There are two types of nuclear reactions that are very prevalent in today's society: fission and fusion.
What are these reactions and how are they used?
Nuclear fission is a reaction where a molecule splits into smaller molecules and excess subatomic particles, and releases energy. This type of reaction happens in nuclear bombs and in nuclear power plants. In nuclear bombs, this reaction is set off by a neutron hitting a nucleus, making it unstable and causing fission. In a bomb, this happens in the vicinity of an amount of a particle, called critical mass, where the reaction becomes a chain reaction, causing an explosion.
In a power plant, the same reaction occurs, but below critical mass, so the power plant doesn't explode. Instead of the reactions energy causing an explosion, it heats water, which turns into steam and turns turbines, providing power to municipalities. Power plants also have safeguards to slow down a fission reaction in case it becomes too fast.
Nuclear fusion is a reaction where two molecules are put together which creates a bigger molecule and excess subatomic particles and releases energy. Nuclear fusion happens on a large scale on the surface of stars, and fusion bombs have been detonated. Fusion bombs operate on many of the same principles as fission bombs, such as needing critical mass to become a chain reaction. Interestingly enough, modern fusion bombs need an initial fission reaction to get enough energy to sustain a fusion reaction for and explosion.
The power producing applications of fusion energy are the next frontier of nuclear energy. The current challenge is inducing a safe fusion reaction and producing more energy than is used in initializing the reaction. This is the first step in being able to harness fusion as a power source, and once fusion becomes a viable energy source, the supply of energy becomes extremely large due to the abundance of molecules to use as fuel.