How can fission and fusion release energy?
At first sight, it doesn’t make sense that both fission and fusion release energy.
The key is in how tightly the nucleons (protons and neutrons) are held together in a nucleus. If a nuclear reaction produces nuclei that are more tightly bound (i.e., at a lower energy) than the originals, then the excess energy will be released.
It turns out that the most tightly bound atomic nuclei are around the size of iron (56 nucleons).
Thus, if you split a nucleus that is much larger than iron (say, uranium with 238 nucleons) into smaller fragments, you will release energy because the smaller fragments are at a lower energy than the original nucleus.
If instead you fuse very light nuclei (say, helium with four nucleons) to get bigger products, energy is again released because the nucleons in the products are more tightly bound than in the original nuclei.
Nuclei lighter than iron release energy by fusion. Nuclei heavier than iron release energy by fission.