Uranium-235 ($\mathbf{^{235}U}$) is a scientific concept, specifically a naturally occurring, radioactive isotope of the element uranium. An isotope is a form of an element with the same number of protons (92 for uranium) but a different number of neutrons; U-235 has 143 neutrons, totaling 235 nucleons. It was discovered in 1935 by Arthur Jeffrey Dempster.
Its significance stems from its unique property of being fissile, meaning its nucleus can readily undergo nuclear fission when struck by a slow, or thermal, neutron. This solved the problem of how to harness the immense energy stored in the atomic nucleus. The process works when a U-235 nucleus captures a neutron, becomes unstable, and splits into two smaller nuclei, releasing a massive amount of thermal energy (about 200 MeV per fission) and two or three additional neutrons. If at least one of these newly released neutrons strikes another U-235 nucleus, a self-sustaining chain reaction is created.
This mechanism is the cornerstone of both nuclear power generation and nuclear weapons. In a nuclear reactor, this chain reaction is controlled using materials like boron or cadmium in control rods to absorb excess neutrons. U-235 is rare, making up only about 0.72% of natural uranium, with the rest being the non-fissile isotope Uranium-238 ($\mathbf{^{238}U}$). Therefore, for use in most reactors, the concentration of U-235 must be increased to 3% to 5% through a process called enrichment. This concept connects directly to the Nuclear Non-Proliferation Treaty (NPT), which regulates the enrichment process due to its dual-use nature, as weapons-grade uranium requires an enrichment of 85% or more. The fundamental scientific concept of U-235 has not changed, but its application and regulation are constantly evolving, with international agreements governing its use for peaceful purposes.