What you'll learn
This revision guide covers everything you need to know about nuclear energy for your WJEC GCSE Physics examination. You will learn about the structure of atoms, how nuclear reactions release energy, and the difference between fission and fusion processes. These topics appear regularly in both Foundation and Higher tier papers, particularly in extended response and calculation questions.
Key terms and definitions
Nuclear fission — the splitting of a large, unstable atomic nucleus (such as uranium-235 or plutonium-239) into two smaller nuclei, releasing energy, neutrons and gamma radiation.
Nuclear fusion — the joining together of two light atomic nuclei (such as hydrogen isotopes) to form a heavier nucleus, releasing energy in the process.
Chain reaction — a self-sustaining sequence of nuclear fission reactions where neutrons released from one fission event go on to cause further fission events.
Control rods — materials (usually boron or cadmium) that absorb neutrons in a nuclear reactor to control the rate of the chain reaction.
Mass defect — the difference between the mass of a nucleus and the sum of the masses of its individual nucleons (protons and neutrons).
Binding energy — the energy required to separate all the nucleons in a nucleus, or the energy released when a nucleus forms from individual nucleons.
Moderator — a material (often water or graphite) used in nuclear reactors to slow down fast-moving neutrons so they can cause further fission.
Critical mass — the minimum mass of fissile material needed to maintain a self-sustaining chain reaction.
Core concepts
The nucleus and nuclear stability
The atomic nucleus contains protons and neutrons (collectively called nucleons). Protons are positively charged and should repel each other due to electrostatic forces, but the strong nuclear force holds the nucleus together. This force:
- Acts over very short distances (approximately 10⁻¹⁵ m)
- Is attractive between nucleons at typical nuclear separations
- Overcomes the electrostatic repulsion between protons
- Does not depend on electric charge
Most nuclei are stable, but some are unstable and undergo radioactive decay. Very large nuclei (atomic number greater than 82) tend to be unstable because the strong nuclear force cannot hold all the nucleons together effectively across the larger nuclear diameter.
The stability of a nucleus depends on the ratio of neutrons to protons. For lighter elements, a 1:1 ratio provides stability, but heavier stable nuclei require more neutrons than protons to counteract the increasing electrostatic repulsion.
Nuclear fission in detail
Nuclear fission occurs when a large, unstable nucleus absorbs a neutron and splits into two smaller nuclei of roughly similar size. The process releases:
- Two or three fast-moving neutrons
- Energy (primarily as kinetic energy of the fission products)
- Gamma radiation
The most commonly used fissile materials are:
- Uranium-235 (U-235) — naturally occurring but only 0.7% of natural uranium
- Plutonium-239 (Pu-239) — manufactured in nuclear reactors
A typical fission equation for uranium-235:
²³⁵U + ¹n → ¹⁴⁴Ba + ⁸⁹Kr + 3¹n + energy
The fission products (barium and krypton in this example) vary, and many different combinations are possible. Each fission event releases approximately 200 MeV of energy.
Chain reactions and control:
When uranium-235 undergoes fission, it releases 2-3 neutrons. If at least one of these neutrons causes another fission event, a chain reaction occurs. In an uncontrolled chain reaction (atomic bomb), the reaction rate increases exponentially. In a controlled chain reaction (nuclear power station), exactly one neutron from each fission goes on to cause another fission, maintaining a steady energy output.
Nuclear reactors control the chain reaction using:
- Control rods — lowered into the reactor to absorb excess neutrons and slow the reaction; raised to allow more fissions
- Moderator — slows down fast neutrons to thermal speeds, making them more likely to cause fission in U-235
- Coolant — removes heat energy from the reactor core (often the same substance as the moderator, such as water)
Nuclear fusion in detail
Nuclear fusion joins light nuclei together to form heavier nuclei. This process powers the Sun and other stars. Common fusion reactions involve isotopes of hydrogen:
- Deuterium (²H) — hydrogen with one neutron
- Tritium (³H) — hydrogen with two neutrons
A typical fusion equation:
²H + ³H → ⁴He + ¹n + energy
Each fusion event releases approximately 17 MeV, which is less than fission per reaction, but fusion fuel is much more abundant and produces less radioactive waste.
Conditions required for fusion:
Fusion requires extremely high temperatures (tens of millions of degrees Celsius) and pressures because:
- Nuclei are positively charged and repel each other (electrostatic repulsion)
- High kinetic energy is needed to overcome this repulsion and bring nuclei close enough for the strong nuclear force to act
- High pressure increases the frequency of collisions between nuclei
These conditions exist naturally in stars, where gravitational pressure provides the necessary conditions. On Earth, achieving controlled fusion for power generation remains technically challenging. Current experimental reactors like JET (Joint European Torus) use magnetic fields to contain the hot plasma.
Mass-energy equivalence and E=mc²
Einstein's equation E = mc² describes the relationship between mass and energy:
- E = energy (joules, J)
- m = mass (kilograms, kg)
- c = speed of light in a vacuum (3.0 × 10⁸ m/s)
In nuclear reactions, a small amount of mass is converted into a large amount of energy. The mass of the products is slightly less than the mass of the reactants — this "missing" mass has been converted to energy.
When nucleons join to form a nucleus, the mass of the nucleus is less than the total mass of the separate nucleons. This mass defect corresponds to the binding energy — the energy that would be needed to separate the nucleus back into individual nucleons.
Nuclei with higher binding energy per nucleon are more stable. Iron-56 has the highest binding energy per nucleon, which explains why:
- Fission of heavy nuclei (uranium, plutonium) releases energy — products are more stable
- Fusion of light nuclei (hydrogen isotopes) releases energy — products are more stable
- Elements heavier than iron cannot release energy through fusion
- Elements lighter than iron cannot release energy through fission
Energy released in nuclear reactions
Nuclear reactions release approximately one million times more energy per kilogram than chemical reactions (such as burning fossil fuels). This makes nuclear energy extremely energy-dense.
For uranium-235 fission:
- 1 kg of U-235 can release approximately 8.0 × 10¹³ J of energy
- This is equivalent to burning approximately 2,500,000 kg (2,500 tonnes) of coal
The energy released in nuclear reactions appears mainly as:
- Kinetic energy of the products (fission fragments or fusion products)
- Kinetic energy of released neutrons
- Electromagnetic radiation (gamma rays)
In a nuclear power station, this energy heats water to produce steam, which drives turbines connected to generators. The energy conversion chain is:
Nuclear energy → thermal energy → kinetic energy → electrical energy
Advantages and disadvantages of nuclear power
Advantages:
- No carbon dioxide emissions during operation (important for climate change mitigation)
- Very high energy density — small amounts of fuel produce large amounts of energy
- Reliable baseload electricity supply (not dependent on weather)
- Existing uranium reserves can last for many decades
- Lower fuel costs compared to fossil fuels per unit of energy produced
Disadvantages:
- Produces radioactive waste that remains hazardous for thousands of years
- High initial construction costs for power stations
- Risk of accidents with potentially catastrophic consequences (Chernobyl, Fukushima)
- Thermal pollution of water used for cooling
- Decommissioning old nuclear power stations is expensive and time-consuming
- Possible target for terrorism
- Limited uranium supplies (though more abundant than oil or gas)
- Public concerns about safety
Worked examples
Example 1: Identifying nuclear equations (Foundation/Higher)
Question: Complete the following nuclear fission equation:
²³⁵U + ¹n → ⁹²Kr + ?Ba + 3¹n
Determine the mass number and atomic number of the barium isotope.
Solution:
Mass numbers must balance: 235 + 1 = 92 + A + 3(1) 236 = 95 + A A = 141
Atomic numbers must balance: 92 + 0 = 36 + Z + 3(0) 92 = 36 + Z Z = 56
Answer: ¹⁴¹Ba (barium-141) [2 marks: 1 for mass number, 1 for atomic number]
Example 2: Energy calculations (Higher)
Question: In a nuclear fusion reaction, 0.025 kg of mass is converted to energy. Calculate the energy released. (Speed of light c = 3.0 × 10⁸ m/s)
Solution:
Use E = mc²
E = 0.025 × (3.0 × 10⁸)² E = 0.025 × 9.0 × 10¹⁶ E = 2.25 × 10¹⁵ J
Answer: 2.25 × 10¹⁵ J (or 2.3 × 10¹⁵ J to 2 s.f.) [3 marks: 1 for correct formula, 1 for correct substitution, 1 for correct answer with unit]
Example 3: Extended response on chain reactions (Higher)
Question: Explain how a chain reaction is controlled in a nuclear reactor. [6 marks]
Mark scheme points:
- Nuclear fission releases 2-3 neutrons [1]
- These neutrons can cause further fission events [1]
- Control rods absorb neutrons [1]
- Control rods are made of boron or cadmium [1]
- Lowering control rods slows the reaction / raising them speeds it up [1]
- Moderator slows down fast neutrons / increases probability of further fission [1]
- For steady power output, exactly one neutron from each fission causes another fission [1]
Quality of written communication also assessed (maximum 6 marks available from 7 possible points)
Common mistakes and how to avoid them
Confusing fission and fusion — Remember: fission = splitting (large → small), fusion = joining (small → large). Think of "fuse together" for fusion.
Forgetting to balance nuclear equations — Both mass numbers (top) and atomic numbers (bottom) must balance on both sides of the equation. Always check both.
Mixing up control rods and moderators — Control rods absorb neutrons to control the reaction rate; moderators slow neutrons down to make fission more likely. They have different functions.
Incorrect units in E=mc² calculations — Mass must be in kilograms (not grams), and the answer will be in joules. Always convert grams to kilograms first.
Stating fusion is easy to achieve on Earth — Fusion requires extreme temperatures (millions of degrees) and is currently not used for commercial power generation. Don't confuse experimental research with working power stations.
Claiming nuclear power produces no waste — Nuclear fission produces highly radioactive waste that remains dangerous for thousands of years. The advantage is no CO₂ emissions, not no waste at all.
Exam technique for "Nuclear Energy"
Command words matter: "State" requires a simple fact; "Explain" needs a reason; "Describe" needs a sequence or process; "Compare" requires similarities and differences. Extended response questions worth 6 marks need well-structured answers covering multiple points.
Show your working in calculations — Even if your final answer is wrong, you can earn method marks for correct formulas and substitutions. Write E = mc² before substituting values.
Use correct scientific terminology — Write "nucleons," "fission products," and "binding energy" rather than vague terms like "bits of atom" or "nuclear stuff." Examiners award marks for precise language.
Balance equations carefully — In nuclear equations, spend time checking that mass numbers and atomic numbers balance. If they don't balance, you'll lose marks even if your physics understanding is correct.
Quick revision summary
Nuclear fission splits heavy nuclei (U-235, Pu-239), releasing energy, neutrons and radiation. Chain reactions are controlled using control rods and moderators in nuclear reactors. Nuclear fusion joins light nuclei (hydrogen isotopes), requiring extremely high temperatures. Both processes convert mass to energy according to E=mc². The binding energy per nucleon explains why fission of heavy elements and fusion of light elements both release energy. Nuclear power produces no CO₂ but creates long-lived radioactive waste. Energy released from nuclear reactions is approximately one million times greater than chemical reactions per kilogram of fuel.