Definition · Plain-language
Nuclear energy
Nuclear energy is the energy stored within the nucleus of an atom, released when nuclei split or join.
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Energy locked in the nucleus
Nuclear energy is the energy held within the nucleus of an atom, where protons and neutrons are bound together by the immensely strong nuclear force. This binding stores energy, and rearranging the nucleus can release a portion of it. The amounts involved dwarf those from chemical reactions like burning, because the nuclear force is far stronger than the electrical forces between atoms. A small mass of nuclear fuel can release as much energy as many tonnes of coal, which is what makes nuclear power so energy-dense.
Fission and fusion
There are two ways to release nuclear energy. Fission splits a heavy nucleus, such as uranium-235, into lighter fragments, releasing energy and neutrons that can split further nuclei in a chain reaction. Fusion joins light nuclei, such as hydrogen isotopes, into a heavier one, releasing even more energy per unit mass — the process that powers the Sun. In both, the products have slightly less mass than the starting materials, and that missing mass becomes energy according to Einstein’s E = mc².
Generating electricity
Nuclear power stations harness controlled fission. Inside a reactor, a carefully managed chain reaction releases heat, which boils water into steam; the steam drives turbines connected to generators that produce electricity. The reaction is kept steady with control rods that absorb neutrons. Nuclear power generates large amounts of electricity with very low carbon emissions, but it produces radioactive waste that must be stored safely for a long time, and requires strict safety measures — the trade-offs at the centre of debates about its use.
Key facts
At a glance
- Definition: energy stored in the nucleus of an atom
- Held by: the strong nuclear force binding protons and neutrons
- Released by: fission (splitting) or fusion (joining) of nuclei
- Energy source: a small mass loss converted to energy via E = mc²
- Power use: stations use controlled fission to make steam and electricity
- Trade-offs: low-carbon and energy-dense, but produces radioactive waste
Common misconceptions
What people often get wrong
Often heard: Nuclear energy and chemical energy are basically the same.
Actually: They are very different. Chemical energy comes from rearranging electrons between atoms; nuclear energy comes from changes in the atomic nucleus and is millions of times more concentrated per unit of fuel.
Often heard: All nuclear power comes from fusion like the Sun.
Actually: Today’s nuclear power stations use fission, splitting heavy atoms. Fusion, the Sun’s process, is not yet a practical source of electricity on Earth and remains experimental.
Often heard: A nuclear power station can explode like a nuclear bomb.
Actually: The fuel in a power reactor is far too low in enrichment to detonate like a weapon. Reactor accidents are serious but are not nuclear explosions; the physics of the two is quite different.
Going deeper
