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Nuclear power plants and the Sun both get their energy from atomic nuclei — but in opposite ways. A reactor splits large nuclei apart. The Sun fuses small nuclei together. How can splitting and joining both release energy? The answer lies in one of the most important graphs in physics.
Fission: splitting heavy nuclei
Nuclear fission is the splitting of a heavy nucleus into two smaller ones.
The classic example: a slow neutron is absorbed by a uranium-235 nucleus. The resulting uranium-236 is unstable and splits, for example:
²³⁵U + n → ¹⁴¹Ba + ⁹²Kr + 3n + energy
(There are many possible pairs of fragments; barium and krypton are one common outcome.)
Each fission releases about 200 MeV of energy — tens of millions of times more than a typical chemical reaction per atom — plus two or three fast neutrons.
Fission was discovered in 1938 by Otto Hahn and Fritz Strassmann in Berlin, and explained physically by Lise Meitner and Otto Frisch early in 1939.
Chain reactions
Those extra neutrons can go on to split more uranium nuclei, each releasing more neutrons. That’s a chain reaction.
- In a nuclear reactor, control rods absorb excess neutrons so that, on average, exactly one neutron from each fission causes another. The reaction runs steadily, heating water to drive turbines.
- In a nuclear weapon, the chain reaction is allowed to grow uncontrollably in a fraction of a second.
Natural uranium is only about 0.7% uranium-235; most reactors need fuel enriched to a few percent.
Fusion: joining light nuclei
Nuclear fusion joins two light nuclei to form a heavier one.
In the core of the Sun, at around 15 million °C, hydrogen nuclei fuse step by step into helium. Overall:
4 ¹H → ⁴He + 2 positrons + 2 neutrinos + energy
Every second the Sun converts about 4 million tonnes of mass into energy this way.
On Earth, fusion research focuses on the easiest reaction, between two heavy isotopes of hydrogen, deuterium and tritium:
²H + ³H → ⁴He + n + 17.6 MeV
Hydrogen bombs use fusion too, triggered by a fission explosion.
Why both release energy: the binding energy curve
The energy comes from nuclear binding energy — the energy that holds protons and neutrons together. If you plot binding energy per nucleon against mass number, the graph rises steeply from hydrogen, peaks around iron-56 and nickel-62, then slowly declines for heavier nuclei.
- Light nuclei are to the left of the peak. Fusing them moves towards the peak → nucleons become more tightly bound → energy is released.
- Heavy nuclei are to the right of the peak. Splitting them also moves towards the peak → energy is released.
The released energy shows up as a tiny loss of mass, through Einstein’s E = mc². The products weigh very slightly less than the reactants.
This same curve explains why stars can only fuse elements up to iron — see the most abundant elements in the universe.
Side by side
| Fission | Fusion | |
|---|---|---|
| What happens | Heavy nucleus splits | Light nuclei join |
| Typical fuel | Uranium-235, plutonium-239 | Deuterium, tritium |
| Conditions | Neutrons, critical mass | Extreme temperature and pressure (~100 million °C on Earth) |
| Energy per kg of fuel | Very high | About 4 times higher than fission |
| Waste | Long-lived radioactive fission products | Mainly short-lived activation of reactor materials; helium |
| Status for power | Commercial since the 1950s | Experimental |
Why fusion power is so hard
Nuclei repel each other strongly, so fusion needs temperatures above 100 million °C — hotter than the Sun’s core, because we can’t use the Sun’s enormous gravity to squeeze the fuel. At those temperatures fuel is a plasma that must be held away from any material wall, using powerful magnetic fields (tokamaks and stellarators) or compressed in a flash by lasers (inertial confinement).
In December 2022, the National Ignition Facility in the USA reported the first laboratory fusion experiment to release more energy from fusion than the laser energy delivered to the fuel — a milestone, though still far from producing net electricity for the grid. Large projects such as ITER in France are designed to demonstrate sustained fusion on a power-plant scale.
Quick answers
Which releases more energy? Per kilogram of fuel, fusion — several times more than fission.
Which is used in nuclear power plants? Fission. Fusion power plants don’t exist yet.
Is fusion radioactive? Deuterium–tritium fusion produces neutrons that make reactor materials radioactive, and tritium is radioactive, but it doesn’t produce the long-lived high-level waste that fission does.
Related
Radioactive decay, the third way nuclei change, is covered in half-life explained and alpha, beta and gamma radiation.
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