Fission

Fission is a process where a large nucleus such as ²³⁵U is caused to split into two smaller nuclei. The process is initiated by the capture of a slow (often called thermal) neutron.

²³⁵U + n --> ²³⁶U^*

(The * on the Uranium-236 indicates that it is in a highly excited state)

The new nucleus then splits into two daughter nuclei. This split can happen in a number of ways but one example might be:

²³⁶U^* --> ¹⁴⁰Xe+ ⁹⁴Sr + 2n

This process releases roughly 200MeV per event, plus two more neutrons. Unfortunately these neutrons are way too energetic to be captured by other nuclei, so in [nuclear reactors]? or [nuclear bombs]? they are slowed down by a moderator in order to produce a [chain reaction]?