The nucleus of an atom of uranium 235 contains 92 protons and 143 neutrons. If we fire another neutron at this nucleus it becomes unstable - a bit like a wobbly jelly and canít hold itself together. In the end the nucleus splits in half and this is called NUCLEAR FISSION.
The word fission just means splitting.
The nucleus splits in half giving two smaller nuclei, called FISSION FRAGMENTS, and about three fast moving neutrons.