W53 (nuclear weapon)
The same "physics package" was used for two nuclear weapons, the W53 warhead and the B53 bomb. Both had a high yield of 9 megatons. In the bomb version, it replaced the 25 Mt W41 bomb. In turn, the ground-penetrating B61-11 will replace the B53 for hard underground targets.
Mated to the UGM-27 Titan II, it was the largest single warhead of any U.S. missile, although there were higher-yielding bombs.
The W53/B53 is a two-stage thermonuclear device. Safeguards are not known, but probably consist of a combination lock on the arming circuit. Its Primary was all highly enriched uranium, with no plutonium, and a lithium-6 deuteride Secondary.  The B53 is described as having both "clean" and "dirty" jackets, but there is no explanation of why the yield is stated as the same if the "dirty" jacket is indeed U-238, which would be expected to become a Tertiary stage and itself fission for additional yield.
While the Titan II has long been retired, some B53 bombs, while not operational with the B-52 bomber that carried them, have been retained in the "enduring stockpile", reserved for attacking deep underground installations. Since a nuclear weapon that penetrates only a few meters into the earth is far more efficient at creating ground shock waves than even a surface burst, the ground-penetrating B-61-11 bomb is seen as a lower-yield replacement for the same mission.
There have been suggestions that high-yield devices also have been retained for a possible need to divert an asteroid from a collision course with the earth.
On 19 September 1980, a U.S. missile crewman, working on a missile in an open silo at Little Rock Air Force Base in Arkansas, dropped a socket wrench weighing perhaps one pound, into the silo, where it hit the outer skin of the fueled rocket at approximately 6:30 PM. At approximately 3 AM the next morning, the rocket exploded, killing 1, injuring 23, and propelling the W53 warhead several hundred feet away. The nuclear weapon demonstrated its safeguards by not partially or fully detonating. That there was no detonation even of the Cyclotol/Composition B high explosives is impressive, as the explosive compression system of the W53 did not use one of the newer, safer insensitive high explosives.
- Sublette, Carey, "The B-53 (Mk-53) Bomb: High yield strategic thermonuclear bomb", Nuclear Weapons Archive
- The Encyclopedia of Arkansas History and Culture, Titan II Missile Explosion