explain why nuclear fission and nuclear fusion release large amounts of energy.

Brief Explanations:

Nuclear fission splits heavy nuclei, nuclear fusion combines light - nuclei. Both processes result in a mass defect. According to Einstein's $E = mc^{2}$, this mass defect is converted into energy. The strong nuclear force is involved in these reactions, and when the nucleus changes its state (either splitting or combining), a large amount of energy is released due to the high - energy nature of the strong nuclear force interactions.

Answer:

Nuclear fission and fusion release large amounts of energy because they involve mass defects that are converted to energy via $E = mc^{2}$, and strong nuclear force interactions change the state of the nucleus.