Which force is responsible for the binding energy that releases in nuclear fission and fusion?

Nuclear fission and fusion both involve changes in how strongly nucleons are bound together. The strong nuclear force provides the binding energy that holds nuclei together, and when the binding arrangement changes, energy can be released. In fusion, light nuclei combine into a more tightly bound nucleus; in fission, a very heavy nucleus splits into parts with higher total binding energy per nucleon. This is the same basic physics behind the Sun and nuclear power plants. The underlying energy scale is enormous compared with chemical reactions.