Which produces more energy, nuclear fusion or nuclear fission?
It depends on the atomic number of the element, fusion produces much more energy for lighter elements and fission produces more energy for heavier elements.
The key to understanding this is called binding energy. When protons and neutrons are bound together by the strong force, the combined mass of the protons and neutrons in a nucleus is always less than the combined mass of the same number of free protons and neutrons.
When two light nuclei are fused together to produce a heavier nucleus, the mass difference is converted into energy. This means that the same amount of energy needs to be added to cause the nucleus to break apart.
The problem is that fusing Hydrogen into Helium releases a large amount of energy. Fusing heavier elements releases less energy.
Nuclei of Iron and heavier elements require additional energy to fuse and fusion reactions require energy rather than produce it.
Elements heavier than Iron produce more energy through fission. Fission is when a large nucleus divides into smaller nuclei releasing energy.
Certain elements such as Uranium-235 can be made to fission easily.
So, the answer is fusion produces more energy if it involves light elements and fission produces more energy if it involves heavy elements.