235U

235U, or uranium-235, is a radioisotope of uranium with 92 protons and 143 neutrons, giving it a mass number of 235. It constitutes about 0.72% of natural uranium, the remainder being mostly 238U. It is produced in trace amounts in nature and is separated from natural uranium through enrichment to increase its proportion for use in nuclear applications.

235U is fissile, meaning it can sustain a nuclear chain reaction when bombarded with neutrons, including the relatively slow (thermal) neutrons that are common in many reactors. The fission of 235U releases energy and additional neutrons, enabling a self-sustaining reaction under appropriate conditions. In contrast, 238U is fertile, meaning it can be transformed into a fissile material through neutron capture in a reactor.

Naturally occurring 235U decays by alpha emission with a half-life of about 703 million years, turning into thorium-231. This long-lived radioactivity contributes to the radiological considerations associated with uranium handling.

Applications and enrichment: In civilian nuclear power, natural uranium is typically enriched to about 3–5% 235U to serve as reactor fuel, particularly for light-water reactors. Higher enrichment levels (significantly above 20–25%) are used for certain reactor types and research purposes, and very high enrichment (often above 85–90%) is associated with weapons-grade material, subject to strict regulation and safeguards.

Safety and regulation: 235U poses radiological and chemical hazards. Handling and use are governed by strict safety protocols and international nonproliferation regimes to prevent uncontrolled release or diversion for unauthorized purposes.