Fission and fusion are two ways to release energy from atomic nuclei via nuclear reaction. The difference between them is in the process: One melds atoms with smaller nuclei together by fusing them while the other breaks them apart into fission products. In either case, the amount of energy involved is so large, millions of times more than from other energy sources, that these nuclear processes only happens in specific conditions.

What Is Nuclear Fusion?

As a verb, fuse is synonymous with "combine" or "blend." It follows that in a nuclear fusion process, two light nuclei fuse together to form a heavier nucleus. For example, two hydrogen atoms can fuse together to form one deuterium.

Tremendously high energy, usually in the form of extreme heat creating very high temperatures, and pressure is required to coax two strongly positive nuclei that would normally repel into a close enough space for fusion to occur, releasing nuclear energy in the process.

As a result, this process only happens inside stars like the sun which have a natural fusion reactor in their cores. Humanity can temporarily create the conditions for nuclear fusion, for instance with a hydrogen bomb, but sustaining such high temperatures necessary for a controlled, ongoing reaction to use as an energy source is not yet possible.

Once nuclear fusion begins, however, it can continue in a self-sustaining chain reaction. This is because the smaller atoms with masses up to that of iron on the periodic table give off more energy when fused than is required to fuse them together (an exothermic reaction). As such, nuclear fusion is the process by which most stars give off energy.

What Is Nuclear Fission?

Fission, which can be defined as the act of splitting something into parts, is the opposite of fusion.

In nuclear fission, a heavy nucleus breaks apart into lighter nuclei. The breakage occurs when a neutron slams into a heavy nucleus, creating very radioactive and unstable byproducts, along with more neutrons, which continue to break down in a nuclear chain reaction.

The energy released from nuclear fission is millions of times more efficient than that released from burning an equivalent mass of coal. Unlike fusion reactions, fission reactions are relatively easy to initiate and control inside nuclear reactors, making them a widespread source of energy.

Examples of Fission and Fusion

• Nuclear reactors: Engineers typically use plutonium or uranium to begin a fission reaction, controlling the rate with water and rods of non-reactive material that absorb free neutrons. The energy released in the fission reactions heats water, and the resulting steam turns turbines that generate electricity for human use.

• Atomic bombs: Nuclear fission reactions occur in atomic bombs. Unlike in a nuclear power plant, the reaction is not controlled, allowing for a rapid chain reaction that results in incredible energies being released at once. The only way humans on Earth can create the conditions necessary for fusion, the right temperature with enough mass smashed together at a high enough pressure, is by initiating fission with a bomb.

• Radioactive decay: Nuclear fission also occurs in radioactive decay, when an element spontaneously emits energy in the form of particles. The half-life ofradioactive decay, or the time for half of the radioactive nuclei in asample to break down, depends on the overall stability of the nucleus.Naturally occurring radioactive material on Earth constantly undergoesfission reactions in this way.

• The core of stars: Nuclear fusion reactions occur naturally under the intense temperature and pressure inside a star. This is the basis of most energy that stars give off.

• Cold fusion: A hypothetical way to create nuclear fusion at "room temperatures," thus making it a viable human-made energy source, cold fusion has never been successfully developed.