P13: Nuclear Fission and Fusion
Nuclear fission, fusion and nuclear power
📋 Key Definitions
Nuclear fission: The splitting of a heavy, unstable nucleus (such as uranium-235 or plutonium-239) into two lighter nuclei, releasing energy and neutrons.
Nuclear fusion: The joining of two light nuclei (such as hydrogen) to form a heavier nucleus (such as helium), releasing a large amount of energy.
Chain reaction: A process in which neutrons released from one fission event cause further fission events in other nuclei, which release more neutrons, and so on.
Control rods: Rods (usually made of boron or cadmium) inside a nuclear reactor that absorb neutrons to control the rate of fission.
Moderator: A substance (usually water or graphite) in a nuclear reactor that slows down fast-moving neutrons so they can be more easily absorbed by uranium-235 nuclei.
💥 Nuclear Fission
Nuclear fission is the process used in nuclear power stations. A heavy nucleus absorbs a neutron, becomes unstable, and splits into two smaller nuclei.
How fission works
- A slow-moving neutron is absorbed by a uranium-235 or plutonium-239 nucleus
- The nucleus becomes unstable and splits into two smaller nuclei (fission fragments)
- Two or three neutrons are released
- A large amount of energy is released (as kinetic energy of the fission fragments)
- The released neutrons can cause further fission events, creating a chain reaction
U-235 + neutron → fission fragments + 2-3 neutrons + energy
The exact fission fragments vary — they are typically a pair of medium-mass nuclei
Why energy is released: The total mass of the products is slightly less than the total mass of the reactants. The “missing” mass has been converted into energy (E = mc²).
Worked Example 1: Fission of uranium-235
Describe the fission process when a uranium-235 nucleus absorbs a neutron.
A uranium-235 nucleus absorbs a slow-moving neutron, becoming uranium-236, which is highly unstable. It splits into two lighter nuclei (e.g. barium-141 and krypton-92), plus 3 neutrons and a large amount of energy. The 3 neutrons can go on to cause further fission events, creating a chain reaction.
🏭 The Nuclear Reactor
A nuclear reactor controls the fission chain reaction to produce a steady output of energy.
| Component | Material | Function |
|---|---|---|
| Fuel rods | Uranium-235 or plutonium-239 | Provide the fissile material for fission |
| Control rods | Boron or cadmium | Absorb neutrons to control the rate of fission. Lowered to slow reaction, raised to speed it up |
| Moderator | Water or graphite | Slow down fast neutrons so they can be absorbed by uranium-235 (slow neutrons are more likely to cause fission) |
| Coolant | Water or carbon dioxide | Transfers thermal energy from the reactor to a heat exchanger, producing steam to drive a turbine |
| Containment | Thick concrete and steel | Prevents radiation escaping and absorbs neutrons |
Control rods are raised and lowered to control the chain reaction. If they are pushed in further, they absorb more neutrons and the reaction slows. If they are pulled out, fewer neutrons are absorbed and the reaction speeds up. In an emergency, the control rods are fully inserted to shut down the reactor.
Worked Example 2: Explaining control rods
The power output of a nuclear reactor is too low. Explain how the control rods should be adjusted to increase it.
The control rods should be raised slightly out of the reactor core. This means they absorb fewer neutrons, so more neutrons are available to cause further fission events. The chain reaction speeds up, releasing more energy per second, and the power output increases.
☀️ Nuclear Fusion
Nuclear fusion is the process that powers the Sun and other stars. It releases much more energy per reaction than fission.
Deuterium + Tritium → Helium + neutron + energy
(Two hydrogen isotopes join to form helium)
Conditions needed for fusion
- Very high temperature (about 100 million °C) — needed so nuclei can overcome their electrostatic repulsion (both are positive)
- Very high pressure — needed to force nuclei close enough together for fusion to occur
Why fusion is difficult to achieve on Earth: The temperatures and pressures needed are so extreme that no material can contain the fusion plasma. Scientists use magnetic fields or lasers to confine the plasma, but it is extremely difficult and no fusion reactor yet produces more energy than it consumes.
Fusion in the Sun: The enormous gravitational pressure inside the Sun creates the extreme conditions needed. Hydrogen nuclei fuse to form helium, releasing the energy that makes the Sun shine.
Worked Example 3: Explaining fusion conditions
Explain why extremely high temperatures are needed for nuclear fusion.
Both nuclei are positively charged, so they repel each other (electrostatic repulsion). At very high temperatures, the nuclei have enough kinetic energy to overcome this repulsion and get close enough for the strong nuclear force to pull them together. Without sufficient temperature, the nuclei would just bounce apart.
⚖️ Fission vs Fusion Comparison
| Property | Nuclear Fission | Nuclear Fusion |
|---|---|---|
| Process | Splitting heavy nuclei | Joining light nuclei |
| Fuel | Uranium-235, plutonium-239 | Hydrogen isotopes (deuterium, tritium) |
| Energy released per reaction | Large | Even larger (per unit mass) |
| Waste products | Radioactive fission fragments — must be stored safely for thousands of years | Helium (not radioactive) — very little waste |
| Fuel availability | Limited supplies of uranium and plutonium | Hydrogen is abundant (from seawater) |
| Used in power stations? | Yes — current nuclear power | Not yet — still experimental |
| Conditions needed | Slow neutrons, controlled chain reaction | Very high temperature and pressure |
| Risk of runaway reaction | Possible (but controlled by control rods) | No — if conditions change, fusion simply stops |
Worked Example 4: Comparing fission and fusion
Give two advantages of nuclear fusion over nuclear fission for power generation.
(1) Fusion produces much less radioactive waste — the product is helium, which is harmless. Fission produces radioactive waste that must be stored for thousands of years.
(2) Fusion fuel (hydrogen from seawater) is abundant and virtually unlimited. Fission fuel (uranium) is limited and must be mined.
(3) A fusion reactor cannot undergo a runaway chain reaction — if conditions change, fusion stops automatically. This makes it inherently safer.
❓ Practice Questions
Q1: Foundation Describe the process of nuclear fission of uranium-235.
Q2: Foundation Explain the function of control rods and the moderator in a nuclear reactor.
Q3: Higher Explain why extremely high temperatures and pressures are needed for nuclear fusion.
Q4: Higher Compare nuclear fission and nuclear fusion. Include two similarities and three differences.
Q5: Foundation Why is nuclear fusion not yet used as a practical energy source on Earth?
✅ Answers
- A uranium-235 nucleus absorbs a slow-moving neutron, becoming unstable. It splits into two smaller nuclei (fission fragments), releasing 2-3 neutrons and a large amount of energy. The released neutrons can cause further fissions, creating a chain reaction.
- Control rods absorb neutrons to control the rate of fission. They are raised to speed up the reaction and lowered to slow it down. The moderator (water or graphite) slows down fast neutrons so they can be more easily absorbed by uranium-235 nuclei to cause fission.
- Both nuclei are positively charged, so they repel each other. Very high temperatures give nuclei enough kinetic energy to overcome this repulsion. Very high pressures force nuclei close enough together for the strong nuclear force to bind them. These conditions are extremely difficult to create and maintain on Earth.
- Similarities: both release large amounts of energy; both involve changes to the nucleus. Differences: fission splits heavy nuclei, fusion joins light nuclei; fission produces radioactive waste, fusion produces helium (harmless); fission is used in current power stations, fusion is not yet practical; fission can have a runaway reaction, fusion stops if conditions change.
- The temperatures (about 100 million degrees) and pressures needed are so extreme that no material can contain the fusion plasma. Current experimental reactors use magnetic fields or lasers to confine the plasma, but they do not yet produce more energy than they consume.
🎯 Exam Tips
- Don't confuse fission (splitting) and fusion (joining) — remember: fission = fracture/split; fusion = fusing/joining
- When describing fission, always mention the chain reaction — the neutrons from one fission cause more fissions
- Control rods absorb neutrons; the moderator slows down neutrons — these are different functions, don't mix them up
- For fusion questions, always explain why high temperature is needed in terms of overcoming electrostatic repulsion between positive nuclei
- The reason fusion is “cleaner” is because the product is helium (not radioactive) — this is a key advantage over fission
- Nuclear power stations use fission, not fusion — don't say fusion when asked about current nuclear power
🔢 Maths Skills
Mathematical Skills
In nuclear equations, the total mass number and total atomic number must balance on both sides. You may need to solve for an unknown product. Energy released can be calculated from mass defect using E = mc², though at GCSE you mainly need to balance equations and compare energy scales.
Maths Example
Uranium-235 absorbs a neutron and fissions into barium-144 (atomic number 56) and an unknown element X, plus 3 neutrons. Find X: Mass number balance: 235 + 1 = 144 + AX + 3(1), so AX = 236 − 144 − 3 = 89. Atomic number balance: 92 = 56 + ZX, so ZX = 36 (krypton-89).
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Nuclear fusion is what happens in current nuclear power stations. Correct: Current nuclear power stations use fission (splitting uranium). Fusion is not yet practical for power generation.
2. Wrong: Control rods speed up the chain reaction by releasing more neutrons. Correct: Control rods ABSORB neutrons to slow down or stop the chain reaction. Raising them speeds it up; lowering them slows it down.
3. Wrong: Fusion produces more radioactive waste than fission. Correct: Fusion produces helium (harmless) with very little radioactive waste. Fission produces highly radioactive waste that must be stored for thousands of years.
✍️ 6-Mark Question
Extended Answer
6 marks: Compare nuclear fission and nuclear fusion. Discuss the process, energy released, waste produced and the conditions needed for each.
Nuclear fission is the splitting of a heavy, unstable nucleus (like uranium-235) into two lighter nuclei when it absorbs a neutron, releasing energy and 2–3 more neutrons that can cause a chain reaction. Nuclear fusion is the joining of two light nuclei (like hydrogen isotopes) to form a heavier nucleus (helium), releasing even more energy per unit mass than fission. Fission requires slow-moving neutrons and a controlled chain reaction using fuel rods, a moderator and control rods. Fusion requires extremely high temperatures (about 100 million °C) and high pressures to overcome electrostatic repulsion between positive nuclei. Fission produces radioactive waste (fission fragments) that must be stored safely for thousands of years. Fusion produces helium, which is harmless and not radioactive, so it produces very little waste. Both release large amounts of energy, but only fission is currently used in power stations.
Mark scheme: 1 mark for fission = splitting heavy nucleus; 1 mark for fusion = joining light nuclei; 1 mark for fission conditions (slow neutrons, chain reaction, control rods); 1 mark for fusion conditions (very high temperature and pressure); 1 mark for fission waste (radioactive, long storage); 1 mark for fusion waste (helium, minimal). (6 marks total)
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
The table shows data for two energy sources: a fission reactor and an experimental fusion reactor (ITER).
| Property | Fission reactor | Fusion reactor (ITER) |
|---|---|---|
| Fuel | Uranium-235 | Deuterium + Tritium |
| Energy per reaction (MeV) | ~200 | ~17.6 |
| Energy per kg of fuel (J) | 8.2 × 10¹³ | 3.4 × 10¹⁴ |
| Radioactive waste | High-level waste, stored 1000+ years | Low-level only, decays in ~100 years |
| Currently operational? | Yes (440 worldwide) | No (still experimental) |
(a) Calculate how many times more energy per kg fusion produces compared to fission.
(b) Explain why fusion produces more energy per kg despite releasing less energy per individual reaction.
(c) Evaluate, using data from the table, whether investment in fusion research is justified despite fission already being operational.
Answers: (a) 3.4 × 10¹⁴ / 8.2 × 10¹³ = 4.1 times more energy per kg. (b) Each fusion reaction releases less energy than fission, but the fuel nuclei (hydrogen) have much smaller mass than uranium nuclei. This means there are far more nuclei per kilogram of fuel, so the total energy per kg is greater. (c) Justified: fusion produces 4× more energy per kg of fuel; fuel (hydrogen from seawater) is virtually unlimited compared to limited uranium supplies; waste is far less dangerous (decays in ~100 years vs 1000+ years); fusion cannot undergo a runaway chain reaction so it is inherently safer. Against: enormous technical challenges; not yet proven to produce net energy; will take decades to become operational; expensive research. Overall, the long-term benefits of clean, safe, abundant energy justify the investment.
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