Answer:
In nuclear fusion, nuclei combine. In nuclear fission, a nucleus breaks apart.
Explanation:
Nuclear fission is the process in which the nucleus of a heavy atom splits into two or more parts releasing energy. These released parts are atoms with lower mass than the initial atom. Neutrons may be released as well.
Nuclear fusion happens when an atom, say A, hits another atom, say B, and their nuclei are combined to form a heavier atom, call it C. This process releases energy as well. Neutrons may be released as well.
The diagrams are attached as images.
