fission+and+fusion

Fission
The fission process occurs when the nucleus of an atom **splits** into two smaller nuclei. This will release energy and in some cases, subatomic particles such as neutrons.



A sustainable (or self-propagating) fission chain reaction occurs when there are enough "fissile" (fissionable) atoms in proximity (called //critical mass//).



This process occurs in nuclear fuel of power plants or atomic weapons.

Watch this brief video on fission:

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Fusion
Fusion occurs when two nuclei combine to form one larger nucleus.

In the example below, two isotopes of hydrogen - deuterium and tritium - undergo fusion to form helium, and release energy and a neutron.



Fusion is what powers the Sun, and many other stars. In the Sun, hydrogen is fused into helium in a complex nuclear process.



Fusion, as you can imagine, requires enormous temperatures and pressures (think, Sun). However, once initiated, it emits enormous amounts of energy. Scientists have yet to mimic a successful, self-propagating fusion reaction in the lab.

However, fusion is the primary means to make new elements. In particle colliders, atoms are smashed together at tremendous speeds. When they collide, their nuclei may fuse, forming large, unstable nuclei.