The W80 is a thermonuclear warhead produced and deployed by the United States. It comes in two versions, the Mod 0 and Mod 1. The Mod 0 is used on nuclear-armed SLCMs - variants of the Tomahawk. The Mod 1 is used on ALCMs and on the newer but soon-to-be-retired AGM-129 ACM Advanced Cruise Missile. The Mod 0 differs from the Mod 1 in that it contains more highly refined plutonium in order to reduce neutron emissions. Since the Mod 0 is used on the SLCM, it must be kept inside the sealed environment of a nuclear submarine along with sensitive electronics and the crew, necessitating the more expensive processing in order to reduce risks.
The W80 is a bullet-shaped device, approximately 31.4 inches in length and 11.8 inches in diameter, weighing around 290 lbs. It is a two-stage fission-fusion weapon, and can be set for either 5 or 150 kilotons yield. It is speculated that this option is selected by disabling or removing the fusion secondary pit from the weapon, as 5 kilotons is the nominal yield of most fission primary ignition devices. Since there is no evidence that the weapon leaves excessive fallout when set in the lower mode, it makes sense that the fusion stage - which doesn't contain heavy radioactives- is simply destroyed without detonating.
The W80 is an evolution of the U.S. B61 warhead, as are several others in the arsenal. It is secured with PAL systems. In 2007, a B-52 Stratofortress carried six AGM-129 ACM missiles from Minot AFB, ND to Barksdale AFB, LA where they were to be decommissioned. Due to procedural errors, the six W80 warheads in those missiles were not removed first, leading to a period of 36 hours where they were officially 'missing' until they were tracked down. As a result of this mishap, the Air Force undertook a series of investigations into nuclear weapon handling, and disciplined several officers involved in the process.