uraniumi235

Uranium-235, often abbreviated as U-235, is a fissile isotope of uranium. It is the only naturally occurring fissile isotope in significant quantities. Naturally occurring uranium is composed of approximately 0.72% U-235 and 99.27% U-238, with trace amounts of other isotopes. The nucleus of a U-235 atom contains 92 protons and 143 neutrons, for a total atomic mass of approximately 235 atomic mass units.

The defining characteristic of U-235 is its ability to sustain a nuclear chain reaction. When a U-235 nucleus absorbs a slow-moving neutron, it becomes unstable and undergoes nuclear fission. This fission process splits the nucleus into two smaller nuclei, releasing a significant amount of energy in the form of kinetic energy of the fission products and gamma rays. Crucially, this fission also releases two to three additional neutrons. If these released neutrons are slowed down, they can be absorbed by other U-235 nuclei, causing them to fission and release more neutrons, thus initiating and sustaining a chain reaction.

This property makes U-235 essential for nuclear power generation and for the creation of nuclear weapons. In nuclear reactors, U-235 is used as fuel. Enriched uranium, which has a higher concentration of U-235 than natural uranium, is typically required for most reactor designs to achieve criticality. In nuclear weapons, highly enriched uranium with a very high percentage of U-235 is used to create a rapid and uncontrolled chain reaction.