1. Neutron Absorption: A slow-moving neutron is absorbed by the nucleus of a uranium-235 atom. This process forms an unstable and excited uranium-236 nucleus.
2. Nuclear Excitation: The absorption of the neutron causes the uranium-236 nucleus to become highly excited and energetic. It reaches a state of great instability and is ready to split into smaller fragments.
3. Nuclear Fission: The excited uranium-236 nucleus undergoes nuclear fission, where it splits into two smaller, more stable nuclei. These fragments, called fission products, are typically krypton-92 and barium-141.
4. Release of Neutrons: During the fission process, several neutrons are released. On average, around 2-3 neutrons are produced per fission event. These released neutrons are crucial for sustaining the chain reaction.
5. Chain Reaction: The neutrons released in the fission process can go on to strike and cause further fission events in neighboring uranium-235 atoms. This sequence of fission reactions leads to a chain reaction where multiple uranium-235 nuclei undergo fission, releasing more neutrons and energy.
6. Critical Mass: For the chain reaction to become self-sustaining, a critical mass of uranium-235 is required. Critical mass refers to the minimum amount of fissile material needed for a sustained fission chain reaction. Uranium-235 is typically enriched to increase the concentration of the isotope and achieve criticality.
7. Control of the Chain Reaction: In nuclear reactors, the chain reaction is carefully controlled and moderated using control rods. These rods absorb neutrons, thus regulating the rate at which the chain reaction progresses and ensuring safe and steady operation of the reactor.
It is worth noting that uranium-235 does not undergo fission spontaneously. The absorption of a neutron provides the necessary energy and instability to initiate the fission process. In nuclear weapons, the rapid and uncontrolled fission of uranium-235 results in a sudden release of enormous energy, leading to the detonation of the weapon.
