U235

Uranium-235 (U-235) is an isotope of the element uranium with 92 protons and 143 neutrons, giving it a mass number of 235. It is one of the few fissile nuclides and plays a central role in both nuclear power and nuclear weapons. U-235 undergoes alpha decay with a half-life of about 703.8 million years, ultimately decaying to lead-207 through a series of intermediate daughters. In nature, uranium contains a small fraction of U-235, typically about 0.7% of total uranium, with the remainder mostly uranium-238.

U-235 is fissile, meaning it can sustain a nuclear chain reaction when it absorbs a slow (thermal) neutron. This property enables its use as a fuel in many nuclear reactors and as a material for nuclear weapons. The energy released per fission is about 200 MeV, making U-235 a highly energy-dense source. When used in reactors, U-235 is often accompanied by other fissile or fertile isotopes, such as plutonium-239 produced from uranium-238.

Enrichment processes increase the proportion of U-235 in uranium. Natural uranium is enriched through methods such as gaseous diffusion or gas centrifugation to levels of roughly 3–5% for most civilian light-water reactors, or to higher concentrations (often above 90%) for weapons-grade material. The isotope is extracted from uranium ore and processed into fuel, where it undergoes fission to release energy.

Historically, U-235 has been central to both civilian nuclear power and military applications. Safety and regulatory controls govern its handling, irradiation, and transport due to its radiological properties and potential for misuse.