Which isotopes are used as fissionable materials in atomic bombs?

Fission weapons are primarily constructed using materials with high concentrations of specific fissile isotopes that can sustain a nuclear chain reaction. The primary isotopes utilized as fissionable materials in atomic bombs include:1

• Uranium-235: This is the only naturally occurring fissile isotope. Because it exists in low concentrations in nature, it must be enriched to high levels—typically around 93.5 percent—for use in weapons.12
• Plutonium-239: This isotope is produced artificially in nuclear reactors through the neutron bombardment of uranium-238. Weapons-grade material typically contains 93 percent or more of this isotope.12
• Uranium-233: While less common than the other two, explosive devices using high concentrations of this isotope have also been constructed and tested.1

While other heavy nuclei are capable of fissioning, these specific isotopes are preferred because they are fissile, meaning they can be split by both fast and slow neutrons, allowing for the rapid growth of fissions required for an explosive yield.13