Nuclear Fission vs Nuclear Fusion: Physicist explains the difference | David Kirtley and Lex Fridman

Quick Overview

Nuclear fusion, which powers the sun by fusing light hydrogen isotopes into helium, is fundamentally different from nuclear fission, which splits heavy elements like Uranium-235 and is the basis for current nuclear reactors; fusion energy release comes from mass defect (E=mc^2), but achieving self-sustaining fusion on Earth requires overcoming immense electrostatic repulsion between positively charged nuclei, a challenge far exceeding the containment required for fission reactors.

Key Points: Nuclear fusion powers stars like our Sun by fusing the most common element, hydrogen, and its isotopes into helium, while nuclear fission splits heavy elements like Uranium-235 or Plutonium-239. The energy released in fusion results from a mass defect, where the mass of the resulting nucleus is less than the sum of its components, converted to energy according to Einstein's $E=mc^2$ (1:31). Fusion fuel is primarily light hydrogen isotopes: deuterium (abundant in seawater) and tritium (rare, manufactured by bombarding lithium with neutrons) (0:55, 6:45). Fission reactions, like the splitting of Uranium-235, can occur at room temperature and are sustained by a chain reaction, whereas fusion requires extreme temperatures (millions of degrees) to overcome the repulsive electromagnetic force between positively charged nuclei (10:57, 11:09). The binding energy per nucleon curve shows that fusion releases energy moving towards iron (Fe), which has the highest binding energy per nucleon, while fission releases energy moving away from heavy elements like Uranium (U) towards iron (2:42). The confinement required for fusion is significantly more challenging than for fission because the strong nuclear force only overcomes electrostatic repulsion at extremely close distances and high kinetic energy (high temperature) (11:11, 12:54). The energy released in fusion is quantified using units of electron volts (eV), contrasting with the energy from chemical bonds which is much smaller (5:33).