P14: Nuclear Fission and Fusion
The processes of nuclear fission and fusion, how nuclear power stations work, chain reactions, and the advantages and disadvantages of nuclear energy.
Nuclear Fission
Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei, along with two or three neutrons and energy. It is the process used in nuclear power stations and nuclear weapons.
The Fission Process
For fission to occur, a large unstable nucleus (usually uranium-235 or plutonium-239) absorbs a neutron. This makes the nucleus even more unstable, causing it to split into two smaller nuclei (fission fragments), releasing energy and more neutrons.
The energy released during fission is far greater than the energy from chemical reactions. A single fission event releases millions of times more energy than burning a fossil fuel molecule.
Fission of Uranium-235
23592U + 10n → 14156Ba + 9236Kr + 310n + energy
A uranium-235 nucleus absorbs a neutron and splits into barium-141 and krypton-92, releasing three neutrons and a large amount of energy.
The products of fission are:
- Two smaller nuclei (fission fragments) — these are typically radioactive
- Two or three neutrons — these can cause further fissions
- Energy — mainly as kinetic energy of the fission fragments, converted to heat
Chain Reactions
The neutrons released in a fission event can go on to be absorbed by other uranium-235 nuclei, causing them to split and release more neutrons. This creates a chain reaction.
Each fission event can trigger multiple further fissions. The number of fissions can grow rapidly, releasing enormous amounts of energy in a very short time.
Controlling the Chain Reaction
In a nuclear power station, the chain reaction must be controlled so that it proceeds steadily — one fission leads to exactly one further fission on average.
- If too many neutrons cause fission: the reaction accelerates — this leads to a nuclear weapon (uncontrolled) or a meltdown
- If too few neutrons cause fission: the reaction dies out and the reactor shuts down
- Control rods (made of boron or cadmium) absorb excess neutrons to control the rate of fission
Control rods are lowered into the reactor core to absorb neutrons and slow the reaction. Raising them allows more neutrons to cause fission, increasing the reaction rate. The position of the control rods is adjusted continuously to maintain a steady power output.
Nuclear Fuels: Uranium and Plutonium
The two main fissile (can undergo fission) isotopes used in nuclear reactors are uranium-235 and plutonium-239.
Uranium-235
- Naturally occurring but only makes up about 0.7% of natural uranium (the rest is uranium-238, which is not fissile)
- Uranium ore is mined and enriched to increase the proportion of U-235 to around 3–5% for use in reactors
- Most commonly used fuel in nuclear power stations worldwide
Plutonium-239
- Not naturally occurring in significant quantities — produced in reactors when uranium-238 absorbs a neutron
- Used in some nuclear reactors and in nuclear weapons
- Created as a by-product in uranium-fuelled reactors and can be reprocessed for use as fuel
Nuclear Power Stations
A nuclear power station uses the energy from controlled nuclear fission to generate electricity. The heat from fission is used to produce steam, which drives a turbine connected to a generator.
How a Nuclear Power Station Works
- Reactor core: Fuel rods (containing uranium-235) undergo controlled fission, producing heat
- Control rods: Absorb neutrons to control the rate of fission
- Coolant: Water or carbon dioxide carries heat from the reactor to the heat exchanger
- Heat exchanger: Heat from the coolant is used to boil water into steam
- Turbine: Steam drives the turbine, which spins the generator
- Generator: Converts kinetic energy into electrical energy
- Condenser: Steam is cooled and condensed back into water to be reused
The only difference between a nuclear power station and a fossil fuel power station is how the water is heated. In a nuclear station, nuclear fission provides the heat. In a fossil fuel station, burning coal, oil, or gas provides the heat. Everything from the turbine onwards is the same.
Key Components
- Fuel rods: Contain enriched uranium dioxide pellets
- Control rods: Made of boron or cadmium; raised or lowered to control the reaction
- Moderator: Slows down fast neutrons so they can be more easily absorbed by uranium-235 (usually water or graphite)
- Containment: Thick concrete and steel shield to absorb radiation and prevent escape of radioactive material
Do not confuse the moderator (slows neutrons down) with the control rods (absorb neutrons to control the reaction rate). Both are essential for safe reactor operation but have different functions.
Nuclear Fusion
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, releasing energy. This is the process that powers the Sun and other stars.
The Fusion Process
Two light nuclei (typically isotopes of hydrogen) are forced together at extremely high temperatures and pressures. They overcome their electrostatic repulsion (both are positively charged) and fuse to form a heavier nucleus, releasing a large amount of energy.
Fusion of Hydrogen Isotopes (Deuterium-Tritium)
21H + 31H → 42He + 10n + energy
Deuterium and tritium fuse to form helium-4 and a neutron, releasing a large amount of energy. This is the most promising reaction for future fusion reactors on Earth.
Fusion in the Sun
In the Sun, hydrogen nuclei (protons) fuse through a series of reactions called the proton-proton chain. The net result is:
4 × 11H → 42He + 2 positrons + 2 neutrinos + energy
This process requires temperatures of about 15 million °C in the Sun's core.
Conditions Needed for Fusion
- Very high temperature: Over 100 million °C for deuterium-tritium fusion on Earth (even hotter than the Sun's core, because we cannot replicate the Sun's immense pressure)
- Very high pressure: To force nuclei close enough together to overcome electrostatic repulsion
- Containment: No material can withstand these temperatures, so magnetic fields (tokamak) or laser confinement are used
Fusion releases much more energy per unit mass than fission. The fuel (hydrogen isotopes) is abundant, and the products are not radioactive (helium is inert and harmless). However, achieving and maintaining the extreme conditions needed for fusion is extremely difficult.
Fusion Reactors
Scientists have been working for decades to achieve controlled nuclear fusion for energy production. The main approaches are:
Tokamak (Magnetic Confinement)
- A doughnut-shaped chamber uses powerful magnetic fields to confine a plasma of hydrogen isotopes
- The magnetic fields keep the extremely hot plasma away from the reactor walls
- The ITER project in France is the world's largest tokamak under construction
- So far, no fusion reactor has produced more energy than it consumes (net energy gain has only recently been demonstrated in laboratory conditions)
Inertial Confinement (Laser Fusion)
- Powerful lasers are fired at a tiny pellet of deuterium and tritium
- The lasers compress and heat the pellet to fusion conditions
- The National Ignition Facility in the USA achieved net energy gain using this method in 2022
Fusion power is not yet a practical energy source. The main challenges are: achieving and maintaining the extreme temperatures and pressures, confining the plasma, and producing more energy than is needed to start and sustain the reaction. Commercial fusion power is still likely decades away.
Fission vs Fusion — Comparison
| Feature | Nuclear Fission | Nuclear Fusion |
|---|---|---|
| Process | Splitting of a heavy nucleus | Joining of light nuclei |
| Fuel | Uranium-235, plutonium-239 | Hydrogen isotopes (deuterium, tritium) |
| Fuel availability | Limited supply of uranium | Abundant (deuterium from seawater) |
| Products | Radioactive waste (fission fragments) | Helium (harmless, inert) |
| Energy released per reaction | Large | Even larger (per unit mass) |
| Conditions needed | Relatively easy to achieve | Extremely high temperature and pressure |
| Current use | Used in power stations worldwide | Not yet viable for power generation |
| Chain reaction | Yes (must be controlled) | No chain reaction |
| Radiation risk | High — radioactive waste and potential meltdown | Low — minimal radioactive waste |
| Nuclear weapons | Fission bombs (atomic bombs) | Fusion bombs (thermonuclear / hydrogen bombs) |
Advantages and Disadvantages of Nuclear Power
Advantages
- Produces no carbon dioxide during operation — does not contribute to climate change
- Very energy-dense fuel — a small amount of uranium produces a huge amount of energy
- Reliable — can generate electricity continuously (unlike wind or solar)
- High power output from a single station
- Fuel supply is sufficient for many decades
Disadvantages
- Produces radioactive waste that remains dangerous for thousands of years and must be stored safely
- Risk of accidents — meltdown can release large amounts of radiation (e.g. Chernobyl, Fukushima)
- Decommissioning is expensive and takes decades
- Uranium mining has environmental impact
- Non-renewable — uranium supplies are finite
- Potential for nuclear proliferation — technology can be used to make weapons
- Large startup costs and long construction times
When evaluating nuclear power, always consider both sides. A common exam question asks you to compare nuclear power with fossil fuels or renewable energy. Remember that nuclear power does not produce CO₂ but does produce radioactive waste.
Nuclear Waste
Radioactive waste from nuclear fission is categorised by its activity level. All waste must be stored safely to prevent harm to humans and the environment.
Categories of Nuclear Waste
- Low-level waste: Contaminated clothing, tools, filters. Disposed of in sealed containers in shallow landfill sites. Decays relatively quickly.
- Intermediate-level waste: Reactor components, chemical sludge. Encased in cement or bitumen inside steel drums and stored in purpose-built facilities.
- High-level waste: Spent fuel rods and fission products. Very hot and highly radioactive. Stored in cooling ponds for years, then vitrified (mixed with glass) and sealed in steel canisters for long-term deep underground storage.
High-level waste remains radioactive for thousands of years. It must be stored in a way that ensures it will not leak or be accessed. Deep geological disposal (hundreds of metres underground in stable rock) is the preferred long-term solution.
The Future of Nuclear Energy
Nuclear energy is likely to play a role in reducing carbon emissions as countries move away from fossil fuels. Current developments include:
- Small Modular Reactors (SMRs): Smaller, cheaper, factory-built reactors that could be deployed more quickly
- Generation IV reactors: New designs that are safer, more efficient, and produce less waste
- Fusion power: If achieved, would provide nearly limitless clean energy with minimal waste, but remains technically very challenging
Both fission and fusion convert mass into energy, as described by Einstein's equation E = mc². In both processes, the total mass of the products is slightly less than the total mass of the reactants. The "lost" mass is converted to energy.
E = mc²
Where E = energy released (J), m = mass difference (kg), c = speed of light (3 × 10⁸ m/s)
Even a tiny mass difference produces an enormous amount of energy because c² is a very large number.
Worked Example: Mass-Energy Conversion in Fission
In a fission reaction, the total mass of the products is 0.001 kg less than the total mass of the reactants. Calculate the energy released.
E = mc² = 0.001 × (3 × 10⁸)² = 0.001 × 9 × 10¹⁶ = 9 × 10¹³ J
This is equivalent to the energy from burning thousands of tonnes of coal.
Practice Questions
1. Describe the process of nuclear fission and explain how a chain reaction is maintained in a nuclear reactor. [5 marks]
Nuclear fission is the splitting of a large, unstable nucleus (such as uranium-235) into two smaller nuclei. When a uranium-235 nucleus absorbs a neutron, it becomes unstable and splits, releasing two or three neutrons, energy, and two smaller nuclei (fission fragments). The neutrons released can be absorbed by other uranium-235 nuclei, causing them to split as well — this is a chain reaction. In a nuclear reactor, control rods (made of boron) are lowered into the core to absorb excess neutrons, ensuring that on average each fission event leads to exactly one further fission, maintaining a steady, controlled chain reaction.
2. Explain the role of the moderator and the control rods in a nuclear reactor. [4 marks]
The moderator (usually water or graphite) slows down fast-moving neutrons produced by fission. Slower (thermal) neutrons are more likely to be absorbed by uranium-235 nuclei, sustaining the chain reaction. Control rods (made of boron or cadmium) absorb neutrons. By raising or lowering the control rods, the number of neutrons available to cause further fission can be increased or decreased, controlling the rate of the chain reaction and therefore the power output of the reactor.
3. Compare nuclear fission and nuclear fusion. Include the fuel used, the products, and the energy released. [5 marks]
Nuclear fission splits heavy nuclei (uranium-235 or plutonium-239) into lighter nuclei, producing radioactive waste and two or three neutrons per fission. Nuclear fusion joins light nuclei (hydrogen isotopes — deuterium and tritium) to form a heavier nucleus (helium), with no long-lived radioactive waste. Both release energy due to a mass difference (E = mc²), but fusion releases more energy per unit mass of fuel than fission. Fission is currently used in power stations worldwide, while fusion requires extremely high temperatures and pressures that have not yet been sustained for practical energy production.
4. State two advantages and two disadvantages of nuclear power compared to fossil fuel power stations. [4 marks]
Advantages: Nuclear power does not produce carbon dioxide during operation (unlike fossil fuels), so it does not contribute to climate change. Nuclear fuel is very energy-dense, so a small amount of fuel produces a large amount of energy, and nuclear stations are reliable (can run continuously). Disadvantages: Nuclear power produces radioactive waste that must be stored safely for thousands of years. There is a risk of serious accidents (e.g. meltdown) that could release harmful radiation. Fossil fuel waste does not remain radioactive.
5. Explain why nuclear fusion is difficult to achieve on Earth and state one reason why it would be advantageous compared to fission. [3 marks]
Fusion requires extremely high temperatures (over 100 million °C) and pressures to overcome the electrostatic repulsion between positively charged hydrogen nuclei. No material can contain such hot plasma, so magnetic fields or lasers must be used for confinement, which is technically very challenging. An advantage of fusion over fission is that the fuel (deuterium from seawater) is virtually unlimited and the product (helium) is not radioactive, so fusion produces no long-lived radioactive waste.
Maths Skills
Energy from Mass: E = mc²
In both fission and fusion, the total mass of the products is slightly less than the total mass of the reactants. This missing mass (mass defect) is converted to energy according to Einstein's equation E = mc². Because c² is extremely large (9 × 10¹&sup6; m²/s²), even a tiny mass difference releases enormous energy.
Worked Example
In one fission event, 3.2 × 10−28 kg of mass is converted to energy. Calculate the energy released.
E = mc² = 3.2 × 10−28 × (3 × 10&sup8;)² = 3.2 × 10−28 × 9 × 10¹&sup6; = 2.88 × 10−11 J
Chain Reaction Mathematics
In a chain reaction, each fission event releases 2–3 neutrons. If every neutron causes another fission, the number of events grows exponentially. After n generations with an average of k neutrons per fission causing further fission, the number of fissions = kn. In a controlled reactor, k is maintained at approximately 1 (steady state). In a weapon, k > 1 leads to runaway growth.
Worked Example
If each fission releases on average 2 neutrons that go on to cause further fission, how many fission events occur after 10 generations starting from a single event?
Number of fissions = 210 = 1024 events after 10 generations.
Common Misconceptions
Fission vs Fusion in Power Stations
Nuclear fusion is how current nuclear power stations work. All current nuclear power stations use nuclear fission — the splitting of heavy nuclei like uranium-235. Nuclear fusion (joining light nuclei together) has not yet been achieved as a practical, sustained energy source on Earth. Fusion requires temperatures over 100 million °C and extreme pressures that are enormously difficult to contain and maintain.
Nuclear Waste
Nuclear waste is just the used fuel rods. Nuclear waste includes everything that has come into contact with radioactive material and become contaminated: protective clothing, tools, filters, cooling water, reactor components, and chemical processing materials. Even the reactor structure itself becomes radioactive over time due to neutron activation. All of this must be classified and disposed of according to its activity level.
6-Mark Extended Question
Compare nuclear fission and nuclear fusion. Explain why fusion is not yet used in power stations despite its potential advantages. [6 marks]
Nuclear fission splits heavy nuclei (e.g. uranium-235) into lighter fragments, releasing energy and neutrons that sustain a chain reaction (1). Nuclear fusion joins light nuclei (e.g. hydrogen isotopes) into a heavier nucleus, releasing even more energy per unit mass (1). Fission is currently used in power stations worldwide because it can be controlled at achievable temperatures and pressures using moderators and control rods (1). Fusion requires temperatures over 100 million °C and extreme pressures to overcome electrostatic repulsion between positive nuclei (1). No material can withstand these temperatures, so magnetic confinement (tokamak) or laser compression must be used, which is technically extremely challenging (1). Despite fusion's advantages — abundant fuel (seawater), no long-lived radioactive waste, and no risk of meltdown — we cannot yet sustain the reaction long enough to produce more energy than is required to start and contain it, making fusion power stations unviable at present (1).
AO3: Analyse and Evaluate
The table below shows the approximate energy released per kilogram of fuel for different energy sources:
| Energy source | Energy per kg of fuel (J) |
|---|---|
| Coal (burning) | 3 × 107 |
| Oil (burning) | 4 × 107 |
| Uranium-235 (fission) | 8 × 1013 |
| Deuterium-tritium (fusion) | 3 × 1014 |
(a) How many times more energy-dense is fission compared to coal? (b) How many times more energy-dense is fusion compared to fission? (c) A 1 GW coal power station burns 10 000 tonnes of coal per day. Estimate how much uranium-235 would provide the same energy.
Evaluation
(a) Fission / coal = 8 × 1013 / 3 × 107 = approximately 2.7 million times more energy-dense.
(b) Fusion / fission = 3 × 1014 / 8 × 1013 = approximately 3.75 times more energy-dense than fission.
(c) Energy from 10 000 tonnes of coal = 10 000 × 1000 × 3 × 107 = 3 × 1014 J. Mass of uranium = 3 × 1014 / 8 × 1013 = 3.75 kg. Just a few kilograms of uranium replaces 10 000 tonnes of coal, demonstrating the extraordinary energy density of nuclear fuel.
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