fission. Control rods absorb neutrons to keep the chain reaction steady.
- 4
If the chain reaction is uncontrolled, energy is released in a very short time (a nuclear weapon). Reactors are designed so that cannot happen: rods, coolant, shielding.
- 5
Nuclear fusion is the joining of light nuclei, typically hydrogen isotopes, to make a heavier nucleus such as helium, releasing energy. This is the energy source of stars.
- 6
Fusion requires extremely high temperature and pressure so that nuclei, which repel electrically, get close enough for the strong nuclear force to bind them.
- 7
Fission in power stations does not produce carbon dioxide in generation but does produce radioactive waste that needs long-term storage. Fusion waste would be much less long-lived, but a practical power station is not yet available.
- 8
The energy comes from a loss of mass: E = mc² is beyond most Combined calculations, but you should know that products have a slightly smaller total mass than the starting nuclei.
Quotations worth analysing
Short evidence. Real method.
“Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei.”
Name the neutron that starts it, the two daughter nuclei, extra neutrons, and the energy released. A split without neutrons will not chain.
“Nuclear fusion is the joining of two light nuclei to form a heavier nucleus.”
The Sun fuses hydrogen to helium. That is not burning in oxygen; it is a nuclear change in the core.
“A chain reaction occurs when neutrons from one fission cause further fissions.”
Control is about how many neutrons go on to cause another fission: too few and it dies; too many and the power rises.
Go deeper
Reactors are chain reactions with a steering wheel
One slow neutron hits uranium-235, the nucleus splits, energy is transferred to the kinetic store of the fragments (which becomes thermal in the fuel), and two or three neutrons fly out. If exactly one of those, on average, causes another fission, the power is steady. The moderator slows neutrons because slow neutrons fission U-235 more effectively. Control rods of boron or cadmium swallow neutrons if the reaction runs too hot. Coolant carries thermal energy to make steam, turn a turbine, turn a generator — the rest is ordinary power-station physics. Waste is the used fuel, still radioactive. In an exam diagram, label fuel rods, moderator, control rods, coolant and concrete shielding, and say what each one does, not just what it is made of.
Go deeper
Fusion is what stars do, and why Earth finds it hard
Two positive nuclei repel. In the Sun’s core the temperature is millions of degrees, so nuclei move fast enough to fuse despite that repulsion. Each fusion to helium releases more energy per kilogram than fission, with no long-lived actinide waste. On Earth we cannot copy the Sun’s gravitational pressure easily, so experiments use magnetic fields or lasers to confine a hot plasma. GCSE does not need the engineering, but it does need the comparison: fission is used now; fusion is the process in stars and a research goal. Do not write that the Sun “burns coal” or that fission and fusion are the same because both are “nuclear”. One splits heavy nuclei; the other joins light ones.
See the idea in action
A fission event can be written in outline as ¹₀n + ²³⁵₉₂U → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n + energy. Check A: 1 + 235 = 236 and 141 + 92 + 3 = 236. Check Z: 0 + 92 = 92 and 56 + 36 + 0 = 92. The three neutrons can go on to cause more fissions; in a reactor, on average one of them does. If each fission transfers about 3 × 10⁻¹¹ J, then 2.0 × 10¹³ fissions transfer 600 J — a tiny mass of fuel stores an enormous energy compared with burning the same mass of coal. Fusion in the Sun: ¹H nuclei eventually form ⁴He, releasing energy that leaves the star as radiation, including the visible and infrared that reach Earth.
Exam technique
Turn knowledge into marks
Use split versus join in the first sentence. For fission, mention neutrons, chain reaction, moderator and control rods. For fusion, mention light nuclei, high temperature, and stars. Do not mix the two processes.
Common mistakes
Do not give these marks away
- 01
Calling the Sun a fission reactor, or saying fusion is how chemical fuels burn.
- 02
Forgetting that extra neutrons are needed for a chain reaction, or mixing up the jobs of moderator and control rods.
- 03
Writing that nuclear power stations emit carbon dioxide from the fission reaction itself.
What is the role of control rods in a nuclear reactor?
AThey slow neutrons down so fission is more likely
BThey absorb neutrons to keep the chain reaction at a steady rate
CThey join hydrogen nuclei to make helium
DThey generate the electricity directly from fission fragments
Show the answer
They absorb neutrons to keep the chain reaction at a steady rate. Moderators slow neutrons. Control rods absorb them. Inserting rods further reduces the number of neutrons available to cause fission, so power falls.
Quick questions
If this is the bit you searched
What is the difference between fission and fusion?
Fission splits a heavy nucleus into two smaller ones. Fusion joins light nuclei into a heavier one. Both can release energy. Stars use fusion; current power stations use fission.
What does a moderator do?
It slows neutrons so they are more likely to be absorbed by uranium-235 and cause fission. Water or graphite is used.
Why is fusion difficult on Earth?
Nuclei are positive and repel. Extremely high temperatures are needed so they get close enough to fuse, and the hot plasma must be confined.
Is nuclear waste from fission the same as fusion?
Fission produces highly radioactive daughter nuclei that can remain hazardous for a long time. Fusion would produce much less long-lived waste, but it is not yet a working power source.