Hydrogen Bomb

[Theotris]

The basics of the Teller–Ulam design. Radiation from a primary fission bomb compresses a secondary section containing both fission and fusion fuel. The compressed secondary is heated from within by a second fission explosion.


Teller–Ulam design

[Theotris]

The Teller–Ulam design is the nuclear weapon design concept used in most of the world's nuclear weapons.[1] Colloquially referred to as "the secret of the hydrogen bomb", because it employs hydrogen fusion to generate neutrons, in most applications the bulk of its destructive energy comes from uranium fission, not hydrogen fusion. It is named for its two chief contributors, Hungarian scientist Edward Teller and Polish scientist Stanisław Ulam, who developed it in 1951, for use by the United States. It was first used in multi-megaton-range thermonuclear weapons. However, it is also the most efficient design concept for small nuclear weapons, and 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-rays coming 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.

The radiation implosion mechanism is a heat engine exploiting the temperature difference between the hot radiation channel, surrounding the secondary, and the relatively cool interior of the secondary. This temperature difference is briefly maintained by a massive heat barrier called the "pusher". The pusher is also an implosion tamper, increasing and prolonging the compression of the secondary, and, if made of uranium, which it usually is, it undergoes fission by capturing the neutrons produced by fusion. In most Teller–Ulam weapons, fission of the pusher dominates the explosion and produces radioactive fission product fallout.

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.

[Theotris]

Public knowledge concerning nuclear weapon design

Detailed knowledge of actual 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, 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, 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, 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). 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.

[Theotris]

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 bomb (a "trigger"), and a secondary section which consists of fusion fuel. 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 of the secondary. The energy released by the primary compresses the secondary through the concept of "radiation implosion", at which point it is heated and undergoes nuclear fusion.

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. Numerous photographs of various thermonuclear bomb exteriors have been declassified.[2]

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; 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, 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 an explosive lens pattern, initiating the nuclear chain reaction that powers the conventional "atomic bomb".

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). 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). 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.) 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 without compression). The tertiary, if one is present, would be set below the secondary and probably be made up of the same materials.[3][4]

[Theotris]

Separating the secondary from the primary is the interstage. The fissioning primary produces three types of energy: 1) expanding hot gases from high explosive charges which implode the primary, 2) the electromagnetic radiation and 3) 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, 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 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.

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".[5] The major components and their arrangement are in the diagram, though details are almost absent; what scattered details it does include are 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; 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, known as radiation implosion. 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 [1]) 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. However, each is tilted so one end is higher than the other relative to the bomb if laid on its side – much like the rifling in a gun barrel. 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 Reliable Replacement Warhead 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".[6] 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.[7]

The "special material" is called "FOGBANK", 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 start up again - Y-12 currently being the sole producer (the "unique facility" referenced). Manufacture involves the moderately toxic, highly volatile solvent called acetonitrile, which presents a hazard for workers (causing three evacuations in March 2006 alone).[8]

[Theotris]

One possible version of the Teller–Ulam configuration.

[Theotris]

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/tritium gas is placed inside the primary's core, it will be compressed during the explosion and a nuclear fusion reaction will occur; 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; the Little Boy and Fat Man bombs had an efficiency of only 1.4% and 14%, 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 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).
   3. 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.

Actual designs of thermonuclear weapons may vary. For example, they 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.

[Theotris]

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-rays which are emitted from the fissioning primary. This energy is then used to compress the secondary. 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 from the primary explosion. This was allegedly Ulam's second concept and was abandoned as unworkable.
    * Radiation pressure exerted by the X-rays. 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. This is currently believed to be the actual mechanism.

[Theotris]

Radiation pressure

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, 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.[9]

[Theotris]

Foam plasma pressure

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.

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 state and beginning the fission chain reaction.
   2. The fissioning primary emits X-rays, which "reflect" along the inside of the casing, irradiating the polystyrene foam.
   3. 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.
   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-6 atom splits into one tritium atom and one alpha particle. 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, 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.

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. 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, for example, the overall explosive force is reduced by approximately half but the amount of fallout is relatively low.

[Theotris]

Foam plasma mechanism firing sequence.
A. Warhead before firing; primary (fission bomb) at top, secondary (fusion fuel) at bottom, all suspended in polystyrene foam.
B. High-explosive fires in primary, compressing plutonium core into supercriticality and beginning a fission reaction.
C. Fission primary emits X-rays which are scattered along the inside of the casing, irradiating the polystyrene foam.
D. Polystyrene foam becomes plasma, compressing secondary, and plutonium sparkplug begins to fission.
E. Compressed and heated, lithium-6 deuteride fuel produces tritium and begins the fusion reaction. The neutron flux produced causes the U-238 tamper to fission. A fireball starts to form.

[Theotris]

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 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. Analyzing the effects of that absorbed energy led to the third mechanism: ablation.

However, an aerogel-type material impregnated with salts of high-Z elements like thorium or uranium could have higher absorption efficiency of the X-ray flux from the primary, thus making foam plasma pressure a viable secondary mechanism for a radiation implosion.

[Theotris]

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 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.

[Theotris]

Ablation mechanism firing sequence.
1. Warhead before firing. The nested spheres at the top are the fission primary; the cylinders below are the fusion secondary device.
2. Fission primary's explosives have detonated and collapsed the primary's fissile pit.
3. The primary's fission reaction has run to completion, and the primary is now at several million degrees and radiating gamma and hard X-rays, heating up the inside of the hohlraum and the shield and secondary's tamper.
4. The primary's reaction is over and it has expanded. The surface of the pusher for the secondary is now so hot that it is also ablating or expanding away, pushing the rest of the secondary (tamper, fusion fuel, and fissile spark plug) inwards. The spark plug starts to fission. Not depicted: the radiation case is also ablating and expanding outwards (omitted for clarity of diagram).
5. The secondary's fuel has started the fusion reaction and shortly will burn up. A fireball starts to form.

[Theotris]

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. 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, 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 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) in the Ivy Mike device and 64 billion bar (6.4 P Pa) in the W-80 device.[9]