Bomb Facts: How Nuclear Weapons are Made

 

A. Plutonium

The world's first nuclear explosion was achieved with plutonium, a man-made element produced in nuclear reactors. Plutonium is created when an atom of uranium-238 absorbs a neutron and becomes plutonium-239. The reactor generates the neutrons in a controlled chain reaction. For the neutrons to be absorbed by the uranium their speed must be slowed by passing them through a substance known as a "moderator." Graphite and heavy water have been used as moderators in reactors fueled by natural uranium. For graphite to succeed as a moderator it must be exceptionally pure; impurities will halt the chain reaction. Heavy water looks and tastes like ordinary water but contains an atom of deuterium instead of an atom of hydrogen. For heavy water to succeed as a moderator, it too must be pure; it must be free of significant contamination by ordinary water, with which it is mixed in nature.

(a) Plutonium needed to make a bomb:

- 4 kilograms: Weight of a solid sphere of plutonium just large enough to achieve a critical mass with a beryllium reflector. Diameter of such a sphere: 2.86 in (7.28 cm). Diameter of a regulation baseball: 2.90 in (7.36 cm).
- 4.4 kilograms: Estimated amount used in Israel's fission bombs.
- 5 kilograms: Estimated amount needed to manufacture a first-generation fission bomb today.
- 6.1 kilograms: Amount used in "Trinity" test in 1945 and in the bomb dropped on Nagasaki.
- 15 kilograms: Weight of a solid sphere of plutonium just large enough to achieve a critical mass without a reflector. Diameter of such a sphere: 4.44 in (11.3 cm). Diameter of a regulation softball: 3.82 in (9.7 cm).

(b) Plutonium generated by various reactors:

- 5.5-8 kilograms/year: North Korea's 20-30 megawatt (thermal) Yongbyon reactor moderated by graphite.
- 12 kilograms/year: Pakistan's 50 megawatt (thermal) Khushab reactor moderated by heavy water.
- 9 kilograms/year: India's 40 megawatt (thermal) Cirus reactor moderated by heavy water.
- 25 kilograms/year: India's 100 megawatt (thermal) Dhruva reactor moderated by heavy water.
- 40 kilograms/year: Israel's more than 100 megawatt (thermal) Dimona reactor moderated by heavy water. 230 kilograms/year: Iran's 1,000 megawatt (electric) Bushehr reactor supplied by Russia and moderated by ordinary water.
- 230 kilograms/year: North Korea's 1,000 megawatt (electric) power reactor to be supplied by a consortium sponsored by the United States and moderated by ordinary water.

(c) Estimated amount of heavy water needed for a small reactor used to make nuclear weapons:

- 19 metric tons: India's 40 megawatt (thermal) Cirus reactor.
- More than 36 metric tons: Israel's more than 100 megawatt (thermal) Dimona reactor.
- 78 metric tons: India's 100 megawatt (thermal) Dhruva reactor.

 

B. Uranium-235

The world's second nuclear explosion was achieved with uranium-235. This isotope is unstable and fissions when struck by a neutron. It is, however, found in natural uranium at a concentration of only 0.7 percent. To be useful in nuclear weapons, the concentration must be increased. This is accomplished by a process known as enrichment. Because the isotopes of uranium are identical chemically, the enrichment process exploits the slight difference in their masses. Nuclear weapons now use a concentration of 93.5 percent uranium-235.

(a) Uranium-235 needed to make a bomb:

- 15 kilograms: Weight of a solid sphere of 100 percent uranium-235 just large enough to achieve a critical mass with a beryllium reflector. Diameter of such a sphere: 4.48 in (11.4 cm). Diameter of a regulation softball: 3.82 in (9.7 cm).
- 16 kilograms: Amount needed for an Iraqi bomb design found by UN inspectors.
- 50 kilograms: Weight of a solid sphere of 100 percent uranium-235 just large enough to achieve a critical mass without a reflector. Diameter of such a sphere: 6.74 in (17.2 cm), comparable to an average honeydew melon.
- 60 kilograms: Reported amount used in Hiroshima bomb "Little Boy."

(b) Various methods used to enrich uranium:

(i) Electromagnetic Isotope Separation (EMIS)

In this process, uranium atoms are ionized (given an electrical charge) then sent in a stream past powerful magnets. The heavier U-238 atoms are deflected less in their trajectory than the lighter U-235 atoms by the magnetic field, so the isotopes separate and can be captured by collectors. The process is repeated until a high concentration of U-235 is achieved. An American version of the EMIS process, featuring "calutrons", was used in the Manhattan Project. EMIS was also the principal process pursued by the Iraqi uranium enrichment effort.

(b) Gaseous Diffusion

In the gaseous diffusion process gaseous uranium hexafluoride (UF6) flows through a porous membrane of nickel or aluminum oxide. Lighter molecules of uranium-235 within the UF6 (235UF6) diffuse through the porous barrier at a faster rate than the heavier molecules of uranium-238 (238UF6). Because the difference in velocities between the two isotopes is small the process must be repeated thousands of times to achieve weapon-usable uranium-235.

(c) Gas Centrifuge

In the gas centrifuge process gaseous UF6 is fed into a cylindrical rotor that spins at a high speed inside an evacuated casing. Centrifugal forces cause the heavier 238UF6 to tend to move closer to the outer wall than the lighter 235UF6, thus partially separating the uranium isotopes. This separation is increased by a relatively slow axial countercurrent flow of gas within the centrifuge that concentrates enriched gas at one end and depleted gas at the other. Numerous stages in the process, employing thousands of centrifuges, are needed to concentrate the uranium-235 to weapon-grade.

(d) Aerodynamic Processes

In the Becker nozzle process a mixture of gaseous UF6 and helium (H2) is compressed and then directed along a curved wall at high velocity. The heavier uranium-238-bearing molecules move preferentially out to the wall relative to those containing uranium-235. At the end of the deflection, the gas jet is split by a knife edge into a light fraction and a heavy fraction, which are withdrawn separately.

(e) Atomic Vapor Laser Isotope Separation (AVLIS)

The AVLIS process uses dye lasers tuned so that only uranium-235 atoms absorb the laser light. As the uranium-235 atom absorbs the laser light, its electrons are excited to a higher energy state. When enough energy is absorbed, a uranium-235 atom will eject an electron and become a positively charged ion. The uranium-235 ions may then be deflected by an electrostatic field to a product collector. The uranium-238 atoms remain neutral and pass through the product collector.

(f) Molecular Laser Isotope Separation (MLIS)

The MLIS separation process consists of two basic steps. In the first step UF6 is excited by an infrared laser system, which selectively excites the UF6 molecules bearing uranium-235 (235UF6), leaving the UF6 molecules bearing uranium-238 unexcited (238UF6). In the second step, photons from a second laser system (infrared or ultraviolet) preferentially dissociate the excited 235UF6 to form uranium pentafluoride (UF5) molecules bearing uranium-235 (235UF5) and free fluorine atoms. The 235UF5 formed from the dissociation precipitates from the gas as a powder that can be filtered from the gas stream.

(g) Thermal Diffusion

Thermal diffusion uses the transfer of heat across a thin liquid or gas to accomplish isotope separation. By cooling a vertical film on one side and heating it on the other, the resultant convection currents will produce an upward flow along the hot surface and a downward flow along the cold surface. Under these conditions, the lighter uranium-235 molecules will diffuse toward the cold surface. These two diffusive motions combined with the convection currents will cause the lighter uranium-235 molecules to concentrate at the top of the film and the heavier uranium-238 molecules to concentrate at the bottom of the film.

 

The First Bombs

United States

"Trinity": World's first nuclear test explosion: July 16, 1945.
Location: Near Alamogordo, New Mexico.
Yield: 21 kilotons.
Fissile material used: Plutonium-239.
Amount: 6.1 kilograms.
Method of detonation: Implosion.
Amount of high-explosive wrapped around plutonium core: 2268 kilograms.
Method of production: Nuclear reactor at the Hanford Reservation.

"Little Boy": First use of nuclear weapon in war: August 6, 1945.
Location: Hiroshima, Japan.
Detonation height: 580 meters.
Delivery mechanism: Airdropped from B-29 bomber named Enola Gay.
Yield: 12.5 kilotons.
Fissile material used: Uranium-235.
Method of detonation: "Gun-type" device.
Method of production: "Calutron" electromagnetic isotope separation.

"Fat Man": Second use of a nuclear weapon in war: August 9, 1945.
Location: Nagasaki, Japan.
Detonation Height: 500 meters.
Delivery mechanism: Airdropped from B-29 bomber named Bock's Car.
Yield: 22 kilotons.
Fissile material used: Plutonium-239.
Method of Detonation: Implosion.
Amount used: 6.2 kilograms.

"Ivy Mike": First hydrogen bomb tested: November 1, 1952.
Location: Elugelab Island, Enewetak Atoll.
Yield: 10.4 megatons.

Soviet Union

"Joe 1": First nuclear test: August 29, 1949.
Location: Semipalatinsk, Kazakhstan.
Yield: 10-20 kilotons.
Fissile material used: Plutonium-239.
Method of detonation: Implosion.
Method of production: Reactor.

"Joe 4": First thermonuclear test: August 12, 1953.
Location: Possibly in Siberia.
Yield: 200-300 kilotons.

Great Britain

"Hurricane": First nuclear test: October 3, 1952.
Location: Off Trimouille Island, Australia.
Yield: 25 kilotons.
Fissile material used: Plutonium-239.
Method of detonation: Implosion.
Method of production: Reactor.
Foreign Assistance: United States.

"Grapple Y": Thought to be the first two-step thermonuclear test: April 28, 1958.
Location: Christmas Island.
Yield: 2 megatons.
Delivery Mechanism: Airdropped from a Valiant XD825 bomber.

France

"Gerboise Bleue": First nuclear test: February 13, 1960.
Location: Reggane Proving Grounds, Algeria.
Yield: 60-70 kilotons.
Fissile material used: Plutonium-239.
Method of detonation: Implosion.
Method of production: Reactor.

"Canopus": First thermonuclear test: August 24, 1968.
Location: Fangataufa Atoll.
Yield: 2.6 megatons.
Foreign assistance: Norway (heavy water to make tritium).

China

"596": First nuclear test: October 16, 1964.
Location: Lop Nor.
Yield: 12.5-22 kilotons.
Fissile material used: Uranium-235.
Method of production: Gaseous diffusion.
Foreign assistance: Soviet Union.

First thermonuclear test: June 17, 1967.
Location: Lop Nor.
Yield: Approximately 3 megatons.
Delivery mechanism: Airdropped from a Hong 6 bomber.

Israel

Estimated date when first bomb was produced: Late 1966.
Fissile material: Plutonium.
Method of production: Dimona reactor imported from France and operated with heavy water supplied by Norway.
Probably conducted a 2-3 kiloton nuclear test on September 22, 1979 in the South Atlantic Ocean in cooperation with South Africa.

India

First nuclear test: May 18, 1974.
Location: Pokhran.
Yield: 2-15 kilotons.
Fissile material used: Plutonium-239.
Method of production: Cirus reactor supplied by Canada and operated with heavy water supplied by the United States.

Second nuclear test "Shakti 1": May 11, 1998.
Location: Pokhran.
Yield: 10-15 kilotons.

Third nuclear test (claimed): May 13, 1998.
Yield: India claimed it tested two nuclear bombs, with a combined yield of 0.8 kilotons; however, there is no seismic evidence of any nuclear explosion.

South Africa

First device built: December 1982.
Total bombs built: Six.
Method of detonation: "Gun-type" device.
Fissile material used: Uranium-235.
Nuclear tests: None.

Dismantlement of bomb program began in November 1989 and was completed in early September 1991, after which South Africa signed a comprehensive safeguards inspection agreement with the IAEA.

Pakistan

Estimated production of first bomb: Late 1987.
First nuclear test: May 28, 1998.
Location: Chagai Hills region.
Yield: 9-12 kilotons Fissile material used: Uranium-235.
Method of production: Gas centrifuge technology smuggled from Europe.
Foreign assistance: China (bomb design), Germany (uranium processing equipment).

Second nuclear test: May 30, 1998.
Yield: 4-6 kilotons.

North Korea

In 1993 U.S. intelligence declared that North Korea had a "better than even" chance of possessing one or two atomic bombs.
North Korea has conducted no known nuclear tests.
Fissile material: Plutonium-239.
Method of production: Graphite reactor near Yongbyon.