Answer:
Nuclear fusion is the process in which multiple atomic nuclei merge together to create a single heavier nucleus, whereas nuclear fission is the exact opposite. It is the process of atomic nuclei spitting into smaller and lighter atoms.
Nuclear fusion is the process of squeezing two lighter atoms together to make heavier atoms; nuclear fission is the process of splitting heavier atoms into lighter ones. In both processes, some of the mass of the original atoms are converted into energy; fusion tends to convert more mass into energy than fission does, so fusion tends to create more energy. Heavier atoms needed for a fission chain reaction tend to be unstable and radioactive, and thus the fission process tends to produce more radioactivity.