The Teller–Ulam design is the nuclear weapon design concept first developed and used by the United States and has since been used in most of the world's nuclear weapons. It is colloquially referred to as "the secret of the hydrogen bomb" because it employs hydrogen fusion, though in most applications the bulk of its destructive energy comes fromuranium fission, not hydrogen fusion. Sony VAIO PCG-3B1M BatterySony VAIO PCG-3C1T Battery
It is named for its two chief contributors, Edward Teller and Stanisław Ulam, who developed it in 1951 for the United States, with certain concepts developed with the contribution of John von Neumann. It was first used in multi-megaton-range thermonuclear weapons. Sony VAIO PCG-3D1M Battery
As it is also the most efficient design concept for small nuclear weapons, today virtually all the nuclear weapons deployed by the five nuclear-weapon states under the NPT use the Teller–Ulam design.
Its essential features, which officially remained secret for nearly three decades, Sony VAIO PCG-3G2M Battery
are: 1) separation of stages into a triggering "primary" explosive and a much more powerful "secondary" explosive, 2) compression of the secondary by X-rayscoming from nuclear fission in the primary, a process called the "radiation implosion" of the secondary, and 3) heating of the secondary, after cold compression, by a second fission explosion inside the secondary. Sony VAIO PCG-5R1M Battery
The radiation implosion mechanism is a heat engine 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, increasing and prolonging the compression of the secondary. Sony VAIO PCG-7141M Battery
If made of uranium—and it usually is—it can capture neutrons 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 fallout. Sony VAIO PCG-7143M Battery
The first test of this principle was the "Ivy Mike" nuclear test in 1952, conducted by the United States. In the Soviet Union, the design was known as Andrei Sakharov'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. Sony VAIO PCG-7151M Battery
Detailed knowledge of fission and fusion weapons is classified to some degree in virtually every industrialized nation. In the United States, such "knowledge" can by default be classified as Restricted Data, even if it is created by persons who are not government employees or associated with weapons programs, Sony VAIO PCG-7154M Battery
in a legal doctrine known as "born secret" (though the constitutional standing of the doctrine has been at times called into question, see United States v. The Progressive). Born secret is rarely invoked for cases of private speculation. The official policy of the United States Department of Energy has been not to acknowledge the leaking of design information, Sony VAIO PCG-7162M Battery
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, with limited success.
Though large quantities of vague data have been officially released, Sony VAIO PCG-7181M Battery
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 from known information, or comparison with similar fields of physics (inertial confinement fusion is the primary example). Sony VAIO PCG-41112M Battery
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. Sony VAIO PCG-7153M Battery
The state of public knowledge about the Teller–Ulam design has been mostly shaped from a few specific incidents outlined in a section below.
The basic principle of the Teller–Ulam configuration is the idea that different parts of a thermonuclear weapon can be chained together in "stages", Sony VAIO PCG-71312M Battery
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 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", Sony VAIO PCG-7144M Battery
at which point it is heated and undergoes nuclear fusion. 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" is thought to have been a three-stage device. Sony VAIO PCG-7191L Battery
Surrounding the other components is a hohlraum 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. Sony VAIO PCG-3C1M Battery
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 by small amounts of fusion fuel (usually 50/50% deuterium/tritium gas) for extra efficiency; Sony VAIO PCG-3F1M Battery
the fusion fuel releases excess neutrons 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, Sony VAIO PCG-3H1M Battery
the plutonium-239 (Pu-239) and/or uranium-235 (U-235) core would be compressed to a smaller sphere by special layers of conventional high explosives arranged around it in anexplosive lens pattern, initiating the nuclear chain reaction that powers the conventional "atomic bomb".Sony VAIO PCG-3J1M Battery
The secondary is usually shown as a column of fusion fuel and other components wrapped in many layers. Around the column is first a "pusher-tamper", a heavy layer of uranium-238 (U-238) or lead which serves to help compress the fusion fuel (and, in the case of uranium, may eventually undergo fission itself). Sony VAIO PCG-8141M Battery
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 experiment to the (over)success of the lithium deuteride-based Castle Bravo experiment). Sony VAIO PCG-8161M Battery
This dry fuel, when bombarded by neutrons, produces tritium, a heavy isotope of hydrogen which can undergo nuclear fusion, along with the deuterium present in the mixture. (See the article on nuclear fusion for a more detailed technical discussion of fusion reactions.) Sony VAIO PCG-3C2M Battery
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 acritical mass without compression). The tertiary, if one is present, would be set below the secondary and probably be made up of the same materials. Sony VAIO PCG-5N2M Battery
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 that was originally the bomb's fissile material and its tamper; 3) Sony VAIO PCG-5P1M Battery
the electromagnetic radiation; and 4) the neutrons 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. Sony VAIO PCG-5S1M Battery
Less than optimal interstage designs have resulted in the secondary failing to work entirely on multiple shots, known as a "fissile fizzle". The Koon shot of Operation Castle 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. Sony VAIO PCG-9Z1M Battery
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 warhead. It was released by Greenpeace in a report titled "Dual Use Nuclear Technology". Sony VAIO PCG-7171M Battery
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 reflector; Sony VAIO PCG-7186M Battery
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; 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. Sony VAIO PCG-81112M Battery
In Ivy Mike, gold was used as a coating over the uranium to enhance the blackbody effect. 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. Sony VAIO PCG-31311M Battery
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) each poking through the outer edge of the reflector with one end in each section; Sony VAIO PCG-8152M Battery
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. Sony VAIO PCG-31111M Battery
A "Polystyrene 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 Program. Sony VAIO PCG-61111M Battery
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". Sony VAIO PCG-51112M Battery
The "toxic, brittle material" is widely assumed to be beryllium, which fits that description and would also moderate the neutron flux 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",Sony VAIO PCG-51211M Battery
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 has been suggested as a possibility. Manufacture stopped for many years; however, the Life Extension Program required it to start up again – Sony VAIO PCG-51212M Battery
Y-12 currently being the sole producer (the "unique facility" referenced). Manufacture involves the moderately toxic and moderately volatile solvent called acetonitrile, which presents a hazard for workers (causing three evacuations in March 2006 alone).
A simplified summary of the above explanation would be: Sony VAIO PCG-41111M Battery
An implosion assembly type of fission bomb is exploded. This is the primary stage. If a small amount of deuterium/tritium gas is placed inside the primary's core, it will be compressed during the explosion and a nuclear fusion reaction will occur; Sony VAIO PCG-41111V Battery
the released neutrons from this fusion reaction will induce further fission in the plutonium-239 or uranium-235 used in the primary stage. The use of fusion fuel to enhance the efficiency of a fission reaction is called boosting. Without boosting, a large portion of the fissile material will remain unreacted; Sony VAIO PCG-61412V Battery
- the Little Boy and Fat Man bombs had anefficiency of only 1.4% and 17%, respectively, because they were unboosted.
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; Sony VAIO PCG-71112M Battery
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 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). Sony VAIO PCG-81111V Battery
The fusion fuel of the secondary stage may be surrounded by depleted uranium or natural uranium, whose U-238 is not fissile and cannot sustain a chain reaction, but which is fissionable when bombarded by the high-energy neutrons released by fusion in the secondary stage. Sony VAIO PCG-81111V Battery
This process provides considerable energy yield (as much as half of the total yield in large devices), but is not considered a tertiary "stage". Tertiary stages are further fusion stages (see below), which have been only rarely used, and then only in the most powerful bombs ever made. Sony VAIO PCG-81212M Battery
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 (or another neutron reflecting material) instead of depleted uranium to prevent further fission from occurring. Sony VAIO PCG-81212V Battery
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, Sony VAIO PCG-51111M Battery
but is thought to be transmitted through the X-rays which are emitted from the fissioning primary. This energy is then used to compress thesecondary. 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: Sony VAIO VPCS13X9E/B battery
- Neutron pressure from the primary explosion. This was allegedly Ulam's first concept and was abandoned as unworkable.
- Blast wave from the primary explosion. This was allegedly Ulam's second concept and was abandoned as unworkable.
Radiation pressure exerted by the X-rays. Sony VAIO VPCS12V9E/B battery
- This was the first idea put forth by Howard Morland in the article in The Progressive.
- X-rays creating a plasma in the radiation case's filler (a polystyrene or "FOGBANK" plastic foam). This was a second idea put forward by Chuck Hansen and later by Howard Morland.
Tamper/Pusher ablation. Sony VAIO VPCS12V9E/B battery
The radiation pressure exerted by the large quantity of X-ray photons 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, Sony VAIO VPCF13M8E/B battery
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 (atmospheres) (7.3 T Pa) for the Ivy Mike design and 1,400 million bar (140 TPa) for the W-80. Sony VAIO VPCF13Z0E/B battery
Foam 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. Sony VAIO VPCM13M1E/L battery
The sequence of firing the weapon (with the foam) would be as follows:
The high explosives surrounding the core of the primary fire, compressing the fissile material into a supercritical state and beginning the fission chain reaction. Sony VAIO VPCM13M1E/W battery
- The fissioning primary emits X-rays, which "reflect" along the inside of the casing, irradiating the polystyrene foam.
The irradiated foam undergoes a phase transition, becoming a hot plasma, pushing against the tamper of the secondary, compressing it tightly, and beginning the fission reaction in the spark plug. Sony VAIO VPCF22M1E battery
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-6 atom splits into one tritium atom and one alpha particle. Sony VAIO VPCF11M1E/H battery
- Then begins a fusion reaction between the tritium and the deuterium, releasing even more neutrons, and a huge amount of energy.
The fuel undergoing the fusion reaction emits a large flux of neutrons, which irradiates the U-238 tamper (or the U-238 bomb casing), causing it to undergo a fission reaction, providing about half of the total energy. Sony VAIO VPCF13M0E/B battery
This would complete the fission-fusion-fission sequence. Fusion, unlike fission, is relatively "clean"—it releases energy but no harmful radioactive productsor large amounts of nuclear fallout. The fission reactions though, especially the last fission reaction, release a tremendous amount of fission products and fallout. Sony VAIO VPCYB2M1E battery
If the last fission stage is omitted, by replacing the uranium tamper with one made of lead, for example, the overall explosive force is reduced by approximately half but the amount of fallout is relatively low. The neutron bomb is a hydrogen bomb without the final fission stage. Sony VAIO VPCYB3V1E battery
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, Sony VAIO VPCY11M1E battery
and also that the known foam materials intrinsically have a very low absorption efficiency of the gamma ray and X-ray 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. Sony VAIO VPCS12L9E/B battery
Analyzing the effects of that absorbed energy led to the third mechanism: ablation.
The proposed tamper-pusher ablation mechanism is that the primary compression mechanism for the thermonuclear secondary is that the outer layers of the tamper-pusher, Sony VAIO VPCF11S1E/B battery
or heavy metal casing around the thermonuclear fuel, are heated so much by the X-ray flux from the primary that they ablate 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. Sony VAIO VPCYB3V1E/R Battery
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, and the effects of that thermal energy are then analyzed. Sony VAIO VPCF23P1E Battery
The energy is mostly deposited within about one X-ray optical thickness 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, Sony VAIO VPCF23N1E Battery
from a basic Newtonian momentum balance, the velocity at which the rest of the tamper implodes inwards.
Applying the more detailed form of those calculations to the Ivy Mike device yields vaporized pusher gas expansion velocity of 290 kilometers per second Sony VAIO VPCY21S1E/L Battery
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. Sony VAIO VPCY21S1E/G battery
The pressure due to the ablating material is calculated to be 5.3 billion bar (530 T Pa) in the Ivy Mike device and 64 billion bar (6.4 P Pa) in the W-80 device.
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. Sony VAIO VPCF24M1E battery
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. HP Compaq HSTNN-105C Battery
United States Department of Defense 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 until energy has distributed evenly and a sufficient fraction has reached the secondary's tamper/pusher. HP Compaq HSTNN-C12C Battery
Richard Rhodes' book Dark Sun stated that a 1-inch-thick (25 mm) layer of plastic foam was fixed to the lead liner of the inside of the Ivy Mike 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: HP Compaq HSTNN-C66C Battery
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, HP Compaq HSTNN-C66C-4 Battery
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.
A number of possible variations to the weapon design have been proposed: HP Compaq HSTNN-C66C-5 Battery
Either the tamper or the casing have been proposed to be made of uranium-235 (highly enriched uranium) in the final fission jacket. The far more expensive U-235 is also fissionable with fast neutrons like the standard U-238, HP Compaq HSTNN-C67C Battery
but its fission-efficiency is higher than natural uranium, which is almost entirely U-238. Using a final fissionable jacket of U-235 would thus be expected to increase the yield of any Teller-Ulam bomb above a U-238 (depleted uranium) or natural uranium jacket design. HP Compaq HSTNN-C67C-4 Battery
- 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. HP Compaq HSTNN-C67C-5 Battery
- X-ray "reflection" is not like light reflecting off of a mirror, 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.