How Nuclear Energy Is Released

Energy stored in the nucleus

At the center of every atom is a nucleus made of protons and neutrons, held together by the strong nuclear force. This force is powerful at very short distances, strong enough to overcome the electrical repulsion between the positively charged protons. Details of these particles are covered in the article on subatomic particles.

Different nuclei are bound with different degrees of tightness. Very heavy nuclei, such as uranium, and very light ones, such as hydrogen, are less tightly bound per particle than medium-sized nuclei such as those near iron. Nuclear reactions that move a nucleus toward the more tightly bound middle release energy. That is the basis of both fission and fusion.

Where the energy comes from

When a nucleus is split or joined, the total mass of the products turns out to be slightly less than the mass of the starting material. The difference is not lost. It is released as energy, in line with the relationship E = mc² from Albert Einstein’s special relativity. Because the speed of light is so large, even a very small amount of mass corresponds to a great deal of energy. This is why nuclear reactions release millions of times more energy per atom than chemical reactions, which only rearrange the outer electrons of atoms.

Nuclear fission

Fission is the splitting of a heavy nucleus into smaller pieces. In a typical reactor, a uranium-235 nucleus absorbs a neutron, becomes unstable and splits into two smaller nuclei, releasing energy and a few more neutrons. Uranium-235 is one of the atomic isotopes of uranium; the other main isotope, uranium-238, is much more common but does not sustain a chain reaction in the same way.

The released neutrons can strike other uranium-235 nuclei and split them, which release more neutrons, and so on. This is a chain reaction. In a nuclear power plant it is carefully controlled:

• Control rods that absorb neutrons are moved in or out to adjust the rate of reaction.
• A moderator, often water, slows neutrons so they are more likely to cause fission.
• The heat produced boils water, and the steam turns a turbine to generate electricity, the same final step used in many fossil fuel plants.

In a nuclear weapon, by contrast, the chain reaction is designed to grow extremely rapidly and without control. A power reactor cannot explode like a weapon, because its fuel is not arranged or enriched that way, though accidents of other kinds can still occur.

Fission has well-known trade-offs. It produces electricity with no direct carbon dioxide emissions while operating, and it provides steady power. It also produces radioactive waste that must be managed safely for a long time, and serious accidents, though rare, have had lasting consequences. Views on its role differ, and the question is part of the wider discussion of energy sources.

Nuclear fusion

Fusion is the joining of light nuclei into a heavier one. The Sun and other stars shine mainly by fusing hydrogen into helium in their cores. In the Sun, the core temperature is around 15 million degrees Celsius, and the pressure from gravity is enormous. How stars evolve over time is described in the stellar life cycle.

For fusion to happen, nuclei must get close enough for the strong force to take over, despite their mutual electrical repulsion. That requires either extremely high temperatures, at which matter becomes a hot, electrically charged gas called plasma, or very high pressure. On Earth, researchers are trying to achieve it with strong magnetic fields that hold hot plasma, or with powerful lasers that compress tiny fuel pellets. Experiments have produced fusion reactions, but making a device that reliably yields more usable energy than it takes to run, and doing so continuously and economically, has not yet been achieved on a commercial scale.

If it can be done, fusion fuel such as forms of hydrogen is plentiful, and the waste is generally considered less long-lived than that from fission. These are hopes rather than guarantees, and timelines are uncertain.

Fission versus fusion

A related phenomenon

Some nuclei are unstable and change by themselves, giving off radiation. This natural radioactive decay was investigated by Marie Curie and others in the early twentieth century. It is a separate process from fission and fusion, though it also involves energy released from the nucleus.