Nuclear Fission and Fusion
A pinhead of uranium can release as much energy as a tonne of coal. The trick is that nuclear reactions turn a tiny bit of mass directly into a vast amount of energy.
Q1 · "Fission" means splitting and "fusion" means joining. Based only on those words, what do you think happens to a nucleus in each process?
Q2 · The Sun has been shining for billions of years. Burning fuel could not last that long. What kind of nuclear reaction do you think powers it, and why?
● Know
- That nuclear fission is the splitting of a large nucleus into smaller ones, releasing energy
- That nuclear fusion is the joining of small nuclei into a larger one, releasing energy
- That both convert a small amount of mass into a large amount of energy ($E = mc^2$)
● Understand
- How a fission chain reaction works and how it is controlled in a reactor
- Why fusion requires extremely high temperatures and pressures
- Why fusion (the Sun's process) is harder to use on Earth than fission
● Can do
- Compare fission and fusion in terms of what happens to the nuclei
- Explain a chain reaction and the role of control rods
- Identify which process powers stars and which powers current reactors
Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei. It happens when a heavy nucleus such as uranium-235 absorbs a neutron, becomes even more unstable, and splits apart. The split releases:
- two smaller "daughter" nuclei,
- two or three free neutrons, and
- a large amount of energy.
The energy comes from a tiny loss of mass: the products together weigh very slightly less than the original, and that "missing" mass becomes energy according to $E = mc^2$. Because $c$ (the speed of light) is huge, even a minuscule mass loss releases an enormous amount of energy — millions of times more than a chemical reaction with the same mass of fuel.
The chain reaction. The neutrons released by one fission can strike other uranium-235 nuclei and make them split, releasing yet more neutrons. This is a chain reaction. In a nuclear power station, control rods absorb some neutrons to keep the reaction steady, so the energy is released gradually and safely as heat — which boils water to drive turbines and generate electricity.
A typical fission of uranium-235 can be written: $^{235}_{92}\text{U} + {}^{1}_{0}\text{n} \rightarrow {}^{141}_{56}\text{Ba} + {}^{92}_{36}\text{Kr} + 3\,{}^{1}_{0}\text{n} + \text{energy}$. Notice it still balances: mass numbers $141 + 92 + 3 = 236 = 235 + 1$, and atomic numbers $56 + 36 = 92$. The three neutrons can go on to split three more nuclei — the start of a chain reaction.
Australia's uranium and the world's reactors: Australia mines about a third of the world's uranium but uses it only for research and medical isotopes, not for power. The uranium-235 in Australian ore is the fuel for fission reactors in many other countries. Decisions about exporting it involve exactly the science of this lesson, plus the safety and environmental issues you will weigh up in Lesson 27.
Fission is not the same as ordinary "burning." A nuclear reactor does not burn uranium chemically; it splits uranium nuclei. That is why fission releases millions of times more energy per kilogram than burning coal — the energy comes from the nucleus, not from chemical bonds.
Nuclear fusion is the opposite of fission: small, light nuclei join together to form a larger nucleus, releasing energy. The classic example is hydrogen nuclei fusing into helium — exactly the process that powers the Sun and the stars, which you met in Lesson 22.
Like fission, fusion releases energy because a small amount of mass is converted to energy via $E = mc^2$. In fact, fusing light nuclei releases even more energy per kilogram than fission, and its only main product (helium) is harmless.
Why fusion is hard. To fuse, two positively charged nuclei must be pushed close enough for the strong nuclear force to grab them — but their electrostatic repulsion fights this fiercely. Overcoming it needs extremely high temperatures (millions of degrees) and enormous pressure, turning matter into a plasma. The Sun achieves this with its colossal gravity squeezing its core. On Earth, recreating those conditions in a machine is a huge engineering challenge.
That is why our power stations today use fission (easier to start and control), while controlled fusion power — clean, abundant and low-waste — is still being developed in giant experimental reactors.
In the Sun, hydrogen nuclei fuse step by step into helium. A simplified version of the fusion that experiments on Earth aim to use is deuterium + tritium → helium-4 + a neutron + energy. Compared with burning coal, fusing the hydrogen in a cup of water could release as much energy as burning tonnes of fuel — with no greenhouse gases and only helium as a by-product.
Australia and fusion research: Australian scientists, including teams at the Australian National University, contribute to international fusion research and to projects like ITER, the giant fusion experiment in France. Australia's plentiful sunshine is itself fusion energy arriving from the Sun — so when you use solar power, you are already using the output of nuclear fusion.
Do not mix up fission and fusion. Fission = splitting a big nucleus (used in today's reactors and based on uranium). Fusion = joining small nuclei (powers the Sun; still experimental on Earth). Both release energy, but in opposite ways.
Quick-fire true or false on fission and fusion.
Fission splits a large nucleus into smaller ones.
Fusion joins small nuclei into a larger one.
Both fission and fusion convert mass into energy.
The Sun is powered by nuclear fusion.
Control rods absorb neutrons to control a chain reaction.
Today's power stations mostly use fusion.
Fusion needs extremely high temperatures and pressures.
Nuclear fission is just chemical burning of uranium.
It helps to compare the two processes side by side.
- What happens: fission splits a large nucleus; fusion joins small nuclei.
- Fuel: fission uses heavy elements like uranium-235 or plutonium; fusion uses light elements, especially isotopes of hydrogen.
- Energy source: both convert a small mass loss into energy ($E = mc^2$).
- Conditions: fission can start at ordinary temperatures once a neutron is absorbed; fusion needs millions of degrees and immense pressure.
- Where used: fission powers today's nuclear reactors; fusion powers the stars and is the goal of future clean-energy reactors.
- Waste: fission produces long-lived radioactive waste; fusion's main product (helium) is harmless, though the process does create some shorter-lived radioactivity in the reactor.
Both are nuclear reactions and both obey conservation: the totals of mass number and atomic number balance, even though a tiny part of the mass becomes energy.
If asked "Which process powers a nuclear power station, and which powers the Sun?", the answer is: a power station uses fission (splitting uranium), while the Sun uses fusion (joining hydrogen). Both release energy from a tiny conversion of mass, but in opposite directions on the periodic table.
Two nuclear futures for Australia: Australia's energy debate touches both processes. Fission is sometimes proposed using Australia's vast uranium reserves, but raises waste and safety questions. Fusion, if perfected, could offer clean baseload power with little waste. Either way, citizens need to understand the difference between fission and fusion to take part in the conversation — which is exactly why this is in the syllabus.
"Fusion is already used in power stations." Not yet. Despite decades of research, no fusion reactor produces commercial electricity. Today's nuclear power is fission. Keep this distinction clear in exam answers.
Connect the key ideas about fission and fusion. Click two connected ideas to explain the link.
Wrong: "Fission and fusion are the same thing." No — fission splits a large nucleus, while fusion joins small nuclei. They are opposite processes that both happen to release energy.
Right: Fission splits a heavy nucleus into smaller ones; fusion joins light nuclei into a heavier one. Both convert a tiny mass loss into a large amount of energy.
Wrong: "A nuclear reactor burns its fuel chemically, like a coal fire." No — a fission reactor splits nuclei, not chemical bonds, which is why it releases vastly more energy per kilogram of fuel.
Right: Energy in a reactor comes from splitting nuclei (a nuclear process), not from chemical burning, so a tiny mass of fuel releases enormous energy.
Wrong: "Mass is destroyed in a nuclear reaction, breaking conservation laws." No — mass is not destroyed; a small amount of mass is converted into energy, and the combined mass–energy is conserved.
Right: A tiny amount of mass is converted to energy via $E = mc^2$. Mass and energy are interchangeable, and the total is conserved.
Australia: Uranium Exporter, Fusion Partner, Sun-Powered Nation
Australia sits at the heart of the global nuclear-energy story in three ways. It exports the uranium that fuels fission reactors overseas, while choosing not to build power reactors at home. It contributes to international fusion research aimed at clean future energy. And it harvests fusion energy every day — every solar panel runs on light produced by fusion in the Sun's core, 150 million kilometres away.
Understanding fission and fusion lets Australians engage with real national decisions: whether to develop nuclear power, how to manage uranium exports responsibly, and how to invest in the clean-energy technologies of the future. The physics in this lesson is the foundation for those debates, which you will explore further in the next lesson.
✍ Copy Into Your Books
▾Fission
- Splitting a large nucleus (e.g. U-235) into smaller ones
- Started by a nucleus absorbing a neutron
- Releases 2–3 neutrons + energy → chain reaction
- Control rods absorb neutrons to keep it steady
- Used in today's nuclear power stations
Fusion
- Joining small nuclei (e.g. hydrogen) into a larger one
- Needs millions °C + huge pressure (plasma)
- Releases even more energy; main product = helium
- Powers the Sun and stars; future clean energy
Shared idea
- Both convert a tiny mass loss into energy: E = mc²
- Far more energy than chemical reactions
- Mass–energy is conserved overall
Fission or Fusion?
Explain the Process
At the start, the hook asked how splitting or joining nuclei can release millions of times more energy than burning fuel.
Now write a clear answer using the words mass, energy and $E = mc^2$. Then revisit your Q2 prediction — was the Sun powered by fission or fusion, and were you right?
Q1. Describe nuclear fission. In your answer, state what happens to the nucleus, name a typical fuel, and explain what a chain reaction is. (3 marks)
Q2. Compare nuclear fission and nuclear fusion. Include what happens to the nuclei, the type of fuel each uses, and where each is currently used. (4 marks)
Q3. A student says nuclear reactions "break the law of conservation of mass." Evaluate this statement and correct it, referring to $E = mc^2$. (3 marks)
Revisit Your Thinking
Go back to your Think First answers. Has your understanding changed?
- Were your predictions about what happens in fission and fusion correct?
- Can you now explain why the Sun's fuel lasts billions of years?
Model answers (click to reveal)
Answers
▾MCQ 1
B — Fission is the splitting of a large, unstable nucleus into smaller nuclei, releasing energy and neutrons.
MCQ 2
C — The Sun is powered by nuclear fusion, joining hydrogen nuclei into helium.
MCQ 3
A — Control rods absorb neutrons, which controls how many further fissions occur and so keeps the chain reaction steady.
MCQ 4
D — In both fission and fusion a tiny amount of mass is converted into a large amount of energy, described by Einstein's equation $E = mc^2$.
MCQ 5
C — Nuclei are positively charged and repel one another strongly. To force them close enough to fuse, fusion requires extremely high temperatures (millions of degrees) and enormous pressure, which is very difficult to achieve on Earth.
Short Answer 1
Model answer: Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei. It is triggered when a heavy nucleus such as uranium-235 absorbs a neutron and becomes unstable. The split releases energy and two or three neutrons. A chain reaction is when those released neutrons strike other uranium-235 nuclei and make them split too, so the reaction sustains and spreads itself.
Short Answer 2
Model answer: In fission a large nucleus splits into smaller ones; in fusion small nuclei join to form a larger one. Fission uses heavy fuels such as uranium-235, while fusion uses light fuels such as isotopes of hydrogen. Fission is used in today's nuclear power stations, whereas fusion powers the Sun and stars and is still being developed as a future energy source on Earth. Both release energy by converting a tiny amount of mass into energy.
Short Answer 3
Model answer: The statement is not quite correct. In a nuclear reaction a very small amount of mass does seem to "disappear," but it is not destroyed — it is converted into a large amount of energy, as described by $E = mc^2$. So the law that should be applied is the conservation of mass–energy together: when the lost mass is counted as energy, the total is conserved. The simple "mass alone is always conserved" rule applies to chemical reactions, not nuclear ones.