Nuclear physics
Uranium-235
A fissile isotope of uranium that constitutes the explosive material in many fission weapons.
Natural uranium is 99.3% non-fissile U-238 and only 0.7% U-235. Weapons-grade uranium is enriched to >90% U-235 — a process requiring centrifuges or gaseous diffusion. The Hiroshima bomb (Little Boy) used about 64 kg of highly enriched uranium-235.
Related terms
Plutonium-239
A fissile isotope of plutonium produced from uranium-238 in nuclear reactors and used in most modern fission weapons.
Critical Mass
The minimum amount of fissile material required to sustain a self-propagating nuclear chain reaction.
Nuclear Fission
The splitting of heavy atomic nuclei (typically uranium-235 or plutonium-239) into lighter fragments, releasing large amounts of energy.