Answer:
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.