What is nuclear fission
A reaction in which a heavy atomic nucleus, like uranium or plutonium, absorbs a neutron and splits into two lighter nuclei. When the nucleus splits, it releases two lighter nuclei along with a few neutrons and a large amount of energy.
The chain reaction of fission
When the neutrons released during fission go on to split other nuclei, the reaction keeps propagating itself — this is called a chain reaction. Slowing this chain reaction down with control rods is the basis of nuclear power generation, while letting it happen explosively all at once is the principle behind an atomic bomb.
What is nuclear fusion
A reaction in which two light atomic nuclei, such as hydrogen, combine into a single heavier nucleus, such as helium. This is the exact opposite direction of fission — fusing two nuclei together requires extremely high temperature and pressure, enough to overcome the strong repulsive force between the nuclei.
Why the sun shines
At the core of the sun, fusion reactions are constantly occurring in which four hydrogen nuclei combine into a single helium nucleus. The resulting helium nucleus has slightly less mass than the four hydrogen nuclei it came from combined, and that small missing mass is converted into an enormous amount of energy, radiating out as the sun's light and heat.
E=mc² and mass-energy conversion
In both fission and fusion, a tiny difference in mass appears before and after the reaction, and that mass converts into energy according to Einstein's mass-energy equivalence formula. This formula — energy (E) equals mass (m) times the speed of light (c) squared — is why even a minuscule change in mass can explain such an enormous amount of energy.
Fission power vs. fusion power
Fission is already commercialized in nuclear power plants around the world, but fusion has not yet succeeded in commercial power generation anywhere. Achieving fusion requires artificially sustaining conditions of extreme temperature and pressure comparable to the sun's interior for a long period, and the technology to do this economically is still under development.
Ambitious projects like ITER
Countries around the world are jointly building large experimental facilities, such as the International Thermonuclear Experimental Reactor (ITER), to test the viability of commercial fusion power. If fusion power is realized, its fuel (hydrogen) would be easy to obtain and it is expected to produce far less high-level radioactive waste than fission — earning it the nickname "the dream energy," with many countries investing heavily in the research.
Comparing radioactive waste
Fission power leaves behind high-level radioactive waste that requires long-term management, while fusion reactions are known to produce relatively far less radioactive waste. That said, the fusion reactor's own structural materials still become radioactive from neutron exposure, so research into fusion waste management continues as well.