A fission device is any assembly of components that can produce an explosion from nuclear fission of weapons-purity nuclear fuel. A fission bomb can be dropped from an airplane, or at least transported to a real target. In contrast, the first nuclear device, exploded at the Trinity test in New Mexico, U.S.A., was not transportable. A fission warhead is a device that is sufficiently small and rugged to be used, operationally, as the warhead of a guided missile, artillery shell, or unguided rocket.
Nuclear weapons include bombs and warheads using either fission or fusion. There is a relevant U.S. naming convention used for the last several decades: each "physics package", or the actual mechanism that produces nuclear energy, has a number, which is prefixed by "B" if a bomb and "W" if a warhead. A link, therefore, could be to W54, or to a weapon before this naming convention started, such as Little Boy.
From a nontechnical standpoint, nuclear fission is the mechanism that causes the intense energy release of a fission weapon. In this context, the nucleus of a radioactive element, such as 239plutonium, is struck by a subatomic particle, a neutron. When the unstable nucleus captures the neutron, it splits into two new nuclei, releases energy, and emits two new neutrons.
If the fission were only of one nucleus, the energy release would be infinitesimal. When the system is constructed such that the emitted neutrons hit other nuclei and cause additional fissions, the process of a chain reaction exists. The size and density of the material needed to sustain a chain reaction defines the critical mass. In a nuclear power reactor, the rate of the chain reaction is carefully controlled, with strict limits on the rate of neutron generation.
In a bomb, however, the more neutrons that can be captured in a short time, the higher the yield. Obviously, the bomb cannot be transported while in a chain reaction. The challenge of fission bomb design is to change the physical state of the fissionable material, such that the rate of generation and capture of neutrons are maximized -- and before the energy released physically disrupts the material.
To change that state, the material needs to be compressed, in an extremely precise manner, by pressure waves created by the explosion of conventional explosives. There are two basic ways to do this:
- gun-type compression, where, conceptually, a "bullet" of fissionable material is fired down a barrel into a "target" of fissionable material. Neither the bullet nor the target form a critical mass by themselves, but they do when combined
- implosion systems, where, conceptually, pressure is applied symmetrically around a spherical and subcritical mass. When the shock waves converge on the subcritical mass, they compress it, increasing its density until it reaches critical mass.
All modern fission weapons, or fission Primaries to trigger fusion reactions, use the implosion process. The design of the explosive system used for implosion is extremely complex; the reason that there was only one bomb test before the attacks on Japan was that it was not certain implosion would work. The weapon used on Hiroshima was gun-type.
A concern today is that non-national terrorists, if they could obtain enough fissionable material, would use the gun-type because it is simpler, although less efficient. That lack of efficiency means that much more fissionable material is needed than in an implosion system, so that implosion still might be attempted. If, however, the implosion system fails to compress symmetrically, it may only scatter radioactive material, or create a fizzle yield of minimal force.
Features specific to fission devices include the fire-resistant pit that prevents contamination from molten plutonium.. Even more basic features, however, protect the high-explosive implosion system, such as the one-point safe criterion that requires synchronized multipoint detonation to achieve a nuclear yield.
For an efficient bomb, there must be a controlled source of neutrons applied to the critical mass, especially for implosion systems. Other refinements maximize the number of neutron captures and fissions before the material flies apart.
In the first U.S. and Soviet designs, the neutron source, codenamed the "urchin" in the American weapon, was a precisely machined shape of beryllium and polonium in the core. It released neutrons when compressed. 210polonium, the radioactive component, has a half-life of only 140 days after being made in a nuclear reaction and thus needed to be replaced frequently. The two elements needed to be kept apart until the neutron burst was desired, and then mixed thoroughly; the mechanical design is complex.
U.S. practice, however, moved to an external "neutron gun", essentially a miniature linear accelerator using a deuterium-tritium reaction; they look vaguely like hair dryers attached to the outside of the sphere.
Chinese and Pakistani designs, however, have used a different technique, with uranium deuteride in the core. This has been reported, but not confirmed, to be an active area of research in the Iranian nuclear program;  Philip Giraldi calls the report on the Iranian application to be a forgery. 
Although the linear accelerator approach is considered a militarily critical technology, it does have nonmilitary applications. A compressible core, however, is only useful in a bomb.
Implosion system design
To improve the performance of a fission device, the most important consideration is maximizing the amount of explosive force directed into the core by the implosion subsystem. There are only microseconds to do this, as the Primary will, soon after an uncontrolled chain reaction starts, break apart with the energy generated by fission.
Early implosion systems used large number of explosive "lenses", formed of a mixture of explosives with fast and slow detonation velocity, each forming a shock wave that moved toward the core. A bomb of this type resembles a soccer ball, with the lenses, typically with redundant detonators, corresponding to the facets of the ball. The number of facets and detonators increased in early bombs, from 30 on the Mark 6 Mod 0, to 60 on the Mark 6 Mod 2, to 92 on the Mark 7. The explosive lenses themselves are usually constructed of layers of fast and slow explosive, to help form the appropriate waveform. In the Manhattan Project, the chemical explosive systems were the responsibility of George Kistiakowsky's teams, making him, a physical chemist, as critical as any of the nuclear physicists. Kistiakowsky was later to become Science Advisor to President Dwight D. Eisenhower and the architect of the Single Integrated Operational Plan.
The explosive lenses themselves are usually constructed of layers of fast and slow explosive, to help form the appropriate waveform. In the Manhattan Project, the chemical explosive systems were the responsibility of George Kistiakowsky's teams, making him, a physical chemist, as critical as any of the nuclear physicists. Kistiakowsky was later to become Science Advisor to President Dwight D. Eisenhower and the architect of the Single Integrated Operational Plan.
Many of the methods of increasing the force into the core, and, indeed, in a fusion Secondary, perhaps counterintuitively depend on carefully placed empty spaces, or spaces filled with plastic foam that will quickly turn into gas. As the implosion explosives detonatee. To achieve such a wave, the first requisite is that the multiple explosive "lenses" surrounding the core detonate simultaneously. This requires precise switching of intense current bursts, which most commonly involves switching devices called krytrons or solid-state equivalents. Krytrons still remain on critical technology export controls, but they are now dual-use.
Another application of precise shock waves is in lithotripters, which are medical devices that use shaped shock waves to pulverize kidney stones, gallstones, and the like. Explosive welding is another area related to manufacturing.
Early linear explosion
The physical constraints of certain bombs, such as atomic field artillery projectiles, gave up efficiency in order to push the oval shape into a solid core. While this required additional fissionable material, the initiator timing was simplified.
Increasing fission efficiency: mechanical compression
Returning to the issue of voids, bomb designer Ted Taylor alluded generally to the then-classified techniques: "When you drive a nail, do you put the head of the hammer on top of the nail and push?"  While Taylor would not elaborate, he probably was referring, at the least, to levitated pits and mass drivers.
Solid core issues
The original designs were solid cores of plutonium, in direct contact with the innermost sphere of a nickel- and gold-plated polonium ball, the "urchin" neutron generator. Solid cores have several disadvantages:
- If the chemical explosive is in contact with the core, the maximum pressure is limited to the internal detonation wave, not more than 400 kilobars.
- Especially with plutonium, the metallurgical characteristics of the metal limit the increase of density possible through pure mechanical compression.
The most basic steps to increasing pressure is to reflect the shock wave back at the core, and by convergence of multiple explosive waves. In a solid core design, implosion starts at the urchin initiator in the center, so the outer layers of the pit will provide some reflection. Reflection occurs whenever there is a change in density. This can be increased, by some extent by inserting one or more layers, of increasing density, between the explosive, the very dense tamper, and the pit. The multilayer technique can be used only 2 or 3 times.
Shock convergence is one of the reasons that the explosive system resembles a soccer ball, made up of many curved segments, each with a detonator in the center, the detonators triggered near-simultaneously. The additional compression is limited by the ratio between the radius of the fissile coure and the outer radius of the implosion system. Unfortunately for the designer, getting a large ratio means the entire system grows in size and weight.
Yet another problem in solid pit systems is the Taylor wave. This wave is a sharp pressure drop immediately behind the shock wave propagating from the chemical explosion. If this wave is not suppressed or reduced, by the time the compression reaches the innermost part of the pit, the outer regions of the pit may have returned to normal pressure. To deal with the Taylor wave problem, more concentric layers come into play: intermediate density (e.g., aluminium) pushers' between the explosive and the tamper. Remember, the explosive detonation propagates in all directions, toward the pit and away from the pit. Pusher layers reflect the away-from-pit wave back at the pit, and tend to cancel the Taylor wave.
Levitated pit issues
Remember that the implosion device is made of multiple concentric spheres or sphere-like structures. One of these is called the tamper, and is physically between the explosives and the fissionable material in the central pit. The levitated pit design puts an air gap between the inside of the tamper and the outside of the pit, the pit held in place only by thin wires or foam. This air gap allows the compression wave to become smoother, and, with the slight delay before it hits the pit, have more time for more explosions to contribute to the shock wave. 
There are additional roles for the tamper. When the pit begins to disassemble as the fission chain reaction begins, the tamper does not immediately vaporize. The denser and tougher the tamper, the more containment (still measured in microseconds) it will give to the pit, increasing the neutron flux density and thus the efficiency of reaction.
Modern "hollow pit" fission systems use a levitated pit that is also hollow and roughly ovoid-shaped. Rather than dozens of "soccer ball" explosive lenses, there are two detonation points at either end, which use flying plate detonators to transfer energy into the pit, which is being formed into a dense sphere. In current weapons, as the sphere compresses, tritium will be injected either into the core, or possibly into concentric fissionable layers for tritium boosting. An external neutron generator fires into the sphere at an appropriate time, so no internal initiator is needed.
Hollow pit designs allowed much more miniaturization. The fact of the non-spherical pit, very much different from early bombs, was highly classified until fairly recently; this was one of the key items believed leaked to China with the W88 design. 
Neutrons are reflected by light atomic weight materials, including the explosives themselves. For greater reflection of neutrons back into the pit during the early part of fission, the roughly spherical explosives are surrounded by a neutron reflector, usually of beryllium. Neutrons generated by the pit, again in a process of microseconds, increases core neutron density.
The U.S. Mark 12 was probably the first bomb deployed with a beryllium reflector.
Improving fission efficiency: tritium boosting
The latter technique can be turned into a method of boosting the efficiency of the entire device, by adding a metered quantity of tritium into a Primary. Under the conditions of implosion, tritium, hit by neutrons, will generate neutrons more efficiently than fissionable materials, although the conditions for a fusion chain reaction do not exist in a boosted fission device. These neutrons have much higher energy than fission neutrons.
There is a concern with having too much internal gas and turning the pit into a hollow sphere with undesirable explosive wave reflection characteristics. To avoid this, the boosting tritium has to be somewhere inside the outermost shell, but still inside the compression system. Sublette suggests that, in U.S. weapons, the gas is "between the outer shell and the levitated pit. Here the collapsing thin shell would create multiple reflected shocks that would efficiently compress the gas to a thin very high density layer. There is evidence that US boosted primaries actually contain the boosting gas within the external shell rather than an inner levitated shell. The W-47 primary used a neutron absorbing safing wire that was withdrawn from the core during weapon arming, but still kept its end flush with the shell to form a gas-tight seal."
In fission and fusion weapons with variable yield, the amount of yield is probably controlled by the amount of tritium injected. While this would appear to be a simple process, it apparently is not; there have been various reports on information that suggest that at least one of the Indian tests had a partial fizzle yield in what appears to have been a boosted fission Primary. 
- Ray Kidder (26 July 1991), Report to the Congress: Assessment of the Safety of U.S. Nuclear Weapons and Related Nuclear Test Requirements, Lawrence Livermore National Laboratory, Report UCRL-LR-107454,
- , Section V, Nuclear Weapons Technology, Militarily Critical Technologies List (MCTL), Part II: Weapons of Mass Destruction Technologies (1998 ed.), U.S. Department of Defensepp. II-5-60 to -64
- Catherine Philp (14 December 2009), "Discovery of UD3 raises fears over Iran’s nuclear intentions", The Times (UK)
- Gareth Porter (29 December 2009), Giraldi: US Intel Found Iran Nuke Document Was Forged, AntiWar.com
- Sublette, Carey, 126.96.36.199.2 Linear Implosion, N4.1 Elements of Fission Weapon Design Cite error: Invalid
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