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The Sun has been burning for 4.6 billion years — without any fuel in the chemical sense. Nuclear power plants produce electricity from a lump of uranium smaller than a football. Both use nuclear reactions that release millions of times more energy than burning the same mass of coal. How does splitting or joining atomic nuclei release such enormous energy?
The Sun has been burning for 4.6 billion years — without any fuel in the chemical sense. Nuclear power plants produce electricity from a lump of uranium smaller than a football. Both use nuclear reactions that release millions of times more energy than burning the same mass of coal. How does splitting or joining atomic nuclei release such enormous energy?
E = mc². Mass can be converted to energy. When nuclei split (fission) or merge (fusion), the products have slightly less mass than the reactants. That tiny missing mass becomes an enormous amount of energy. This is the physics behind both nuclear power and the stars.
Nuclear reactions release energy due to mass defect: products have less mass than reactants. The binding energy per nucleon peaks at iron-56. Fission releases energy from heavy nuclei (above the peak); fusion releases energy from light nuclei (below the peak). Energy is calculated from E = Δmc².
The mass defect arises because nucleons are more tightly bound in the product nuclei than the reactants. The missing mass (typically a fraction of a percent of total mass) is released as energy, predominantly as kinetic energy of the products and gamma rays. Fission: a heavy nucleus (U-235, Pu-239) absorbs a neutron and splits into two medium-mass fragments plus 2-3 neutrons. These neutrons can trigger further fissions — a chain reaction. Controlled in nuclear reactors (power), uncontrolled in atomic bombs. ~200 MeV per fission event. Fusion: light nuclei (deuterium + tritium → helium + neutron) join at extremely high temperatures (~100 million°C) needed to overcome electrostatic repulsion. The Sun burns hydrogen to helium via the proton-proton chain, releasing ~26 MeV per cycle. ~4 million tonnes of mass are converted to energy every second. Controlled fusion (tokamaks, inertial confinement) remains the goal of fusion energy research.