Fusion vs fission: what's the difference?

Fusion and fission are opposite ways of releasing energy from an atom's nucleus. Fission is a heavy atom splitting apart. Fusion is two light atoms being forced into a single heavier one. Both give energy out. Iron is the dividing line: elements lighter than iron release energy by joining, and heavier ones release it by splitting.

Both processes change the nucleus of an atom and both release energy. They run in opposite directions. In fission, a heavy nucleus such as uranium splits into lighter ones. In fusion, light nuclei such as hydrogen are forced into a heavier one. Iron is the dividing line. Uranium sits well above it and hydrogen well below, which is why one pays out by splitting and the other by joining. Fission is the one we have engineered, and it runs every nuclear power station on the grid. Heavy atoms like uranium are already unstable, so a reactor mostly manages a process that wants to happen anyway. Fusion powers the sun, but the technology is still experimental. Certain reactors have demonstrated reactions sustaining for around twenty minutes, but none has yet generated usable power.

Fission is like a dead tree that will fall on its own eventually. Manage the fall, cut it, burn it when you choose, and you get heat instead of a rotting log. The tree differs in where the energy sits: burning wood releases it, while splitting a nucleus is itself the release. Fusion is the reverse problem, closer to compressing a stiff spring. It fights back harder the further you push, so getting there takes enormous force. Atoms differ in one crucial way: once they touch, they give back far more than it took to push them.