The B53 with a yield of 9 Mt is one of the most powerful nuclear weapons built by the United States, and one of the last very high-yield thermonuclear bombs in U.S. service.
Development of the weapon began in 1955 by Los Alamos Scientific Laboratory
, based on the earlier Mk 21
and Mk 46
weapons. In March 1958 the Strategic Air Command
issued a request for a new Class C
(less than five tons, megaton-range) bomb to replace the earlier Mk 41
. A revised version of the Mk 46
became the TX-53
in 1959. The development TX-53 warhead was apparently never tested, although an experimental TX-46 predecessor design was detonated 28 June 1958
Oak, which detonated at a yield of 8.9 Megatons
The Mk 53 entered production in 1962 and was built through June 1965. About 340 bombs were built. It entered service aboard B-47 Stratojet, B-52 Stratofortress, and B-58 Hustler bomber aircraft in the mid-1960s. From 1968 it was redesignated B53.
The B53 was 12 ft 6 in (3.8 m) long with a diameter of 50 in (1.27 m). It weighed 8,850 lb (4,015 kg), including the 800-900 pound (350-400 kg) parachute
system and the honeycomb aluminum nose cone
to enable the bomb to survive laydown delivery
. It had five parachutes: one 5 foot (1.5 m) pilot chute, one 16 foot (4.9 m) extractor chute, and three 48 foot (14.6 m) main chutes. Chute deployment depends on delivery mode, with the main chutes used only for laydown delivery (for free-fall delivery, the entire system was jettisoned).
The warhead of the B53 uses oralloy (highly enriched uranium) instead of plutonium for fission, with a mix of lithium-6 deuteride fuel for fusion. The explosive "lens" is a mixture of RDX and TNT, which is not insensitive. Two variants were made: the B53-Y1, a dirty weapon using a U-238-encased secondary, and the B53-Y2 "clean" version with a non-fissile (lead or tungsten) secondary casing. Explosive yield was approximately nine megatons.
It was intended as a bunker buster
weapon, using a surface blast after laydown deployment to transmit a shock wave
through the earth to collapse its target. Attacks against the Soviet deep underground leadership shelters in the Chekhov/Sharapovo area south of Moscow envisaged multiple B53/W53 exploding at ground level. It has since been supplanted in such roles by the earth-penetrating B61 Mod 11
. A bomb that penetrates the surface delivers much more of its explosive energy into the ground and therefore needs a much smaller yield to produce the same effects.
The B53 was intended to be retired in the 1980s, reducing the stockpile to 25 weapons by 1987. On 5 August 1987 SAC decided to halt the retirement and return 25 more weapons to service, for 50. Those weapons are no longer in active service, but are retained as part of the "Hedge" portion of the Enduring Stockpile.
The W53 warhead of the Titan II ICBM used the same physics package as the B53, without the air drop-specific components like the parachute system, reducing its mass to 8,136 lb (3,690 kg). With a yield of 9 Megatons, it was the highest yield warhead ever deployed on a US missile. About 60 W53 warheads were constructed between December 1962 and December 1963.
On September 19, 1980 a fuel leak caused a Titan II to explode within its silo in Arkansas, throwing the W53 warhead some distance away. It did not explode or leak any radiation.
With the retirement of the Titan fleet, disassembly of the W53 warheads was completed by about 1988.
Assuming a detonation at optimum height, a 9 megaton blast would result in a fireball some 1 to 1.6 kilometres in diameter (0.6 to 1 mile) lasting 12 seconds. The radiated heat would be sufficient to cause lethal burns
to any unprotected person within 28.7 kilometres (17.8 miles). Blast effects would be sufficient to collapse most residential and industrial structures within a 14.9 kilometre (9.2 mile) radius; within 5.7 kilometres (3.5 miles) virtually all above-ground structures would be destroyed and blast effects would inflict near 100% fatalities. Within 4.7 kilometres (2.9 miles) a 500 rem dose of ionising radiation
would be received by the average person, sufficient to cause a 50% to 90% casualty rate independent of thermal or blast effects at this distance.