The Mark 18 nuclear bomb, also known as the SOB or Super Oralloy Bomb, was an American nuclear bomb design which was the highest yield fission bomb produced by the US. The Mark 18 had a design yield of 500 kilotons. Noted nuclear weapon designer Ted Taylor was the lead designer for the Mark 18.
The Mark 18 was tested once, in the Ivy King

Ivy King

Ivy King was the largest pure fission nuclear bomb ever tested by the United States. The bomb was tested during the Truman administration as part of Operation Ivy...

 nuclear test at the Enewetak

Enewetak

Enewetak Atoll is a large coral atoll of 40 islands in the Pacific Ocean, and forms a legislative district of the Ralik Chain of the Marshall Islands. Its land area totals less than , surrounding a deep central lagoon, in circumference...

 atoll in the Pacific Ocean

Pacific Ocean

The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

. The test was a complete success at full yield.

Description

The Mark 18 bomb design used an advanced 92-point implosion system, derived from the Mark 13 nuclear bomb

Mark 13 nuclear bomb

The Mark 13 nuclear bomb and its variant, the W-13 nuclear warhead, were experimental nuclear weapons developed by the United States from 1951 to 1954...

 and its ancestors the Mark 6 nuclear bomb

Mark 6 nuclear bomb

The Mark 6 nuclear bomb was an American nuclear bomb based on the earlier Mark 4 nuclear bomb and its predecessor, the Mark 3 Fat Man nuclear bomb design.The Mark 6 was produced from 1951-1955 and saw service until 1962...

, Mark 4 nuclear bomb

Mark 4 nuclear bomb

The Mark 4 nuclear bomb was an American nuclear bomb design produced starting in 1949 and in use until 1953.The Mark 4 was based on the earlier Mark 3 Fat Man design, used in the Trinity test and the bombing of Nagasaki...

, and Fat Man Mark 3 nuclear bomb

Fat Man

"Fat Man" is the codename for the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the second of the only two nuclear weapons to be used in warfare to date , and its detonation caused the third man-made nuclear explosion. The name also refers more...

 of World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. Its normal mixed uranium

Uranium

Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

/plutonium

Plutonium

Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...

 fissile core

Pit (nuclear weapon)

The pit is the core of an implosion weapon – the fissile material and any neutron reflector or tamper bonded to it. Some weapons tested during the 1950s used pits made with U-235 alone, or in composite with plutonium, but all-plutonium pits are the smallest in diameter and have been the standard...

 ("pit") was replaced with over 60 kg of pure highly enriched uranium or HEU. With a natural uranium tamper

Tamper

Tamper can mean:*Tamp, a device used to compact or flatten an aggregate or other powdered or granular material, like ground coffee or gravel*A tool used to pack tobacco into a smoking pipe, as well as to flatten or scoop the ash; usually nail-shaped and sometimes combined into a pipe tool*Ballast...

 layer, the bomb had over four critical mass

Critical mass

A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...

es of fissile material in the core, and was unsafe: the accidental detonation of even one of the detonator triggers, would likely cause a significant (many kilotons of energy yield) explosion. An aluminum/boron

Boron

Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

 chain designed to absorb neutron

Neutron

The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...

s was placed in the fissile pit to reduce the risk of accidental high yield detonation, and removed during the last steps of the arming sequence.

Deployment

Beginning in March 1953, the United States deployed a number of Mark 18 bombs. A total of 90 were manufactured and placed in service.

The weapon had a short lifetime, and was replaced by thermonuclear weapons

Teller-Ulam design

The Teller–Ulam design is the nuclear weapon design concept used in most of the world's nuclear weapons. It is colloquially referred to as "the secret of the hydrogen bomb" because it employs hydrogen fusion, though in most applications the bulk of its destructive energy comes from uranium fission,...

 in the mid-1950s. The Mark 18 weapons were all modified into lower yield Mark 6 nuclear bomb

Mark 6 nuclear bomb

The Mark 6 nuclear bomb was an American nuclear bomb based on the earlier Mark 4 nuclear bomb and its predecessor, the Mark 3 Fat Man nuclear bomb design.The Mark 6 was produced from 1951-1955 and saw service until 1962...

 variants in 1956.

See also

• List of nuclear weapons
• Ivy King
  Ivy King
  Ivy King was the largest pure fission nuclear bomb ever tested by the United States. The bomb was tested during the Truman administration as part of Operation Ivy...
  
  
• Nuclear weapon design
  Nuclear weapon design
  Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three basic design types...
  
  
• Mark 13 nuclear bomb
  Mark 13 nuclear bomb
  The Mark 13 nuclear bomb and its variant, the W-13 nuclear warhead, were experimental nuclear weapons developed by the United States from 1951 to 1954...
  
  
• Mark 6 nuclear bomb
  Mark 6 nuclear bomb
  The Mark 6 nuclear bomb was an American nuclear bomb based on the earlier Mark 4 nuclear bomb and its predecessor, the Mark 3 Fat Man nuclear bomb design.The Mark 6 was produced from 1951-1955 and saw service until 1962...
  
  
• Mark 4 nuclear bomb
  Mark 4 nuclear bomb
  The Mark 4 nuclear bomb was an American nuclear bomb design produced starting in 1949 and in use until 1953.The Mark 4 was based on the earlier Mark 3 Fat Man design, used in the Trinity test and the bombing of Nagasaki...
  
  
• Fat Man Mark 3 nuclear bomb
  Fat Man
  "Fat Man" is the codename for the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the second of the only two nuclear weapons to be used in warfare to date , and its detonation caused the third man-made nuclear explosion. The name also refers more...