B53 nuclear bomb

The  Mk/B53 was a high-yield bunker buster thermonuclear weapon developed by the United States during the Cold War. Deployed on Strategic Air Command bombers, the B53, with a yield of 9 MtonTNT, was the most powerful weapon in the U.S. nuclear arsenal after the last B41 nuclear bombs were retired in 1976.

The B53 was the basis of the W-53 warhead carried by the Titan II Missile, which was decommissioned in 1987. Although not in active service for many years before 2010, fifty B53s were retained during that time as part of the "Hedge" portion of the Enduring Stockpile until its complete dismantling in 2011. The last B53 was disassembled on October 25, 2011, a year ahead of schedule.

With its retirement, the largest bomb currently in service in the U.S. nuclear arsenal is the B83, with a maximum yield of 1.2 MtonTNT. The B53 was replaced in the bunker-busting role by a variant of the two-stage B61 nuclear bomb.

History
Development of the weapon began in 1955 by Los Alamos National 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 as Hardtack 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-52G Stratofortress, and B-58 Hustler bomber aircraft in the mid-1960s. From 1968 it was redesignated B53.

Some early versions of the bomb were dismantled beginning in 1967. About 50 bomb and 54 Titan warhead versions were in service through 1980. After the Titan II program ended, the remaining W-53s were retired in the late 1980s. The B53 was also intended to be retired in the 1980s, but 50 units remained in the active stockpile until the deployment of the B61-11 in 1997. At that point the obsolete B53s were slated for immediate disassembly; however, the process of disassembling the units was greatly hampered by safety concerns as well as a lack of resources. In 2010 authorization was given to disassemble the 50 bombs at the Pantex plant in Texas. The process of dismantling the last remaining B53 bomb in the stockpile commenced on October 25, 2011 and was completed soon afterwards.

Specifications
The B53 was 12 ft long with a diameter of 50 in. It weighed 8850 lb, including the 800 to 900 lb parachute system and the honeycomb aluminum nose cone to enable the bomb to survive laydown delivery. It had five parachutes: one 5 ft pilot chute, one 16 ft extractor chute, and three 48 ft  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 used oralloy (highly enriched uranium) instead of plutonium for fission, with a mix of lithium-6 deuteride fuel for fusion. The explosive lens comprised a mixture of RDX and TNT, which was 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.

Role
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 to deliver 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, but 50 units remained in the active stockpile until the deployment of the B61-11 in 1997. At that point the obsolete B53s were slated for immediate disassembly; however, the process of disassembling the units was greatly hampered by safety concerns as well as a lack of resources. The last remaining B53 bomb began the disassembly processes on Tuesday, 25 October 2011 at the Energy Department’s Pantex Plant.

W53
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 about 6200 lb. The 8,140 pound Mark-6 re-entry vehicle containing the W53 warhead was about 123 inches (3.1 meters) long, 7.5 feet (2.2 meters) in diameter and was mounted atop a spacer which was 8.3 feet (2.5 meters) in diameter at the missile interface (compared to the missile's core diameter of 10 feet). With a yield of 9 Megatons, it was the highest yield warhead ever deployed on a US missile. About 65 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. Due to the safety measures built into the weapon, it did not explode or release any radioactive material. 52 active missiles were deployed in silos prior to the beginning of the retirement program in October 1982. With the retirement of the Titan fleet, disassembly of the W53 warheads was completed by about 1988.

Effects
Assuming a detonation at optimum height, a 9 megaton blast would result in a fireball with an approximate 4 to 5 km diameter. The radiated heat would be sufficient to cause lethal burns to any unprotected person within a 28.7 km radius (995 sqmi). Blast effects would be sufficient to collapse most residential and industrial structures within a 14.9 km radius (300 sqmi); within 5.7 km virtually all above-ground structures would be destroyed and blast effects would inflict near 100% fatalities. Within 4.7 km a 500 rem dose of ionizing 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.

Artifacts

 * B53 on display in the free introduction exhibit room at the Atomic Testing Museum, Las Vegas, Nevada
 * B53 on display at the Wings Over the Rockies Air and Space Museum, Denver, Colorado
 * Mark 53 casing is on display in the Cold War Gallery at the National Museum of the United States Air Force in Dayton, Ohio
 * B53 casing in display yard of The National Museum of Nuclear Science & History, Albuquerque, New Mexico