Teller-Ulam design

Teller-Ulam design

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The Teller–Ulam design is the 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...

 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
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

, though in most applications the bulk of its destructive energy comes from 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...

 fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

, not hydrogen fusion. It is named for its two chief contributors, Edward Teller
Edward Teller
Edward Teller was a Hungarian-American theoretical physicist, known colloquially as "the father of the hydrogen bomb," even though he did not care for the title. Teller made numerous contributions to nuclear and molecular physics, spectroscopy , and surface physics...

 and Stanisław Ulam, who developed it in 1951 for the United States, with certain concepts developed with the contribution of John von Neumann
John von Neumann
John von Neumann was a Hungarian-American mathematician and polymath who made major contributions to a vast number of fields, including set theory, functional analysis, quantum mechanics, ergodic theory, geometry, fluid dynamics, economics and game theory, computer science, numerical analysis,...

. It was first used in multi-megaton
TNT equivalent
TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...

-range thermonuclear weapons. As it is also the most efficient design concept for small nuclear weapons; today virtually all the nuclear weapons deployed by the five major nuclear-armed nations use the Teller–Ulam design.

Its essential features, which officially remained secret for nearly three decades, are: 1) separation of stages into a triggering "primary" explosive and a much more powerful "secondary" explosive, 2) compression of the secondary by X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s coming from nuclear fission in the primary, a process called the "radiation implosion
Radiation implosion
The term radiation implosion describes the process behind a class of devices which use high levels of electromagnetic radiation to compress a target...

" of the secondary, and 3) heating of the secondary, after cold compression, by a second fission explosion inside the secondary.

The radiation implosion mechanism is a heat engine
Heat engine
In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...

 exploiting the temperature difference between the secondary's hot, surrounding radiation channel and its relatively cool interior. This temperature difference is briefly maintained by a massive heat barrier called the "pusher", which also serves as an implosion tamper
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...

, increasing and prolonging the compression of the secondary. If made of uranium—and it usually is—it can capture 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 produced by the fusion reaction and undergo fission itself, increasing the overall explosive yield. In many Teller–Ulam weapons, fission of the pusher dominates the explosion and produces radioactive fission product
Fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...

 fallout
Fallout
Fallout or nuclear fallout is the residual radiation hazard from a nuclear explosion.Fallout may also refer to:*Fallout , a 1997 post-apocalyptic computer role-playing game released by Interplay Entertainment...

.

The first test of this principle was the "Ivy Mike
Ivy Mike
Ivy Mike was the codename given to the first United States test of a thermonuclear weapon, in which a major part of the explosive yield came from nuclear fusion. It was detonated on November 1, 1952 by the United States at on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy...

" nuclear test in 1952, conducted by the United States. In the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

, the design was known as Andrei Sakharov
Andrei Sakharov
Andrei Dmitrievich Sakharov was a Soviet nuclear physicist, dissident and human rights activist. He earned renown as the designer of the Soviet Union's Third Idea, a codename for Soviet development of thermonuclear weapons. Sakharov was an advocate of civil liberties and civil reforms in the...

's "Third Idea", first tested in 1955. Similar devices were developed by the United Kingdom, China, and France, though no specific code names are known for their designs.

Public knowledge concerning nuclear weapon design



Detailed knowledge of fission and fusion weapons is classified
Classified information
Classified information is sensitive information to which access is restricted by law or regulation to particular groups of persons. A formal security clearance is required to handle classified documents or access classified data. The clearance process requires a satisfactory background investigation...

 to some degree in virtually every industrialized nation. In the United States, such "knowledge" can by default be classified as Restricted Data
Restricted Data
Restricted Data is a category of classified information in the United States. Specifically, it is defined by the Atomic Energy Act of 1954 as:The concept was initially introduced, with similar wording, in the Atomic Energy Act of 1946...

, even if it is created by persons who are not government employees or associated with weapons programs, in a legal doctrine known as "born secret
Born secret
"Born secret" and "born classified" are both terms which refer to a policy of information being classified from the moment of its inception, usually regardless of where it was being created, usually in reference to specific laws in the United States that are related to information that describes...

" (though the constitutional standing of the doctrine has been at times called into question, see United States v. The Progressive
United States v. The Progressive
United States of America v. Progressive, Inc., Erwin Knoll, Samuel Day, Jr., and Howard Morland is the name of a lawsuit against the magazine The Progressive by the U.S. government in 1979...

). Born secret is rarely invoked for cases of private speculation. The official policy of the United States Department of Energy
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...

 has been not to acknowledge the leaking of design information, as such acknowledgment would potentially validate the information as accurate. In a small number of prior cases, the U.S. government has attempted to censor weapons information in the public press
Prior restraint
Prior restraint or prior censorship is censorship in which certain material may not be published or communicated, rather than not prohibiting publication but making the publisher answerable for what is made known...

, with limited success.

Though large quantities of vague data have been officially released, and larger quantities of vague data have been unofficially leaked by former bomb designers, most public descriptions of nuclear weapon design details rely to some degree on speculation, reverse engineering
Reverse engineering
Reverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation...

 from known information, or comparison with similar fields of physics
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 (inertial confinement fusion
Inertial confinement fusion
Inertial confinement fusion is a process where nuclear fusion reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium....

 is the primary example). Such processes have resulted in a body of unclassified knowledge about nuclear bombs which is generally consistent with official unclassified information releases, related physics, and is thought to be internally consistent, though there are some points of interpretation which are still considered open. The state of public knowledge about the Teller–Ulam design has been mostly shaped from a few specific incidents outlined in a section below.

Basic principle


The basic principle of the Teller–Ulam configuration is the idea that different parts of a thermonuclear weapon can be chained together in "stages", with the detonation of each stage providing the energy to ignite the next stage. At a bare minimum, this implies a primary section which consists of a fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

 bomb (a "trigger"), and a secondary section which consists of fusion fuel. The energy released by the primary compresses the secondary through a process called "radiation implosion
Radiation implosion
The term radiation implosion describes the process behind a class of devices which use high levels of electromagnetic radiation to compress a target...

", at which point it is heated and undergoes nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

. Because of the staged design, it is thought that a tertiary section, again of fusion fuel, could be added as well, based on the same principle as the secondary; the AN602 "Tsar Bomba
Tsar Bomba
Tsar Bomba is the nickname for the AN602 hydrogen bomb, the most powerful nuclear weapon ever detonated. It was also referred to as Kuz'kina Mat , in this usage meaning "something that has not been seen before"....

" is thought to have been a three-stage device.


Surrounding the other components is a hohlraum
Hohlraum
In radiation thermodynamics, a hohlraum is a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity. This idealized cavity can be approximated in practice by making a small perforation in the wall of a hollow container of any opaque material...

 or radiation case, a container which traps the first stage or primary's energy inside temporarily. The outside of this radiation case, which is also normally the outside casing of the bomb, is the only direct visual evidence publicly available of any thermonuclear bomb component's configuration. Numerous photographs of various thermonuclear bomb exteriors have been declassified.

The primary is thought to be a standard implosion method fission bomb, though likely with a core boosted
Boosted fission weapon
A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released in the fission, as well as inducing the fission reactions...

 by small amounts of fusion fuel (usually 50/50% deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

/tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

 gas) for extra efficiency; the fusion fuel releases excess 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 when heated and compressed, inducing additional fission. Generally, a research program with the capacity to create a thermonuclear bomb has already mastered the ability to engineer boosted fission. When fired, the plutonium-239
Plutonium-239
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...

 (Pu-239) and/or uranium-235
Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...

 (U-235) core would be compressed to a smaller sphere by special layers of conventional high explosives arranged around it in an explosive lens
Explosive lens
An explosive lens—as used, for example, in nuclear weapons—is a highly specialized explosive charge, a special type of a shaped charge. In general, it is a device composed of several explosive charges that are shaped in such a way as to change the shape of the detonation wave passing through it,...

 pattern, initiating the nuclear chain reaction
Nuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...

 that powers the conventional "atomic bomb".

The secondary is usually shown as a column
Column
A column or pillar in architecture and structural engineering is a vertical structural element that transmits, through compression, the weight of the structure above to other structural elements below. For the purpose of wind or earthquake engineering, columns may be designed to resist lateral forces...

 of fusion fuel and other components wrapped in many layers. Around the column is first a "pusher-tamper", a heavy layer of uranium-238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...

 (U-238) or lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

 which serves to help compress the fusion fuel (and, in the case of uranium, may eventually undergo fission itself). Inside this is the fusion fuel itself, usually a form of lithium deuteride, which is used because it is easier to weaponize than liquified tritium/deuterium gas (compare the success of the cryogenic deuterium-based Ivy Mike
Ivy Mike
Ivy Mike was the codename given to the first United States test of a thermonuclear weapon, in which a major part of the explosive yield came from nuclear fusion. It was detonated on November 1, 1952 by the United States at on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy...

 experiment to the (over)success of the lithium deuteride-based Castle Bravo
Castle Bravo
Castle Bravo was the code name given to the first U.S. test of a dry fuel thermonuclear hydrogen bomb device, detonated on March 1, 1954 at Bikini Atoll, Marshall Islands, as the first test of Operation Castle. Castle Bravo was the most powerful nuclear device ever detonated by the United States ,...

 experiment). This dry fuel, when bombarded by 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, produces tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

, a heavy isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 which can undergo nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

, along with the deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 present in the mixture. (See the article on nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

 for a more detailed technical discussion of fusion reactions.) Inside the layer of fuel is the "spark plug", a hollow column of fissile material (plutonium-239 or uranium-235) which, when compressed, can itself undergo nuclear fission (because of the shape, it is not a 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...

 without compression). The tertiary, if one is present, would be set below the secondary and probably be made up of the same materials.

Separating the secondary from the primary is the interstage. The fissioning primary produces four types of energy: 1) expanding hot gases from high explosive charges which implode the primary; 2) superheated plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 that was originally the bomb's fissile material and its tamper; 3) the electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

; and 4) the 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 from the primary's nuclear detonation. The interstage is responsible for accurately modulating the transfer of energy from the primary to the secondary. It must direct the hot gases, plasma, electromagnetic radiation and neutrons toward the right place at the right time. Less than optimal interstage designs have resulted in the secondary failing to work entirely on multiple shots, known as a "fissile fizzle". The Koon
Castle Koon
The Koon shot of Operation Castle was a test of a University of California Radiation Laboratory designed thermonuclear device.The 'dry' two-stage device was known as "Morgenstern". It was tested on 7 April 1954. The predicted yield of Morgenstern, which had a highly innovative secondary stage, was...

 shot of Operation Castle
Operation Castle
Operation Castle was a United States series of high-energy nuclear tests by Joint Task Force SEVEN at Bikini Atoll beginning in March 1954...

 is a good example; a small flaw allowed the neutron flux from the primary to prematurely begin heating the secondary, weakening the compression enough to prevent any fusion.

There is very little detailed information in the open literature about the mechanism of the interstage. One of the best sources is a simplified diagram of a British thermonuclear weapon similar to the American W76
W76
The W76 is a United States thermonuclear warhead. It was manufactured from 1978-1987, and is still in service .The W-76 is carried inside a Mk-4 re-entry vehicle. U.S...

 warhead. It was released by Greenpeace
Greenpeace
Greenpeace is a non-governmental environmental organization with offices in over forty countries and with an international coordinating body in Amsterdam, The Netherlands...

 in a report titled "Dual Use Nuclear Technology". The major components and their arrangement are in the diagram, though details are almost absent; what scattered details it does include likely have intentional omissions and/or inaccuracies. They are labeled "End-cap and Neutron Focus Lens" and "Reflector Wrap"; the former channels neutrons to the U-235/Pu-239 Spark Plug while the latter refers to an X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

 reflector; typically a cylinder made out of an X-ray opaque material such as uranium with the primary and secondary at either end. It does not reflect like a mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

; instead, it gets heated to a high temperature by the X-ray flux from the primary, then it emits more evenly spread X-rays which travel to the secondary, causing what is known as radiation implosion
Radiation implosion
The term radiation implosion describes the process behind a class of devices which use high levels of electromagnetic radiation to compress a target...

. Next comes the "Reflector/Neutron Gun Carriage". The reflector seals the gap between the Neutron Focus Lens (in the center) and the outer casing near the primary. It separates the primary from the secondary and performs the same function as the previous reflector. There are about six neutron guns (seen here from Sandia National Laboratories
Sandia National Laboratories
The Sandia National Laboratories, managed and operated by the Sandia Corporation , are two major United States Department of Energy research and development national laboratories....

) each poking through the outer edge of the reflector with one end in each section; all are clamped to the carriage and arranged more or less evenly around the casing's circumference. The neutron guns are tilted so the neutron emitting end of each gun end is pointed towards the central axis of the bomb. Neutrons from each neutron gun pass through and are focussed by the neutron focus lens towards the centre of primary in order to boost the initial fissioning of the plutonium. A "Polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...

 Polarizer/Plasma Source" is also shown (see below).

The first U.S. government document to mention the interstage was only recently released to the public promoting the 2004 initiation of the Reliable Replacement Warhead
Reliable Replacement Warhead
The Reliable Replacement Warhead was a proposed new American nuclear warhead design and bomb family that was intended to be simple, reliable and to provide a long-lasting, low maintenance future nuclear force for the United States...

 Program. A graphic includes blurbs describing the potential advantage of a RRW on a part by part level, with the interstage blurb saying a new design would replace "toxic, brittle material" and "expensive 'special' material... [which require] unique facilities". The "toxic, brittle material" is widely assumed to be beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...

, which fits that description and would also moderate the neutron flux
Neutron flux
The neutron flux is a quantity used in reactor physics corresponding to the total length travelled by all neutrons per unit time and volume . The neutron fluence is defined as the neutron flux integrated over a certain time period....

 from the primary. Some material to absorb and re-radiate the X-rays in a particular manner may also be used.
The "special material" is thought to be a substance called "FOGBANK
Fogbank
FOGBANK is a code name given to a material used in nuclear weapons such as the W76, W78 and W80.FOGBANK's precise nature is classified; in the words of former Oak Ridge general manager Dennis Ruddy, "The material is classified. Its composition is classified...

", an unclassified codename, though it is often referred to as "THE fogbank" (or "A Fogbank") as if it were a subassembly instead of a material. Its composition is classified, though aerogel
Aerogel
Aerogel is a synthetic porous material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and thermal conductivity...

 has been suggested as a possibility. Manufacture stopped for many years; however, the Life Extension Program required it to start up again – Y-12
Y-12 National Security Complex
The Y-12 National Security Complex is a United States Department of Energy National Nuclear Security Administration facility located in Oak Ridge, Tennessee, near the Oak Ridge National Laboratory...

 currently being the sole producer (the "unique facility" referenced). Manufacture involves the moderately toxic and moderately volatile solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

 called acetonitrile
Acetonitrile
Acetonitrile is the chemical compound with formula . This colourless liquid is the simplest organic nitrile. It is produced mainly as a byproduct of acrylonitrile manufacture...

, which presents a hazard for workers (causing three evacuations in March 2006 alone).

Summary


A simplified summary of the above explanation would be:
  1. An implosion assembly type of fission bomb is exploded. This is the primary stage. If a small amount of deuterium
    Deuterium
    Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

    /tritium
    Tritium
    Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

     gas is placed inside the primary's core, it will be compressed during the explosion and a nuclear fusion
    Nuclear fusion
    Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...

     reaction will occur; the released neutrons from this fusion reaction will induce further fission in the plutonium-239
    Plutonium-239
    Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...

     or uranium-235
    Uranium-235
    - References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...

     used in the primary stage. The use of fusion fuel to enhance the efficiency of a fission reaction is called boosting
    Boosted fission weapon
    A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released in the fission, as well as inducing the fission reactions...

    . Without boosting, a large portion of the fissile material will remain unreacted; the Little Boy
    Little Boy
    "Little Boy" was the codename of the atomic bomb dropped on Hiroshima on August 6, 1945 by the Boeing B-29 Superfortress Enola Gay, piloted by Colonel Paul Tibbets of the 393rd Bombardment Squadron, Heavy, of the United States Army Air Forces. It was the first atomic bomb to be used as a weapon...

     and Fat Man
    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...

     bombs had an efficiency of only 1.4% and 17%, respectively, because they were unboosted.
  2. Energy released in the primary stage is transferred to the secondary (or fusion) stage. The exact mechanism whereby this happens is unknown. This energy compresses the fusion fuel and sparkplug; the compressed sparkplug becomes critical and undergoes a fission chain reaction, further heating the compressed fusion fuel to a high enough temperature to induce fusion, and also supplying neutrons that react with lithium
    Lithium
    Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

     to create tritium for fusion. Generally, increasing the kinetic energy of gas molecules contained in a limited volume will increase both temperature and pressure (see gas laws
    Gas laws
    The early gas laws were developed at the end of the 18th century, when scientists began to realize that relationships between the pressure, volume and temperature of a sample of gas could be obtained which would hold for all gases...

    ).
  3. The fusion fuel of the secondary stage may be surrounded by depleted uranium
    Depleted uranium
    Depleted uranium is uranium with a lower content of the fissile isotope U-235 than natural uranium . Uses of DU take advantage of its very high density of 19.1 g/cm3...

     or natural uranium
    Natural uranium
    Natural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight. In terms of the amount of radioactivity, approximately 2.2 % comes from uranium-235, 48.6 % uranium-238, and 49.2 %...

    , whose U-238
    Uranium-238
    Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...

     is not fissile
    Fissile
    In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...

     and cannot sustain a chain reaction
    Chain reaction
    A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....

    , but which is fissionable
    Fissile
    In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...

     when bombarded by the high-energy neutrons
    Neutron temperature
    The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is...

     released by fusion in the secondary stage.


Thermonuclear weapons may or may not use a boosted primary stage, use different types of fusion fuel, and may surround the fusion fuel with beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...

 (or another neutron reflecting material
Neutron reflector
A neutron reflector is any material that reflects neutrons. This refers to elastic scattering rather than to a specular reflection. The material may be graphite, beryllium, steel, and tungsten carbide, or other materials...

) instead of depleted uranium to prevent further fission from occurring.

The remaining secret: how the secondary is compressed


The basic idea of the Teller–Ulam configuration is that each "stage" would undergo fission or fusion (or both) and release energy, much of which would be transferred to another stage to trigger it. How exactly the energy is "transported" from the primary to the secondary has been the subject of some disagreement in the open press, but is thought to be transmitted through the X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s which are emitted from the fissioning primary. This energy is then used to compress the secondary. The crucial detail of how the X-rays create the pressure is the main remaining disputed point in the unclassified press. There are five proposed theories:
  • Neutron pressure from the primary explosion. This was allegedly Ulam's first concept and was abandoned as unworkable.
  • Blast wave
    Blast wave
    A blast wave in fluid dynamics is the pressure and flow resulting from the deposition of a large amount of energy in a small very localised volume. The flow field can be approximated as a lead shock wave, followed by a 'self-similar' subsonic flow field. In simpler terms, a blast wave is an area of...

     from the primary explosion. This was allegedly Ulam's second concept and was abandoned as unworkable.
  • Radiation pressure
    Radiation pressure
    Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...

     exerted by the X-rays. This was the first idea put forth by Howard Morland
    Howard Morland
    Howard Morland is a United States journalist and activist against nuclear weapons who in 1979 became famous for apparently discovering the "secret" of the hydrogen bomb and publishing it after a lengthy censorship attempt by the Department of Energy...

     in the article in The Progressive
    The Progressive
    The Progressive is an American monthly magazine of politics, culture and progressivism with a pronounced liberal perspective on some issues. Known for its pacifism, it has strongly opposed military interventions, such as the US-led invasion of Iraq in 2003. The magazine also devotes much coverage...

    .
  • X-rays creating a plasma
    Plasma (physics)
    In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

     in the radiation case's filler (a polystyrene
    Polystyrene
    Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...

     or "FOGBANK
    Fogbank
    FOGBANK is a code name given to a material used in nuclear weapons such as the W76, W78 and W80.FOGBANK's precise nature is classified; in the words of former Oak Ridge general manager Dennis Ruddy, "The material is classified. Its composition is classified...

    " plastic foam). This was a second idea put forward by Chuck Hansen
    Chuck Hansen
    Chuck Hansen compiled, over a period of 30 years, the world's largest private collection of documents on how America developed the atomic bomb. These documents were obtained through the U.S...

     and later by Howard Morland.
  • Tamper
    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...

    /Pusher ablation
    Ablation
    Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...

    .

Radiation pressure


The radiation pressure
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...

 exerted by the large quantity of X-ray photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s inside the closed casing might be enough to compress the secondary. For two thermonuclear bombs for which the general size and primary characteristics are well understood, the Ivy Mike test bomb and the modern W-80 cruise missile warhead variant of the W-61 design, the radiation pressure was calculated to be 73 million bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

 (atmospheres) (7.3 T Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

) for the Ivy Mike design and 1,400 million bar (140 TPa) for the W-80.

Foam plasma pressure


Foam
Foam
-Definition:A foam is a substance that is formed by trapping gas in a liquid or solid in a divided form, i.e. by forming gas regions inside liquid regions, leading to different kinds of dispersed media...

 plasma pressure is the concept which Chuck Hansen introduced during the Progressive case, based on research which located declassified documents listing special foams as liner components within the radiation case of thermonuclear weapons.

The sequence of firing the weapon (with the foam) would be as follows:
  1. The high explosives surrounding the core of the primary fire, compressing the fissile material into a supercritical
    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...

     state and beginning the fission chain reaction
    Chain reaction
    A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....

    .
  2. The fissioning primary emits X-ray
    X-ray
    X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

    s, which "reflect" along the inside of the casing, irradiating the polystyrene foam.
  3. The irradiated foam undergoes a phase transition
    Phase transition
    A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....

    , becoming a hot plasma
    Plasma (physics)
    In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

    , pushing against the tamper of the secondary, compressing it tightly, and beginning the fission reaction in the spark plug.
  4. Pushed from both sides (from the primary and the spark plug), the lithium deuteride fuel is highly compressed and heated to thermonuclear temperatures. Also, by being bombarded with neutrons, each lithium
    Lithium
    Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...

    -6 atom splits into one tritium
    Tritium
    Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

     atom and one alpha particle
    Alpha particle
    Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...

    . Then begins a fusion reaction between the tritium and the deuterium, releasing even more neutrons, and a huge amount of energy.
  5. The fuel undergoing the fusion reaction emits a large flux of neutrons
    Neutron flux
    The neutron flux is a quantity used in reactor physics corresponding to the total length travelled by all neutrons per unit time and volume . The neutron fluence is defined as the neutron flux integrated over a certain time period....

    , which irradiates the U-238
    Uranium-238
    Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...

     tamper (or the U-238 bomb casing), causing it to undergo a fission reaction, providing about half of the total energy.


This would complete the fission-fusion-fission sequence. Fusion, unlike fission, is relatively "clean"—it releases energy but no harmful radioactive products or large amounts of nuclear fallout
Nuclear fallout
Fallout is the residual radioactive material propelled into the upper atmosphere following a nuclear blast, so called because it "falls out" of the sky after the explosion and shock wave have passed. It commonly refers to the radioactive dust and ash created when a nuclear weapon explodes...

. The fission reactions though, especially the last fission reaction, release a tremendous amount of fission products and fallout. If the last fission stage is omitted, by replacing the uranium tamper with one made of lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

, for example, the overall explosive force is reduced by approximately half but the amount of fallout is relatively low. The neutron bomb
Neutron bomb
A neutron bomb or enhanced radiation weapon or weapon of reinforced radiation is a type of thermonuclear weapon designed specifically to release a large portion of its energy as energetic neutron radiation rather than explosive energy...

 is a hydrogen bomb without the final fission stage.

Current technical criticisms of the idea of "foam plasma pressure" focus on unclassified analysis from similar high energy physics fields which indicate that the pressure produced by such a plasma would only be a small multiplier of the basic photon pressure within the radiation case, and also that the known foam materials intrinsically have a very low absorption efficiency of the gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...

 and X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

 radiation from the primary. Most of the energy produced would be absorbed by either the walls of the radiation case and/or the tamper around the secondary. Analyzing the effects of that absorbed energy led to the third mechanism: ablation
Ablation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...

.

Tamper-pusher ablation


The tamper-pusher ablation proposed mechanism is that the primary compression mechanism for the thermonuclear secondary is that the outer layers of the tamper-pusher, or heavy metal casing around the thermonuclear fuel, are heated so much by the X-ray flux from the primary that they ablate
Ablation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...

 away, exploding outwards at such high speed that the rest of the tamper recoils inwards at a tremendous velocity, crushing the fusion fuel and the spark plug.

Rough calculations for the basic ablation effect are relatively simple: the energy from the primary is distributed evenly onto all of the surfaces within the outer radiation case, with the components coming to a thermal equilibrium
Thermal equilibrium
Thermal equilibrium is a theoretical physical concept, used especially in theoretical texts, that means that all temperatures of interest are unchanging in time and uniform in space...

, and the effects of that thermal energy are then analyzed. The energy is mostly deposited within about one X-ray optical thickness
Optical depth
Optical depth, or optical thickness, is a measure of transparency. Optical depth is defined by the negative logarithm of the fraction of radiation that is not scattered or absorbed on a path...

 of the tamper/pusher outer surface, and the temperature of that layer can then be calculated. The velocity at which the surface then expands outwards is calculated and, from a basic Newtonian momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...

 balance, the velocity at which the rest of the tamper implodes inwards.

Applying the more detailed form of those calculations to the Ivy Mike
Ivy Mike
Ivy Mike was the codename given to the first United States test of a thermonuclear weapon, in which a major part of the explosive yield came from nuclear fusion. It was detonated on November 1, 1952 by the United States at on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy...

 device yields vaporized pusher gas expansion velocity of 290 kilometers per second and an implosion velocity of perhaps 400 kilometers per second if 3/4 of the total tamper/pusher mass is ablated off, the most energy efficient proportion. For the W-80 the gas expansion velocity is roughly 410 kilometers per second and the implosion velocity 570 kilometers per second. The pressure due to the ablating material is calculated to be 5.3 billion bar (530 T Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

) in the Ivy Mike device and 64 billion bar (6.4 P Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

) in the W-80 device.

Comparing the implosion mechanisms


Comparing the three mechanisms proposed, it can be seen that:
Mechanism TPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

)
Ivy Mike
Ivy Mike
Ivy Mike was the codename given to the first United States test of a thermonuclear weapon, in which a major part of the explosive yield came from nuclear fusion. It was detonated on November 1, 1952 by the United States at on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy...

 
W80
radiation pressure 7.3 140
plasma pressure 35 750
ablation pressure 530 6400


The calculated ablation pressure is one order of magnitude greater than the higher proposed plasma pressures and nearly two orders of magnitude greater than calculated radiation pressure. No mechanism to avoid the absorption of energy into the radiation case wall and the secondary tamper has been suggested, making ablation apparently unavoidable. The other mechanisms appear to be unneeded.

United States Department of Defense
United States Department of Defense
The United States Department of Defense is the U.S...

 official declassification reports indicate that foamed plastic materials are or may be used in radiation case liners, and despite the low direct plasma pressure they may be of use in delaying the ablation
Ablation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...

 until energy has distributed evenly and a sufficient fraction has reached the secondary's tamper/pusher.

Richard Rhodes
Richard Rhodes
Richard Lee Rhodes is an American journalist, historian, and author of both fiction and non-fiction , including the Pulitzer Prize-winning The Making of the Atomic Bomb , and most recently, The Twilight of the Bombs...

' book Dark Sun stated that a 1 inches (25.4 mm) layer of plastic foam was fixed to the lead liner of the inside of the Ivy Mike
Ivy Mike
Ivy Mike was the codename given to the first United States test of a thermonuclear weapon, in which a major part of the explosive yield came from nuclear fusion. It was detonated on November 1, 1952 by the United States at on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy...

 steel casing using copper nails. Rhodes quotes several designers of that bomb explaining that the plastic foam layer inside the outer case is to delay ablation and thus recoil of the outer case: if the foam were not there, metal would ablate from the inside of the outer case with a large impulse, causing the casing to recoil outwards rapidly. The purpose of the casing is to contain the explosion for as long as possible, allowing as much X-ray ablation of the metallic surface of the secondary stage as possible, so it compresses the secondary efficiently, maximizing the fusion yield. Plastic foam has a low density, so causes a smaller impulse when it ablates than metal does.

Design variations


A number of possible variations to the weapon design have been proposed:
  • Either the tamper or the casing have been proposed as being made of uranium-238 for the final fission stage.
  • In some descriptions, additional internal structures exist to protect the secondary from receiving excessive neutrons from the primary.
  • The inside of the casing may or may not be specially machined to "reflect" the X-rays. X-ray "reflection" is not like light reflecting off of a mirror
    Mirror
    A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

    , but rather the reflector material is heated by the X-rays, causing the material itself to emit X-rays, which then travel to the secondary.


Two special variations exist which will be discussed in a further section: the cryogenically
Cryogenics
In physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...

 cooled liquid deuterium device used for the Ivy Mike
Ivy Mike
Ivy Mike was the codename given to the first United States test of a thermonuclear weapon, in which a major part of the explosive yield came from nuclear fusion. It was detonated on November 1, 1952 by the United States at on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy...

 test, and the putative design of the W88
W88
The W88 is a United States thermonuclear warhead, with an estimated yield of 475 kiloton , and is small enough to fit on MIRVed missiles. The W88 was designed at the Los Alamos National Laboratory in the 1970s. In 1999 the director of Los Alamos who had presided over its design described it as...

 nuclear warhead — a small, MIRVed
Multiple independently targetable reentry vehicle
A multiple independently targetable reentry vehicle warhead is a collection of nuclear weapons carried on a single intercontinental ballistic missile or a submarine-launched ballistic missile . Using a MIRV warhead, a single launched missile can strike several targets, or fewer targets redundantly...

 version of the Teller–Ulam configuration with a prolate
Spheroid
A spheroid, or ellipsoid of revolution is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters....

 (egg
Oval
An oval is any curve resembling an egg or an ellipse, such as a Cassini oval. The term does not have a precise mathematical definition except in one area oval , but it may also refer to:* A sporting arena of oval shape** a cricket field...

 or watermelon
Watermelon
Watermelon is a vine-like flowering plant originally from southern Africa. Its fruit, which is also called watermelon, is a special kind referred to by botanists as a pepo, a berry which has a thick rind and fleshy center...

 shaped) primary and an elliptical secondary. Most bombs do not apparently have tertiary stages — the U.S. is only thought to have produced one such model, the heavy but highly efficient (i.e., nuclear weapon yield
Nuclear weapon yield
The explosive yield of a nuclear weapon is the amount of energy discharged when a nuclear weapon is detonated, expressed usually in the equivalent mass of trinitrotoluene , either in kilotons or megatons , but sometimes also in terajoules...

 per unit bomb weight) 25 Mt B41 nuclear bomb
B41 nuclear bomb
The B41 was a thermonuclear weapon deployed by the United States Strategic Air Command in the early 1960s. It was the most powerful nuclear bomb ever developed by the United States with a theoretical maximum yield of 25 megatons...

, and the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 is thought to have used multiple stages in their 50 megaton (100 Mt in use) Tsar Bomba
Tsar Bomba
Tsar Bomba is the nickname for the AN602 hydrogen bomb, the most powerful nuclear weapon ever detonated. It was also referred to as Kuz'kina Mat , in this usage meaning "something that has not been seen before"....

. If any hydrogen bombs have been made from configurations other than those based on the Teller–Ulam design, the fact of it is not publicly known, with the possible exception of the Soviet early Sloika design.

In essence, the Teller–Ulam configuration relies on at least two instances of implosion occurring: first, the conventional (chemical) explosives in the primary would compress the fissile core, resulting in a fission explosion many times more powerful than that which chemical explosives could achieve alone. Second, the radiation from the fissioning of the primary would be used to compress and ignite the secondary, resulting in a fusion explosion many times more powerful than the fission explosion alone. This chain of compression could then be continued with an arbitrary number of secondaries, and would end with the fissioning of the natural uranium
Natural uranium
Natural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight. In terms of the amount of radioactivity, approximately 2.2 % comes from uranium-235, 48.6 % uranium-238, and 49.2 %...

 tamper, something which could not normally be achieved without the neutron flux
Neutron flux
The neutron flux is a quantity used in reactor physics corresponding to the total length travelled by all neutrons per unit time and volume . The neutron fluence is defined as the neutron flux integrated over a certain time period....

 provided by the fusion reactions in the secondary. Such a design can be scaled up to an arbitrary strength, potentially to the level of a "doomsday device
Doomsday device
A doomsday device is a hypothetical construction — usually a weapon, or collection of weapons — which could destroy all life on a planet, particularly the Earth, or destroy the planet itself, bringing "doomsday", a term used for the end of planet Earth...

", but usually such weapons are not more than a dozen megatons, which is generally considered enough to destroy even the largest practical targets.

American developments


The idea of a thermonuclear fusion bomb ignited by a smaller fission bomb was first proposed by Enrico Fermi
Enrico Fermi
Enrico Fermi was an Italian-born, naturalized American physicist particularly known for his work on the development of the first nuclear reactor, Chicago Pile-1, and for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics...

 to his colleague Edward Teller
Edward Teller
Edward Teller was a Hungarian-American theoretical physicist, known colloquially as "the father of the hydrogen bomb," even though he did not care for the title. Teller made numerous contributions to nuclear and molecular physics, spectroscopy , and surface physics...

 in 1941 at the start of what would become the Manhattan Project
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...

. Teller spent most of the Manhattan Project attempting to figure out how to make the design work, to some degree neglecting his assigned work on the Manhattan Project fission bomb program. His difficult and devil's advocate
Devil's advocate
In common parlance, a devil's advocate is someone who, given a certain argument, takes a position he or she does not necessarily agree with, just for the sake of argument. In taking such position, the individual taking on the devil's advocate role seeks to engage others in an argumentative...

 attitude in discussions led Oppenheimer to sidetrack him and other "problem" physicists into the super program to smooth his way.
Stanislaw Ulam, a coworker of Teller's, made the first key conceptual leaps towards a workable fusion design. Ulam's two innovations which rendered the fusion bomb practical were that compression of the thermonuclear fuel before extreme heating was a practical path towards the conditions needed for fusion, and the idea of staging or placing a separate thermonuclear component outside a fission primary component, and somehow using the primary to compress the secondary. Teller then realized that the gamma and X-ray radiation produced in the primary could transfer enough energy into the secondary to create a successful implosion and fusion burn, if the whole assembly was wrapped in a hohlraum
Hohlraum
In radiation thermodynamics, a hohlraum is a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity. This idealized cavity can be approximated in practice by making a small perforation in the wall of a hollow container of any opaque material...

or radiation case. Teller and his various proponents and detractors later disputed the degree to which Ulam had contributed to the theories underlying this mechanism. Indeed, shortly before his death, and in a last-ditch effort to discredit Ulam's contributions, Teller claimed that one of his own "graduate students" had proposed the mechanism.

The "George" shot of Operation Greenhouse
Operation Greenhouse
Operation Greenhouse was the fifth American nuclear test series, the second conducted in 1951 and the first to test principles that would lead to developing thermonuclear weapons . Conducted at the new Pacific Proving Ground, all of the devices were mounted in large steel towers, to simulate air...

 in 1951 tested the basic concept for the first time on a very small scale, raising expectations to a near certainty that the concept would work.

On November 1, 1952, the Teller–Ulam configuration was tested at full scale in the "Ivy Mike
Ivy Mike
Ivy Mike was the codename given to the first United States test of a thermonuclear weapon, in which a major part of the explosive yield came from nuclear fusion. It was detonated on November 1, 1952 by the United States at on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy...

" shot at an island in the Enewetak Atoll, with a yield of 10.4 megatons
TNT equivalent
TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...

 (over 450 times more powerful than the bomb dropped on Nagasaki during 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...

). The device, dubbed the Sausage, used an extra-large fission bomb as a "trigger" and liquid deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

—kept in its liquid state by 20 short ton
Short ton
The short ton is a unit of mass equal to . In the United States it is often called simply ton without distinguishing it from the metric ton or the long ton ; rather, the other two are specifically noted. There are, however, some U.S...

s (18 metric tons) of cryogenic
Cryogenics
In physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...

 equipment—as its fusion fuel, and weighed around 80 short tons (70 metric tons) altogether.

The liquid deuterium fuel of Ivy Mike was impractical for a deployable weapon, and the next advance was to use a solid lithium deuteride
Lithium hydride
Lithium hydride is the inorganic compound with the formula LiH. It is a colorless solid, although commercial samples are gray. Characteristic of a salt-like, or ionic, hydride, it has a high melting point and is not soluble in any solvent with which it does not react...

 fusion fuel instead. In 1954 this was tested in the "Castle Bravo
Castle Bravo
Castle Bravo was the code name given to the first U.S. test of a dry fuel thermonuclear hydrogen bomb device, detonated on March 1, 1954 at Bikini Atoll, Marshall Islands, as the first test of Operation Castle. Castle Bravo was the most powerful nuclear device ever detonated by the United States ,...

" shot (the device was code-named the Shrimp), which had a yield of 15 megatons (2.5 times higher than expected) and is the largest U.S. bomb ever tested.

Efforts in the United States soon shifted towards developing miniaturized Teller–Ulam weapons which could easily outfit intercontinental ballistic missile
Intercontinental ballistic missile
An intercontinental ballistic missile is a ballistic missile with a long range typically designed for nuclear weapons delivery...

s and submarine-launched ballistic missile
Submarine-launched ballistic missile
A submarine-launched ballistic missile is a ballistic missile capable of delivering a nuclear warhead that can be launched from submarines. Modern variants usually deliver multiple independently targetable reentry vehicles each of which carries a warhead and allows a single launched missile to...

s. By 1960, with the W47
W47
The W47 was an American thermonuclear warhead used on the Polaris A-1 sub-launched ballistic missile system. Various models were in service from 1960 through the end of 1974...

 warhead deployed on Polaris
UGM-27 Polaris
The Polaris missile was a two-stage solid-fuel nuclear-armed submarine-launched ballistic missile built during the Cold War by Lockheed Corporation of California for the United States Navy....

 ballistic missile submarine
Ballistic missile submarine
A ballistic missile submarine is a submarine equipped to launch ballistic missiles .-Description:Ballistic missile submarines are larger than any other type of submarine, in order to accommodate SLBMs such as the Russian R-29 or the American Trident...

s, megaton-class warheads were as small as 18 inches (0.5 m) in diameter and 720 pounds (320 kg) in weight. It was later found in live testing that the Polaris warhead did not work reliably and had to be redesigned. Further innovation in miniaturizing warheads was accomplished by the mid-1970s, when versions of the Teller–Ulam design were created which could fit ten or more warheads on the end of a small MIRVed
Multiple independently targetable reentry vehicle
A multiple independently targetable reentry vehicle warhead is a collection of nuclear weapons carried on a single intercontinental ballistic missile or a submarine-launched ballistic missile . Using a MIRV warhead, a single launched missile can strike several targets, or fewer targets redundantly...

 missile (see the section on the W88 below).

Soviet developments




The first Soviet fusion design, developed by Andrei Sakharov
Andrei Sakharov
Andrei Dmitrievich Sakharov was a Soviet nuclear physicist, dissident and human rights activist. He earned renown as the designer of the Soviet Union's Third Idea, a codename for Soviet development of thermonuclear weapons. Sakharov was an advocate of civil liberties and civil reforms in the...

 and Vitaly Ginzburg
Vitaly Ginzburg
Vitaly Lazarevich Ginzburg ForMemRS was a Soviet theoretical physicist, astrophysicist, Nobel laureate, a member of the Russian Academy of Sciences and one of the fathers of Soviet hydrogen bomb...

 in 1949 (before the Soviets had a working fission bomb), was dubbed the Sloika, after a Russian layer cake
Layer cake
A layer cake is a cake consisting of multiple layers, usually held together by frosting or another type of filling, such as jam or other preserves. Most cake recipes can be made into layer cakes; butter cakes and sponge cakes are common choices...

, and was not of the Teller–Ulam configuration. It used alternating layers of fissile material and lithium deuteride fusion fuel spiked with tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

 (this was later dubbed Sakharov's "First Idea"). Though nuclear fusion might have been technically achievable, it did not have the scaling property of a "staged" weapon. Thus, such a design could not produce thermonuclear weapons whose explosive yields could be made arbitrarily large (unlike U.S. designs at that time). The fusion layer wrapped around the fission core could only moderately multiply the fission energy (modern Teller–Ulam designs can multiply it 30-fold). Additionally, the whole fusion stage had to be imploded by conventional explosives, along with the fission core, multiplying the bulk of chemical explosives needed substantially.

Their first Sloika design test, Joe 4
Joe 4
Joe 4 was an American nickname for the first Soviet test of a thermonuclear weapon on August 12, 1953. It utilized a scheme in which fission and fusion fuel were "layered", a design known as the Sloika model in the Soviet Union...

, was detonated in 1953 with a yield equivalent to 400 kilotons of TNT
TNT equivalent
TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...

 (15%–20% from fusion).
Attempts to use a Sloika design to achieve megaton-range results proved unfeasible. After the U.S. tested the "Ivy Mike
Ivy Mike
Ivy Mike was the codename given to the first United States test of a thermonuclear weapon, in which a major part of the explosive yield came from nuclear fusion. It was detonated on November 1, 1952 by the United States at on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy...

" bomb in November 1952, proving that a multimegaton bomb could be created, the Soviets searched for an additional design. The "Second Idea", as Sakharov referred to it in his memoirs, was a previous proposal by Ginzburg in November 1948 to use lithium deuteride in the bomb, which would, in the course of being bombarded by neutrons, produce tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

 and free deuterium. In late 1953 physicist Viktor Davidenko achieved the first breakthrough, that of keeping the primary and secondary parts of the bombs in separate pieces ("staging"). The next breakthrough was discovered and developed by Sakharov and Yakov Zel'dovich
Yakov Borisovich Zel'dovich
Yakov Borisovich Zel'dovich was a prolific Soviet physicist born in Belarus. He played an important role in the development of Soviet nuclear and thermonuclear weapons, and made important contributions to the fields of adsorption and catalysis, shock waves, nuclear physics, particle physics,...

, that of using the X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s from the fission bomb to compress the secondary before fusion ("radiation implosion"), in early 1954. Sakharov's "Third Idea", as the Teller–Ulam design was known in the USSR, was tested in the shot "RDS-37
RDS-37
RDS-37 was the Soviet Union's first "true" hydrogen bomb, first tested on November 22, 1955. The weapon had a nominal yield of approximately 3 megatons. It was scaled down to 1.6 megatons for the live test....

" in November 1955 with a yield of 1.6 megatons.

The Soviets demonstrated the power of the "staging" concept in October 1961, when they detonated the massive and unwieldy Tsar Bomba
Tsar Bomba
Tsar Bomba is the nickname for the AN602 hydrogen bomb, the most powerful nuclear weapon ever detonated. It was also referred to as Kuz'kina Mat , in this usage meaning "something that has not been seen before"....

, a 50 megaton hydrogen bomb that derived almost 97% of its energy from fusion. It was the largest nuclear weapon developed and tested by any country, but was far too large for military purpose to use as a weapon – even though it was dropped from an airplane
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...

 in its lone test over Novaya Zemlya
Novaya Zemlya
Novaya Zemlya , also known in Dutch as Nova Zembla and in Norwegian as , is an archipelago in the Arctic Ocean in the north of Russia and the extreme northeast of Europe, the easternmost point of Europe lying at Cape Flissingsky on the northern island...

.

British developments



In 1954 work began at Aldermaston
Atomic Weapons Establishment
The Atomic Weapons Establishment is responsible for the design, manufacture and support of warheads for the United Kingdom's nuclear deterrent. AWE plc is responsible for the day-to-day operations of AWE...

 to develop the British fusion bomb, with Sir William Penney
William Penney, Baron Penney
William George Penney, Baron Penney OM, KBE PhD, DSc, , FRS, FRSE, FIC, Hon FCGI was a British mathematician who was responsible for the development of British nuclear technology, following World War II...

 in charge of the project. British knowledge on how to make a thermonuclear fusion bomb was rudimentary, and at the time the United States was not exchanging any nuclear knowledge because of the Atomic Energy Act of 1946
Atomic Energy Act of 1946
The Atomic Energy Act of 1946 determined how the United States federal government would control and manage the nuclear technology it had jointly developed with its wartime allies...

. However, the British were allowed to observe the American Castle tests
Operation Castle
Operation Castle was a United States series of high-energy nuclear tests by Joint Task Force SEVEN at Bikini Atoll beginning in March 1954...

 and used sampling aircraft in the mushroom cloud
Mushroom cloud
A mushroom cloud is a distinctive pyrocumulus mushroom-shaped cloud of condensed water vapor or debris resulting from a very large explosion. They are most commonly associated with nuclear explosions, but any sufficiently large blast will produce the same sort of effect. They can be caused by...

s, providing them with clear, direct evidence of the compression produced in the secondary stages by radiation implosion.

Because of these difficulties, in 1955 British prime minister Anthony Eden
Anthony Eden
Robert Anthony Eden, 1st Earl of Avon, KG, MC, PC was a British Conservative politician, who was Prime Minister from 1955 to 1957...

 agreed to a secret plan, whereby if the Aldermaston scientists failed or were greatly delayed in developing the fusion bomb, it would be replaced by an extremely large fission bomb.

In 1957 the Operation Grapple
Operation Grapple
Operation Grapple, and operations Grapple X, Grapple Y and Grapple Z, were the names of British nuclear tests of the hydrogen bomb. They were held 1956—1958 at Malden Island and Christmas Island in the central Pacific Ocean. Nine nuclear detonations took place during the trials, resulting in...

 tests were carried out. The first test, Green Granite was a prototype fusion bomb, but failed to produce equivalent yields compared to the Americans and Soviets, only achieving approximately 300 kilotons. The second test Orange Herald
Orange Herald
- Technical :Orange Herald was a fusion boosted British fission nuclear weapon, comprising a U-235 primary surrounded by lithium deuteride. 'Herald' was suitable for mounting on a missile, utilizing 117 kg of U-235...

was the modified fission bomb and produced 700 kilotons—making it the largest fission explosion ever. At the time almost everyone (including the pilots of the plane that dropped it) thought that this was a fusion bomb. This bomb was put into service in 1958. A second prototype fusion bomb Purple Granite was used in the third test, but only produced approximately 150 kilotons.

A second set of tests was scheduled, with testing recommencing in September 1957. The first test was based on a "… new simpler design. A two stage thermonuclear bomb which had a much more powerful trigger". This test Grapple X Round C was exploded on November 8 and yielded approximately 1.8 megatons. On April 28, 1958 a bomb was dropped that yielded 3 megatons—Britain's most powerful test. Two final air burst tests on September 2 and September 11, 1958, dropped smaller bombs that yielded around 1 megaton each.

American observers had been invited to these kinds of tests. After their successful detonation of a megaton-range device (and thus demonstrating their practical understanding of the Teller–Ulam design "secret"), the United States agreed to exchange some of their nuclear designs with the United Kingdom, leading to the 1958 US–UK Mutual Defence Agreement. Instead of continuing with their own design, the British were given access to the design of the smaller American Mk 28 warhead
B28 nuclear bomb
The B28, originally Mark 28, was a thermonuclear bomb carried by U.S. tactical fighter bombers and bomber aircraft. From 1962 to 1972 under the NATO nuclear weapons sharing program, American B28s also equipped six Europe-based Canadian CF-104 squadrons known as the RCAF Nuclear Strike Force...

 and were able to manufacture copies.

Chinese developments



The People's Republic of China detonated its first device using a Teller–Ulam design June 17, 1967 ("Test No. 6"), a mere 32 months after detonating its first fission weapon (the shortest fission-to-fusion development in history), with a yield of 3.31 Mt.

French developments



Very little is known about the French development of the Teller–Ulam design beyond the fact that they detonated a 2.6 Mt device in the "Canopus
Canopus (nuclear test)
Canopus was the code name for France's first two-stage thermonuclear test, conducted on August 24, 1968 at Fangataufa atoll...

" test in August 1968.

Israel



Israel is alleged to possess thermonuclear weapons of the Teller–Ulam design, but is not known to have tested any nuclear devices.

India




India's first nuclear test occurred on May 18, 1974, which initially surprised the world. The first test, codename Smiling Buddha
Smiling Buddha
The Smiling Buddha, formally designated as Pokhran-I, was the codename given to Republic of India's first nuclear test explosion that took place at the long-constructed Indian Army base, Pokhran Test Range at Pokhran municipality, Rajasthan state on 18 May 1974 at 8:05 a.m....

, was not a thermonuclear device according to the Bhabha Atomic Research Centre
Bhabha Atomic Research Centre
The Bhabha Atomic Research Centre is India's primary nuclear research facility based in Mumbai. It has a number of nuclear reactors, all of which are used for India's nuclear power and research programme.- History :...

. On May 11, 1998, India reportedly detonated a thermonuclear bomb in its Operation Shakti
Pokhran-II
Pokharan-II refers to test explosions of five nuclear devices, three on 11 May and two on 13 May 1998, conducted by India at the Pokhran test range. These nuclear tests resulted in a variety of sanctions against India by a number of major states....

 tests ("Shakti-1", specifically). Dr. Samar Mubarakmand
Samar Mubarakmand
Samar Mubarakmand , , is a Pakistani nuclear physicist, who served as the founding chairman of National Engineering and Scientific Commission from 2001 till 2007. Samar Mubarak-Mand launched the Missile Integration Programme in 1987 which was successfully completed in 2005...

 asserted that Shakti-1 was a successful test, but if it was a thermonuclear device as claimed, then it failed to produce certain results that were to be expected of a thermonuclear device.

Director for the 1998 test site preparations, Dr. K. Santhanam, reported the yield of thermonuclear explosion was lower than expected, although his statement has been disputed by other Indian scientists involved in the test. Indian sources, using local data and citing a US Geologic Survey report compiling seismic data from 125 IRIS stations across the world, argue that the magnitudes suggested a combined yield of up to 60 kilotonnes, consistent with the Indian announced total yield of 56 kilotonnes However, several independent experts have reported lower yields for the nuclear test and remained skeptical about the claims, and others have argued that even the claimed 50 kiloton yield was low for confirmation of a thermonuclear design.

Pakistan



According to the scientific data received and published by PAEC
Pakistan Atomic Energy Commission
The Pakistan Atomic Energy Commission, , is an administrative governmental and autonomous science and technology governmental department of Pakistan, responsible for development of nuclear energy and development of nuclear power sector in Pakistan...

, the Corps of Engineers
Pakistan Army Corps of Engineers
The Pakistan Army Corps of Engineers, , is an active military administrative staff corps, and a major science and technology command of the Pakistan Army...

, and Kahuta Research Laboratories
Kahuta Research Laboratories
The Khan Research Laboratories ,, formerly known as Engineering Research Laboratories , is a multi-program Pakistan's weapons science and engineering research and development institute and nuclear research facility...

 (KRL), in May 1998, Pakistan carried out 6 underground
Underground nuclear testing
Underground nuclear testing refers to test detonations of nuclear weapons that are performed underground. When the device being tested is buried at sufficient depth, the explosion may be contained, with no release of radioactive materials to the atmosphere....

 nuclear tests
Nuclear testing
Nuclear weapons tests are experiments carried out to determine the effectiveness, yield and explosive capability of nuclear weapons. Throughout the twentieth century, most nations that have developed nuclear weapons have tested them...

 in Chagai Hills
Chagai Hills
The Chagai Hills is a range of granite hills in the Chagai District in Pakistan's Balochistan province.-Location:The Chagai Hills lie in a desert area in the northernmost part of Chagai District north of Pakistan's Ras Koh Hills and south of Afghanistan's Helmand and Nimruz provinces.- Topography...

 and Kharan Desert
Kharan Desert
The Kharan Desert is a sand desert situated in the Balochistan province of Pakistan.Kharan desert is Pakistan's second nuclear test site, and the second nuclear test — Codename Chagai-II — was conducted and supervised by the Pakistan Atomic Energy Commission in May 30, 1998.The desert is...

 in Balochistan Province (See the code-names of the tests, Chagai-I
Chagai-I
The Chagai-I was a codename referring to the five underground nuclear tests conducted by Pakistan at 15:15hrs in 28th May of 1998. It was named Chagai-I, as the tests were conducted in the Chagai District...

and Chagai-II). None of these boosted fission devices
Boosted fission weapon
A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate, and thus yield, of a fission reaction. The neutrons released by the fusion reactions add to the neutrons released in the fission, as well as inducing the fission reactions...

 were the thermonuclear weapon design, according to KRL and PAEC.

North Korea



North Korea's two nuclear tests (2006 and 2009) were relatively low yield and do not appear to have been of a thermonuclear weapon design.

Public knowledge


The Teller–Ulam design was for many years considered one of the top nuclear secrets, and even today it is not discussed in any detail by official publications with origins "behind the fence" of classification
Classified information
Classified information is sensitive information to which access is restricted by law or regulation to particular groups of persons. A formal security clearance is required to handle classified documents or access classified data. The clearance process requires a satisfactory background investigation...

. United States Department of Energy
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...

 (DOE) policy has been, and continues to be, that they do not acknowledge when "leaks" occur, because doing so would acknowledge the accuracy of the supposed leaked information.

Aside from images of the warhead casing, most information in the public domain about this design is regulated to a few terse statements by the DOE and the work of a few individual investigators.

DOE statements


In 1972 the United States government declassified a statement that "The fact that in thermonuclear (TN) weapons, a fission 'primary' is used to trigger a TN reaction in thermonuclear fuel referred to as a 'secondary'", and in 1979 added, "The fact that, in thermonuclear weapons, radiation from a fission explosive can be contained and used to transfer energy to compress and ignite a physically separate component containing thermonuclear fuel." To this latter sentence they specified that "Any elaboration of this statement will be classified." The only statement which may pertain to the spark plug was declassified in 1991: "Fact that fissile and/or fissionable materials are present in some secondaries, material unidentified, location unspecified, use unspecified, and weapons undesignated." In 1998 the DOE declassified the statement that "The fact that materials may be present in channels and the term 'channel filler,' with no elaboration", which may refer to the polystyrene foam (or an analogous substance).

Whether these statements vindicate some or all of the models presented above is up for interpretation, and official U.S. government releases about the technical details of nuclear weapons have been purposely equivocating in the past (see, e.g., Smyth Report
Smyth Report
The Smyth Report was the common name given to an administrative history written by physicist Henry DeWolf Smyth about the Allied World War II effort to develop the atomic bomb, the Manhattan Project...

). Other information, such as the types of fuel used in some of the early weapons, has been declassified, though of course precise technical information has not been.

The Progressive case


]

Most of the current ideas on the workings of the Teller–Ulam design came into public awareness after the Department of Energy
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...

 (DOE) attempted to censor
Prior restraint
Prior restraint or prior censorship is censorship in which certain material may not be published or communicated, rather than not prohibiting publication but making the publisher answerable for what is made known...

 a magazine article by U.S. antiweapons activist Howard Morland
Howard Morland
Howard Morland is a United States journalist and activist against nuclear weapons who in 1979 became famous for apparently discovering the "secret" of the hydrogen bomb and publishing it after a lengthy censorship attempt by the Department of Energy...

 in 1979 on the "secret of the hydrogen bomb". In 1978, Morland had decided that discovering and exposing this "last remaining secret" would focus attention onto the arms race
Arms race
The term arms race, in its original usage, describes a competition between two or more parties for the best armed forces. Each party competes to produce larger numbers of weapons, greater armies, or superior military technology in a technological escalation...

 and allow citizens to feel empowered to question official statements on the importance of nuclear weapons and nuclear secrecy. Most of Morland's ideas about how the weapon worked were compiled from highly accessible sources—the drawings which most inspired his approach came from none other than the Encyclopedia Americana
Encyclopedia Americana
Encyclopedia Americana is one of the largest general encyclopedias in the English language. Following the acquisition of Grolier in 2000, the encyclopedia has been produced by Scholastic....

. Morland also interviewed (often informally) many former Los Alamos
Los Alamos National Laboratory
Los Alamos National Laboratory is a United States Department of Energy national laboratory, managed and operated by Los Alamos National Security , located in Los Alamos, New Mexico...

 scientists (including Teller and Ulam, though neither gave him any useful information), and used a variety of interpersonal strategies
Social engineering (security)
Social engineering is commonly understood to mean the art of manipulating people into performing actions or divulging confidential information...

 to encourage informational responses from them (i.e., asking questions such as "Do they still use spark plugs?" even if he was not aware what the latter term specifically referred to).

Morland eventually concluded that the "secret" was that the primary and secondary were kept separate and that radiation pressure
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...

 from the primary compressed the secondary before igniting it. When an early draft of the article, to be published in The Progressive
The Progressive
The Progressive is an American monthly magazine of politics, culture and progressivism with a pronounced liberal perspective on some issues. Known for its pacifism, it has strongly opposed military interventions, such as the US-led invasion of Iraq in 2003. The magazine also devotes much coverage...

magazine, was sent to the DOE after falling into the hands of a professor who was opposed to Morland's goal, the DOE requested that the article not be published, and pressed for a temporary injunction. The DOE argued that Morland's information was (1) likely derived from classified sources, (2) if not derived from classified sources, itself counted as "secret" information under the "born secret
Born secret
"Born secret" and "born classified" are both terms which refer to a policy of information being classified from the moment of its inception, usually regardless of where it was being created, usually in reference to specific laws in the United States that are related to information that describes...

" clause of the 1954 Atomic Energy Act
Atomic Energy Act of 1954
The Atomic Energy Act of 1954, 42 U.S.C. § 2011 et seq., is a United States federal law that is, according to the Nuclear Regulatory Commission, "the fundamental U.S...

, and (3) was dangerous and would encourage nuclear proliferation
Nuclear proliferation
Nuclear proliferation is a term now used to describe the spread of nuclear weapons, fissile material, and weapons-applicable nuclear technology and information, to nations which are not recognized as "Nuclear Weapon States" by the Treaty on the Nonproliferation of Nuclear Weapons, also known as the...

.

Morland and his lawyers disagreed on all points, but the injunction was granted, as the judge in the case felt that it was safer to grant the injunction and allow Morland, et al., to appeal, which they did in United States v. The Progressive
United States v. The Progressive
United States of America v. Progressive, Inc., Erwin Knoll, Samuel Day, Jr., and Howard Morland is the name of a lawsuit against the magazine The Progressive by the U.S. government in 1979...

(1979).

Through a variety of more complicated circumstances, the DOE case began to wane as it became clear that some of the data they were attempting to claim as "secret" had been published in a students' encyclopedia a few years earlier. After another H-bomb speculator, Chuck Hansen
Chuck Hansen
Chuck Hansen compiled, over a period of 30 years, the world's largest private collection of documents on how America developed the atomic bomb. These documents were obtained through the U.S...

, had his own ideas about the "secret" (quite different from Morland's) published in a Wisconsin newspaper, the DOE claimed that The Progressive case was moot, dropped its suit, and allowed the magazine to publish its article, which it did in November 1979. Morland had by then, however, changed his opinion of how the bomb worked, suggesting that a foam medium (the polystyrene) rather than radiation pressure was used to compress the secondary, and that in the secondary there was a spark plug of fissile material as well. He published these changes, based in part on the proceedings of the appeals trial, as a short erratum in The Progressive a month later. In 1981, Morland published a book about his experience, describing in detail the train of thought which led him to his conclusions about the "secret".

Morland's work is interpreted as being at least partially correct because the DOE had sought to censor it, one of the few times they violated their usual approach of not acknowledging "secret" material which had been released; however, to what degree it lacks information, or has incorrect information, is not known with any confidence. The difficulty that a number of nations had in developing the Teller–Ulam design (even when they apparently understood the design, such as with the United Kingdom), makes it somewhat unlikely that this simple information alone is what provides the ability to manufacture thermonuclear weapons. Nevertheless, the ideas put forward by Morland in 1979 have been the basis for all the current speculation on the Teller–Ulam design.

Variations


There have been a few variations of the Teller–Ulam design suggested by sources claiming to have information from inside of the fence of classification. Whether these are simply different versions of the Teller–Ulam design, or should be understood as contradicting the above descriptions, is up for interpretation.

Richard Rhodes' "Ivy Mike" device in Dark Sun


In his 1995 book Dark Sun: The Making of the Hydrogen Bomb, author Richard Rhodes
Richard Rhodes
Richard Lee Rhodes is an American journalist, historian, and author of both fiction and non-fiction , including the Pulitzer Prize-winning The Making of the Atomic Bomb , and most recently, The Twilight of the Bombs...

 describes in detail the internal components of the "Ivy Mike
Ivy Mike
Ivy Mike was the codename given to the first United States test of a thermonuclear weapon, in which a major part of the explosive yield came from nuclear fusion. It was detonated on November 1, 1952 by the United States at on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy...

" Sausage device, based on information obtained from extensive interviews with the scientists and engineers who assembled it. According to Rhodes, though there was polystyrene in the "Mike" device, it was not used as a plasma source — the radiation from the primary itself was enough to compress the secondary. Whether or not this would apply only to the "Mike" device, or the Teller–Ulam design in general, is not known, and potentially casts some doubt onto the role of the foam, and to the exact mechanism of radiation "transport".

W88 revelations



In 1999 a reporter for the San Jose Mercury News
San Jose Mercury News
The San Jose Mercury News is a daily newspaper in San Jose, California. On its web site, however, it calls itself Silicon Valley Mercury News. The paper is owned by MediaNews Group...

reported that the U.S. W88
W88
The W88 is a United States thermonuclear warhead, with an estimated yield of 475 kiloton , and is small enough to fit on MIRVed missiles. The W88 was designed at the Los Alamos National Laboratory in the 1970s. In 1999 the director of Los Alamos who had presided over its design described it as...

 nuclear warhead, a small MIRVed
Multiple independently targetable reentry vehicle
A multiple independently targetable reentry vehicle warhead is a collection of nuclear weapons carried on a single intercontinental ballistic missile or a submarine-launched ballistic missile . Using a MIRV warhead, a single launched missile can strike several targets, or fewer targets redundantly...

 warhead used on the Trident II SLBM
Submarine-launched ballistic missile
A submarine-launched ballistic missile is a ballistic missile capable of delivering a nuclear warhead that can be launched from submarines. Modern variants usually deliver multiple independently targetable reentry vehicles each of which carries a warhead and allows a single launched missile to...

, had a prolate
Spheroid
A spheroid, or ellipsoid of revolution is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters....

 (egg
Oval
An oval is any curve resembling an egg or an ellipse, such as a Cassini oval. The term does not have a precise mathematical definition except in one area oval , but it may also refer to:* A sporting arena of oval shape** a cricket field...

 or watermelon
Watermelon
Watermelon is a vine-like flowering plant originally from southern Africa. Its fruit, which is also called watermelon, is a special kind referred to by botanists as a pepo, a berry which has a thick rind and fleshy center...

 shaped) primary (code-named Komodo) and a spherical secondary (code-named Cursa) inside a specially shaped radiation case (known as the "peanut" for its shape). A story four months later in The New York Times
The New York Times
The New York Times is an American daily newspaper founded and continuously published in New York City since 1851. The New York Times has won 106 Pulitzer Prizes, the most of any news organization...

by William Broad reported that in 1995, a supposed double agent
Double agent
A double agent, commonly abbreviated referral of double secret agent, is a counterintelligence term used to designate an employee of a secret service or organization, whose primary aim is to spy on the target organization, but who in fact is a member of that same target organization oneself. They...

 from the People's Republic of China delivered information indicating that China knew these details about the W88 warhead, supposedly through espionage. (This line of investigation eventually resulted in the abortive trial of Wen Ho Lee
Wen Ho Lee
Dr. Wen Ho Lee is a Taiwan-born Taiwanese American scientist who worked for the University of California at the Los Alamos National Laboratory. He created simulations of nuclear explosions for the purposes of scientific inquiry, as well as for improving the safety and reliability of the US nuclear...

.) If these stories are true, it would explain the reported higher yield of the W88, 475 kilotons, compared with only 300 kilotons for the earlier W87
W87
The W87 is an American thermonuclear missile warhead. It was created for use on the MX or Peacekeeper ICBM, 50 of which, with up to 12 warheads per missile, were deployed during the 1986-2005 period...

 warhead.

The reentry
Atmospheric reentry
Atmospheric entry is the movement of human-made or natural objects as they enter the atmosphere of a celestial body from outer space—in the case of Earth from an altitude above the Kármán Line,...

 cones
Nose cone
The term nose cone is used to refer to the forwardmost section of a rocket, guided missile or aircraft. The cone is shaped to offer minimum aerodynamic resistance...

 for the two warheads are the same size, 1.75 meters (69 in) long, with a maximum diameter of 55 cm. (22 in). The higher yield of the W88 implies a larger secondary, which produces most of the yield. Putting the secondary, which is heavier than the primary, in the wider part of the cone allows it to be larger, but it also moves the center of mass
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...

 aft
Aft
Aft, in naval terminology, is an adjective or adverb meaning, towards the stern of the ship, when the frame of reference is within the ship. Example: "Able Seaman Smith; lay aft!". Or; "What's happening aft?"...

, potentially causing aerodynamic stability problems during reentry. Dead-weight ballast must be added to the nose to move the center of mass forward.

To make the primary small enough to fit into the narrow part of the cone, its bulky insensitive high explosive charges must be replaced with more compact "non-insensitive" high explosives which are more hazardous to handle. The higher yield of the W88, which is the last new warhead produced by the United States, thus comes at a price of higher warhead weight and higher workplace hazard.

External links


Principles

History