핵융합

Nuclear fusion combines small nuclei like hydrogen into larger ones such as helium, releasing enormous energy. Unlike fission, which splits heavy nuclei, fusion merges light nuclei with low binding energies into heavier ones with higher binding energies. The energy difference between products and reactants generates the power. Fusion of one gram of helium-4 produces more energy than fission of one gram of uranium-235.

Fusion reactions require temperatures of 40 million kelvins or higher because nuclei must overcome strong electrostatic repulsion to collide and fuse. At these thermonuclear temperatures, all molecules dissociate into atoms, which ionize to form plasma. The high kinetic energy allows nuclei to approach each other closely enough for the strong nuclear force to bind them together.

The sun powers itself through hydrogen-to-helium fusion, where four hydrogen nuclei fuse to produce one helium nucleus and two positrons. This net fusion reaction converts mass into approximately 1.7 × 10⁹ to 2.6 × 10⁹ kilojoules of energy per mole of helium-4 produced. This process, called the hydrogen-burning process, sustains main-sequence stars like our sun.

During helium fusion, two helium nuclei combine into beryllium-8, which is highly unstable and easily reverses. As the reaction accelerates, beryllium-8 becomes more abundant and fuses with helium-4, producing excited-state carbon-12 that relaxes to stable carbon-12. In massive stars, carbon-12 and helium-4 initiate a chain of fusion reactions producing elements up to magnesium-24.

Fusion reactors remain impractical because no solid materials withstand the extreme temperatures required, and mechanical devices cannot contain plasma. Scientists use magnetic fields in tokamak reactors or focused laser beams to contain plasma, but these techniques remain technically challenging. Currently, no self-sustaining fusion reactors operate worldwide, though small-scale controlled reactions have been briefly achieved.

Elements heavier than nickel-56 form through multiple neutron- or proton-capture events during supernovae explosions. As fusion reactions create heavier nuclides, the decreasing difference in binding energies between reactants and products yields less energy. Nickel-56, which has one of the highest binding energies per nucleon, marks the endpoint of stellar fusion processes.

Fusion of one gram of helium-4 produces approximately 6.5 × 10⁸ kilojoules, exceeding the 8.5 × 10⁷ kilojoules from one gram of uranium-235 fission. Fusion reactants—heavy isotopes of hydrogen like deuterium and tritium—are far more abundant and less expensive than uranium-235. This combination of superior energy density and accessible fuel makes fusion theoretically attractive for future energy production.