Each nuclear weapon in the U.S. arsenal contains a “pit,” a hollow shell of plutonium clad in a corrosion- resistant metal, which is surrounded by chemical explosive. When the weapon is detonated, the explosives compress the pit into a supercritical mass and a fission chain reaction is triggered. All the pits in the current U.S. nuclear weapons stockpile were manufactured at the Department of Energy’s Rocky Flats Plant in Colorado, which was shut down in 1989 because of flagrant violations of safety and environmental regulations. During the Cold War, warheads were replaced by new designs well before the end of their design lifetimes. With the end of the Soviet-U.S. arms race, however, the need for new weapon designs also ended, and the longevity of the pits has become an issue. The pits in current U.S. warheads are expected slowly to deteriorate and at some point they will have to be replaced if the warheads are to remain in the stockpile.