Answer- Answer for this is nucleus.
Explanation- Reaction called fission reaction is done to split the atom. When an atom splits due to this reaction a lot of energy is generated from the nucleus of the atom.
The nucleus contains protons and neutrons and when the atom is bombarded with particles the electrons, neutrons, and protons present in an atom split and the nucleus of the atom produces a large amount of energy that is in the form of heat. Some more particles are formed during this process which needs to be disposed properly.