Nuclear Fusion
The process of combining light atomic nuclei into heavier ones, releasing enormous energy. Powers stars; pursued for decades as a clean energy source on Earth.
Nuclear fusion is the process by which two light atomic nuclei combine into a heavier nucleus, releasing energy because the resulting nucleus has slightly less mass than the sum of the originals (E=mc²). Fusion powers all stars, including the Sun, where hydrogen nuclei fuse into helium under enormous gravitational pressure.
Fusion produces about four times more energy per unit fuel mass than nuclear fission. It produces no long-lived radioactive waste, has fuel (deuterium from seawater, lithium-derived tritium) that is essentially inexhaustible, and cannot have a runaway meltdown.
The challenge is replicating fusion conditions on Earth. Doing so requires temperatures of ~100 million °C (hotter than the Sun's core). NIF achieved scientific ignition in December 2022 — the first time fusion produced more energy than the lasers delivered to the fuel. Commercial fusion power plants are widely estimated 15-30+ years away.