Nuclear physics
Plutonium-239
A fissile isotope of plutonium produced from uranium-238 in nuclear reactors and used in most modern fission weapons.
Pu-239 has a smaller critical mass than U-235 and is the preferred material for compact fission weapons. It cannot be assembled by gun-type designs (it would predetonate), so plutonium weapons use implosion. The Nagasaki bomb (Fat Man) used about 6.2 kg of plutonium.