Nuclear fission
Nuclear & Particle Physics - OCR A-Level Physics
Key Definition
Nuclear fission
The splitting of a heavy nucleus (e.g. uranium-235 or plutonium-239) into two smaller daughter nuclei of roughly equal mass, releasing neutrons and energy.
The splitting of a heavy nucleus (e.g. uranium-235 or plutonium-239) into two smaller daughter nuclei of roughly equal mass, releasing neutrons and energy.
Key Definition
Nuclear fusion
The combining of two light nuclei to form a heavier nucleus, releasing energy. Requires extremely high temperatures to overcome the electrostatic repulsion between the positively charged nuclei.
The combining of two light nuclei to form a heavier nucleus, releasing energy. Requires extremely high temperatures to overcome the electrostatic repulsion between the positively charged nuclei.
- Fission is triggered by a thermal neutronA neutron slowed to thermal equilibrium with its surroundings (~0.025 eV), with a high probability of causing fission. being absorbed by a heavy nucleus (induced fission).
- The fission fragments have a higher binding energyThe capacity to do work. Measured in joules (J). per nucleonThe binding energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). required to completely separate a nucleus into its individual protons and neutrons. Equal to the mass defectThe difference between the total mass of the individual nucleons and the actual mass of the nucleus. This mass is converted to binding energy. multiplied by c². of a nucleus divided by its nucleon number (mass number). Higher values indicate greater nuclear stability. than the parent, so energy is released.
- Each fission releases 2-3 fast neutronsHigh-energy neutrons released during fission; they must be slowed by a moderator before they can cause further fission., which can trigger further fissions - a chain reactionA self-sustaining series of fission reactions where neutrons released by one fission trigger further fissions..
- In a nuclear reactor, the chain reaction is controlled by control rodsRods (e.g. boron) inserted into a nuclear reactor to absorb neutrons and regulate the rate of the chain reaction. (absorb neutrons) and a moderatorA material (e.g. water, graphite) that slows fast neutrons to thermal speeds so they can cause further fission. (slows neutrons to thermal energies for more effective absorption).
- Fusion requires temperatures of ~10⁸ K (stellar cores or tokamak plasmas) to give nuclei enough kinetic energyThe energy an object possesses due to its motion. to overcome Coulomb repulsionThe electrostatic repulsion between positively charged protons in a nucleus; overcome by the strong nuclear force..
- Fusion of hydrogen isotopes (deuterium + tritium → helium-4 + neutron) releases about 4 times more energy per unit mass than fission.