An interactive exploration of nuclear energy
This module explores the powerful nuclear processes of fission and fusion. Through interactive simulations, you'll discover how these reactions release vast amounts of energy, the principles behind nuclear power, and the energy source of stars.
Use the navigation bar above to jump between sections. Each section includes interactive simulations — click the Play buttons to watch them in action!
Inside every atom there is a nucleus held together by incredibly powerful forces. When a nucleus is split apart (fission) or when small nuclei are joined together (fusion), some of that binding energy is released. Even radioactive decay (when an unstable nucleus emits particles on its own) releases energy.
The key idea is simple: nuclear reactions release far more energy than chemical reactions like burning fuels. Let's see just how much more!
Click the buttons below to compare the energy from one single nuclear reaction to everyday things you already know about:
A tiny piece of nuclear fuel the size of your fingertip contains as much energy as roughly 1 tonne (1,000 kg) of coal. That's why nuclear reactions are so powerful — a small amount of fuel produces an enormous amount of energy!
Nuclear fission occurs when a heavy nucleus like Uranium-235 absorbs a neutron, becomes unstable, and splits into two smaller daughter nuclei (e.g., Krypton and Barium), releasing 2–3 additional neutrons and a large amount of kinetic energy.
n + 235U → 92Kr + 141Ba + 3n + Energy (~200 MeV)
A chain reaction occurs when neutrons released from one fission event cause further fissions in nearby U-235 nuclei. If at least one neutron per fission causes another fission, the reaction is self-sustaining.
Control rods (made of boron or cadmium) absorb neutrons to regulate the fission rate. Moderators (e.g., water, graphite) slow neutrons to increase fission probability. Shielding (thick concrete and steel) protects people from radiation.
Nuclear fusion combines light nuclei (like Deuterium and Tritium) to form a heavier nucleus (Helium-4), releasing even more energy per unit mass than fission. This is the process that powers the Sun and all stars.
2H + 3H → 4He + n + Energy (~17.6 MeV)
Fusion requires extreme temperatures (millions of °C) so nuclei have enough kinetic energy to overcome their electrostatic repulsion. Adjust the temperature below to see the effect.
| Feature | Fission | Fusion |
|---|---|---|
| Process | Splitting a heavy nucleus | Combining light nuclei |
| Fuel | Uranium-235, Plutonium-239 | Deuterium, Tritium (hydrogen isotopes) |
| Energy / Reaction | ~200 MeV | ~17.6 MeV (but far more per kg of fuel) |
| Conditions | Neutron bombardment + critical mass | Millions of °C temperature + immense pressure |
| Byproducts | Long-lived radioactive waste | Helium + neutrons (minimal long-lived waste) |
| In nature | Extremely rare (e.g., Oklo reactor) | Powers all stars |
| Technology | Established (nuclear power plants) | Experimental (ITER, NIF) |
This curve explains why both fission (of heavy nuclei) and fusion (of light nuclei) release energy. Elements near Iron-56 sit at the peak — any reaction that moves nuclei toward iron releases energy.
1. Which process involves splitting a heavy nucleus like U-235?
2. What is the primary role of a moderator in a fission reactor?
3. What are the products of U-235 fission?
4. Why does fusion require extremely high temperatures?
5. What powers the Sun?
6. What do control rods in a nuclear reactor do?
7. On the Binding Energy per Nucleon curve, which element sits near the peak?