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Uranium-238 and uranium-235 are two isotopes of the element uranium, differing in the number of neutrons in the nucleus (U-238 has 146 neutrons, U-235 has 143). Both occur naturally, but in very different abundances and with different nuclear properties that shape their uses in energy, research, and weapons.

In nature, uranium is mainly U-238, making up about 99.3% of natural uranium, while U-235 accounts for about 0.7%. The half-life of U-238 is about 4.47 billion years, and that of U-235 is about 704 million years. Both isotopes decay by alpha emission as part of longer decay chains that ultimately lead to stable lead isotopes (U-238 to lead-206, U-235 to lead-207).

U-235 is fissile, meaning it can sustain a nuclear chain reaction when exposed to thermal neutrons. This property underpins most civilian nuclear reactors and nuclear weapons. U-238, by contrast, is not fissile with thermal neutrons, but it is fertile: it readily captures a neutron to form U-239, which decays to neptunium-239 and then to plutonium-239, a fissile material. This breeding capability is central to some reactor designs and to plutonium production.

Because natural uranium contains only a small fraction of U-235, enrichment processes increase the U-235 concentration for reactor fuel, typically to about 3–5% for light-water reactors. Higher fractions (often exceeding 90%) are used in some weapons programs. U-238 itself has applications as shielding and in some industrial processes, and it factors into considerations of reactor physics, fuel cycle economics, and radioactive waste management.

In summary, U-238 serves mainly as a long-lived, non-fissile component and fertile material, while U-235 is the primary fissile isotope responsible for sustaining nuclear chain reactions.