What is a fundamental difference between nuclear fission and fusion?

A fundamental difference between nuclear fission and fusion is that fission involves the splitting of heavy atomic nuclei into lighter nuclei, while fusion combines light atomic nuclei to form a heavier nucleus. In fission, an unstable nucleus breaks apart when it absorbs a neutron, releasing energy in the process, as well as more neutrons that can induce further fission in nearby nuclei. This process is commonly used in nuclear power plants and atomic bombs.

In contrast, fusion is the process that powers the stars, including our Sun, where light nuclei (such as hydrogen isotopes) collide at extremely high temperatures and pressures to form helium and release a significant amount of energy. This process requires immense conditions typically found in stellar environments.

The other choices do not accurately describe the key differences between fission and fusion, as they either misrepresent the energy output or the contexts in which these processes occur. For instance, while fusion does release more energy on a per-mass basis than fission, the processes do not release equal energy. Furthermore, fusion is not common in power plants yet, as harnessing it effectively for energy production remains a challenge, and while fission occurs in power plants, it is fusion that naturally occurs in stars.