Answer 1

Nuclear fission
Nuclear fission is the process by which the heavy nucleus of a radioactive atom splits up into smaller nuclei when bombarded with low-energy neutrons.

Example:
When uranium-235 atoms are bombarded with slow-moving neutrons, the heavy uranium nucleus breaks up to produce two medium-weight atoms, barium-139 and krypton-94, with the emission of three neutrons. A tremendous amount of energy is produced in this process.
Reaction: 

`""_92^238U+""_0^1n"fission"/""> 56^(Ba^139)+""_36Kr^94+""3_0^1n+`Tremendous amount of energy