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Nuclear weapon designs are often divided into two classes, based on the dominant source of the nuclear weapon's energy.

• Fission bombs derive their power from nuclear fission, where heavy nuclei (uranium or plutonium) are bombarded by neutrons and split into lighter elements, more neutrons and energy. These newly created neutrons then bombard other nuclei, which then split and bombard other nuclei, and so on, creating a nuclear chain reaction which releases large amounts of energy. These are historically called atomic bombs, atom bombs, or A-bombs, though this name is not precise due to the fact that chemical reactions release energy from atomic bonds and fusion is no less atomic than fission. Despite this possible confusion, the term atom bomb has still been generally accepted to refer specifically to nuclear weapons, and most commonly to pure fission devices.

• Fusion bombs are based on nuclear fusion where light nuclei such as deuterium and lithium combine together into heavier elements and release large amounts of energy. Weapons which have a fusion stage are also referred to as hydrogen bombs or H-bombs because of their primary fuel, or thermonuclear weapons because fusion reactions require extremely high temperatures for a chain reaction to occur.

The distinction between these two types of weapon is blurred by the fact that they are combined in nearly all complex modern weapons: a smaller fission bomb is first used to reach the necessary conditions of high temperature and pressure which are required for fusion. Fusion elements may also be present in the core of fission devices as well as they generate additional neutrons which increases the efficiency (known as "boosting") of the fission reaction. Additionally, most fusion weapons derive a substantial portion of their energy (often around half of the total yield) from a final stage of fissioning which is enabled by the fusion reactions. Since the distinguishing feature of both fission and fusion weapons is that they release energy from transformations of the atomic nucleus, the best general term for all types of these explosive devices is nuclear weapon.

Other specific types of nuclear weapon design which are commonly referred to by name include: neutron bomb (enhanced radiation weapon), cobalt bomb, and salted bomb.

Fission weapons

The simplest nuclear weapons are pure fission bombs, also historically known as atomic bombs. These were the first types of nuclear weapons built during the Manhattan Project and they are a building block for all advanced nuclear weapons designs.

Critical mass

A mass of fissile material is called critical when it is capable of a sustained chain reaction, which depends upon the size, shape and purity of the material as well as what surrounds the material. A numerical measure of whether a mass is critical or not is available as the neutron multiplication factor, k, where

k = f − l

where f is the average number of neutrons released per fission event and l is the average number of neutrons lost by either leaving the system or being captured in a non-fission event. When k = 1 the mass is critical, k < 1 is subcritical and k > 1 is supercritical. A fission bomb works by rapidly changing a subcritical mass of fissile material into a supercritical assembly, causing a chain reaction which rapidly releases large amounts of energy.

In practice the mass is not made slightly supercritical, but goes from slightly subcritical (k = 0.9) to highly supercritical (k = 2 or 3), so that each neutron creates several new neutrons and the chain reaction advances more quickly. The main challenge in producing an efficient explosion using nuclear fission is to keep the bomb together long enough for a substantial fraction of the available nuclear energy to be released.

Prior to detonation an atomic weapon consists of one or two pieces of weapon grade fissionable material which are subcritical. The main method is the implosion method, where there is one sphere of material, which is compressed to make it supercritical. A simpler method is the gun method, where there are two pieces, one subcritical because of the limited mass, the other subcritical because of being unfavorably shaped (or in small-yield weapons, also because of mass), but such that the pieces together are favorably shaped: the first piece fits in a hole of the second piece. (More generally, also e.g. two subcritical hemispheres might be used, or more than two pieces.)

To produce an efficient atomic detonation, the fissile material must be brought into its optimal supercritical state very rapidly. This must be done to avoid predetonation; this means that the fission chain reaction starts to soon: the material will heat up and expand rapidly before being in its optimal state. Thus only a small, sometimes very small part of the fissionable material actually undergoes fission, and the yield is correspondingly low.

For a quick start of the chain reaction at the right moment a neutron trigger / initiator is used, see below.

The majority of the technical difficulties of designing and manufacturing a fission weapon are based on the need to both reduce the time of assembly of a supercritical mass to a minimum and reduce the number of stray (pre-detonation) neutrons to a minimum.

The isotopes desirable for a nuclear weapon are those which have a high probability of fission reaction, yield a high number of excess neutrons, have a low probability of absorbing neutrons without a fission reaction, and do not release a large number of spontaneous neutrons. The primary isotopes which fit these criteria are U-235, Pu-239 and U-233.

Enriched materials

Uranium-235 and plutonium-239 have so far been the fissionable materials used in atomic bombs. In most cases, producing or procuring enriched materials has historically been the most difficult part in new weapons development programs. During the Manhattan Project, for example, around 90% of the total program budget was devoted to facilities to produce enriched materials.

Uranium-235

Naturally occurring uranium consists mostly of the isotope U-238 (99.29%), with only a small part being the fissile isotope U-235 (0.71%). The U-238 isotope has a high probability of absorbing a neutron without a fission, and also a higher rate of spontaneous fission. As such, the presence of too much U-238 in a fission weapon will impede the chain reaction and result in a fizzle. For weapons, uranium is enriched through one of many varieties of isotope separation, most of which rely on the fact that U-235 is slightly lighter than U-238. This is often the most difficult part of any nuclear weapons production program, as all forms of isotope enrichment require considerable technological investment and capability.

Uranium which is more than 80% U-235 is called highly enriched uranium (HEU), and weapons grade uranium is at least 93.5% U-235. U-235 has a spontaneous fission rate of 0.16 fissions/s-kg. which is low enough to make super critical assembly relatively easy. The critical mass for an unreflected sphere of U-235 is about 50 kg, which is a sphere with a diameter of 17 cm. This size can be reduced to about 15 kg with the use of a neutron reflector surrounding the sphere.

Plutonium-239

Plutonium (atomic number 94, two more than uranium) occurs naturally only in small amounts within uranium ores. Military or scientific production of plutonium is achieved by exposing purified U-238 to a strong neutron source (e.g., in a breeder reactor). When U-238 absorbs a neutron the resulting U-239 isotope then beta decays twice into Pu-239. Pu-239 has a higher probability for fission than U-235, and a larger number of neutrons produced per fission event, resulting in a smaller critical mass. Pure Pu-239 also has a reasonably low rate of neutron emission due to spontaneous fission (10 fission/s-kg), making it feasible to assemble a supercritical mass before predetonation.

In practice, however, reactor-bred plutonium produced will invariably contain a certain amount of Pu-240 due to the tendency of Pu-239 to absorb an additional neutron during production. Pu-240 has a high rate of spontaneous fission events (415,000 fission/s-kg), making it an undesirable contaminant. It is because of this limitation that plutonium-based weapons must be implosion-type, rather than gun-type (see below). Weapons-grade plutonium must contain no more than 7% Pu-240; this is achieved by only exposing U-238 to neutron sources for short periods of time to minimize the Pu-240 produced. The critical mass for an unreflected sphere of plutonium is 16 kg, but through the use of a neutron reflecting tamper the pit of plutonium in a fission bomb is reduced to 10 kg, which is a sphere with a diameter of 10 cm.

Roughly the following values apply: there are 80 generations of the chain reaction, each requiring the time a neutron with a speed of 10,000 km/s needs to travel 10 cm; this takes 80 times 0.01 µs. Thus, the supercritical mass has to be kept together for 1 µs.

Combination methods

As mentioned above, there are two techniques for assembling a supercritical mass. Broadly speaking, one brings two sub-critical masses together and the other compresses a sub-critical mass into a supercritical one.

Gun method

The simplest technique for assembling a supercritical mass is to shoot one piece of fissile material as a projectile against a second part as a target, usually called the gun method (sometimes also as gun-type method), in reference to the fact that it is shooting the material through an artillery barrel as it would a projectile. Usually this is most easily explained as two sub-critical hemispheres shot together to make a supercritical sphere. This is roughly how the "Little Boy" weapon which was detonated over Hiroshima worked, using uranium-235 as its enriched material. In detail, it most likely utilized some arrangement of a uranium "bullet" powered by a cordite charge into a target of uranium rings, rather than using two hemispheres.

The use of "rings" had two advantages: it allowed the target to confidently remain subcritical (the hollow column served to keep the material from having too much contact with other material), and it allowed sub-critical assemblies to be tested using the same "bullet" but with just one ring.

In the Little Boy design the, U-235 "bullet" was a mass of around 24 kg, and it was 16 cm long, with a diameter of 10 cm, and the target had a mass of around 36 kg. The barrel, weighing 450 kg, was that of an anti-aircraft gun, with a 16 cm outer diameter, and the 7.5 cm inside diameter bored out to 10 cm. Its length is 180 cm, which allowed the bullet to accelerate to its final speed of 300 m/s before coming into contact with the target. If the bullet was too slow in arriving to the target, the reaction would start before the material could be fully joined, it would have caused a fizzle, a pre-detonation which would cause the material blows itself apart before creating much of an explosion. As it was, the reaction already would have begun to start when the bullet was at a distance of 25 cm from the target.

In the case of Little Boy the 20% U-238 in the uranium had 70 spontaneous fissions per second. With the fissional material in a supercritical state, each gave a large probability of detonation: each fission creates on average 2.52 neutrons, which each have a probability of more than 1/2.52 to create another fission. During the 1.35 ms of supercriticality prior to full assembly, there was a 10% probability of a fission, with a somewhat less probability of pre-detonation.

Initially, the Manhattan Project planners had planned to use a similar design for their plutonium bomb (known then as the "Thin Man"). However it was discovered that the impurities in reactor-bred plutonium (discussed previously) would have made it impossible to accelerate the bullet to a speed which would have prevented pre-detonation. For this reason the attention of the project was then turned to the development of an implosion-type weapon for use with plutonium (described below).

The gun method has also been applied for nuclear artillery shells. An advantage is that it is easier to keep the diameter small.

However, for technologically advanced states the gun-type method is now essentially obsolete, for reasons of efficiency and safety (discussed below). With regard to the risk of proliferation and use by terrorists, the relatively simple design is a concern, as it does not require as much fine engineering or manufacturing as other methods. With enough highly-enriched uranium (not itself an easy thing to acquire), nations or groups with relatively low levels of technological sophistication could create an inefficient—though still quite powerful—gun-type nuclear weapon.

The scientists who designed the "Little Boy" weapon were confident enough of its likely success that they did not field-test a design first before using it in war (though they did perform non-destructive tests with sub-critical assemblies, as part of their experiments known as tickling the dragon's tail). In any event, it could not be tested before being deployed as there was only sufficient U-235 available for one device.

Implosion method

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The more difficult, but in many ways superior, method of combination is referred to as the implosion method and uses conventional explosives surrounding the material to rapidly compress the mass to a supercritical state. This compression increase the density by a factor 2 to 3, or 4 . In addition,many modern atomic weapons use a hollow core pit. When a hollow core pit is used: the spherical implosion wave both collapses the cavity, and it also increases the density of the pit.

The spherical compression of the pit takes 1 to 4 microseconds. Implosion assembles the nuclear core up to 30 times faster than does the gun method. The maximium implosive copression of the core last until the core expands thermodynamicly enough to become subcritical. Then the self sustaining nuclear fission chain reaction comes to an end.

Alpha phase Pu-239 may contain a 1% fraction of Pu-240 inside of it. Pu-240 atoms freqauntly undergoe spontaneous nuclear fission.This produce many free neutrons inside of a pit. This can cause a gun-type device to begin fissioning its fuel before the pit is fully assembled. This can lead to an inefficient atomic explosion.

To avoid predetonation of the pit , the more technically difficult implosion method is generally used for plutonium fueled A-bombs. The A-bomb bomb used in the "Trinity" shot. and the subsequent "Fat Man" weapon detonated over Nagasaki were fueled by Plutonium 239.

An advantage of increasing the density, in addition to increasing the neutron multiplication factor k of the fissionable assembly, is that the generation time between a neutron being released and causing a new fission, decreases. This makes the process faster, enabling more fissions before the assembly is torn apart.

This precision compression of the pit creates a need for very precise design and machining of the pit and explosive lenses. The milling machines used are so precise that they could cut the polished surfaces of eyeglass lenses. There are strong suggestions that modern implosion devices use non-spherical configurations as well, such as ovoid shapes ("watermelons").

Pit

Casting and then machining plutonium is difficult not only because of its toxicity but also because plutonium has many different metallic phases and changing phases as it cools distorts the metal. This is normally overcome by alloying it with 3–3.5 molar% (0.9–1.0% by weight) gallium which makes it take up its delta phase over a wide temperature range. When cooling from molten it then only suffers a single phase change, from its epsilon phase to the delta one instead of four changes. Other trivalent metals would also work, but gallium has a small neutron absorption cross section and helps protect the plutonium against corrosion. A drawback is that gallium compounds themselves are corrosive and so if the plutonium is recovered from dismantled weapons for conversion to plutonium oxide for power reactors, there is the difficulty of removing the gallium. Modern pits are often composites of plutonium and uranium-235.

Because plutonium is chemically reactive and toxic if inhaled or enters the body by any other means, it is usual to plate the completed pit with a thin layer of inert metal. In the first weapons, nickel was used but gold is now preferred.

Explosive lenses

It is not good enough to pack explosive in a spherical shell around the tamper and detonate it simultaneously at several places because the tamper and plutonium pit will simply be squeezed out of the gaps in the detonation front. This is due to the fact that the simultaneous detonation of all parts of the high explosive is required to create a successful spherical implosion wave.To achieve simultaneous detonation of all of the high explosive, the high explosive needs to be detonated simultaneously at up to 100 exactly symmetrical points. The implosion shock wave must be carefully shaped into a perfect sphere centred on the pit and travelling inwards. This is done by carefully shaping it using a spherical shell made of closely fitting and accurately shaped bodies of explosives of different propagation speeds to form explosive lenses.

There are two general ways to construct the spherical shell of high explosives around the pit. The first method is to cast a hollow sphere of a high explosive such as TNT around the pit. The second method for doing this: is to place three layers of overlapping, and specially shaped high explosive charges around the pit. This method of assembling the spherical shell of high explosive requires the use of PBXs, which are plastic bonded high explosives such as C-4.

The lenses must be accurately shaped, chemically pure and homogenous for precise control of the speed of the detonation front. The casting and testing of these lenses was a massive technical challenge in the development of the implosion method in the 1940s, as was measuring the speed of the shock wave and the performance of prototype shells. It also required electric exploding-bridgewire detonators to be developed which would explode at exactly the same moment so that the explosion starts at the centre of each of the lenses simultaneously (within less than 100 nanoseconds).

The bomb dropped onto Nagasaki used 32 symetrical high explosive detonators to set it off .These detonators were wired into 8 exploding bridge wire type series circuits The exploding-bridgewire detonators were later replaced by slapper detonators, a similar but improved design that is now used in modern weapons, both nuclear and conventional.

Pusher

The explosion shock wave might be of such short duration that only a fraction of the pit is compressed at any instance as it passes through it. A pusher shell made out of low density metal such as aluminium, beryllium, or an alloy of the two metals (aluminium being easier and safer to shape but beryllium reflecting neutrons back into the core) may be needed and is located between the explosive lens and the tamper. It works by reflecting some of the shock wave backwards which has the effect of lengthening it. The tamper or reflector might be designed to work as the pusher too, although a low density material is best for the pusher but a high density one for the tamper. To maximize efficiency of energy transfer, the density difference between layers should be minimized.

Some U.S. weapons since the 1950s have employed a concept called pit "levitation," whereby an air gap is introduced between the pusher and the pit. The effect of this is to allow the pusher to gain momentum before it hits the core, which allows for more efficient and complete compression. A common analogy is that of a hammer and a nail: leaving space between the hammer and nail before striking greatly increases the compressive power of the hammer (as compared to putting the hammer right on top of the nail before beginning to push).

Many modern nuclear weapons use a hollow sphere of Pu-239, or U-235 that is placed inside of a hollow sphere of beryllium, tungsten carbide, or U-238 that serves as the tamper. This may also be placed inside of a hollow pusher sphere made of aluminium, steel, or other metallic material. Additionaly a gram or so of tritium and/or deuterium gas may be injected into the hollow core prior to implosion to achieve a "boosting" effect from small amounts of nuclear fusion (see below).

Tamper reflector

A tamper is an optional layer of dense material (typically natural or depleted uranium or tungsten) surrounding the fissile material. It reduces the critical mass and increases the efficiency by its inertia which delays the expansion of the reacting material.

The tamper prolongs the very short time the material holds together under the extreme pressures of the explosion, and thereby increases the efficiency, i.e. the fraction of the fissile material that actually fissions. The high density has more effect on that than high tensile strength. A coincidence that is fortunate from the point of view of the weapon designer is that materials of high density are also excellent reflectors of neutrons.

While the effect of a tamper is to increase the efficiency both by reflecting neutrons and by delaying the expansion of the core, the effect on the efficiency is not as great as the effect on the critical mass. The reason for this is that the process of reflection is relatively time consuming and may not occur extensively before the chain reaction is terminated.

Neutron reflector

A neutron reflector layer is an optional layer commonly found as the closest layer surrounding the fissile material. This may be the same material used in the tamper, or a separate material. While many tamper materials are adequate reflectors, the best reflector (beryllium metal) makes an extremely poor tamper.

The most efficient reflector is believed to be beryllium metal, then beryllium oxide and tungsten carbide roughly equally efficient, then uranium, then tungsten, then copper, then water, then graphite and iron which are roughly equally efficient.

There is a design tradeoff in choosing to employ a tamper, reflector, or combined material. The weight of the combined pit assembly (any pusher, tamper, reflector, and the fissile material all together) has to be accelerated inwards by the implosion assembly explosives. The larger the pit assembly, the more explosives are needed to implode it to a given velocity and pressure. Early implosion nuclear weapons used heavy pushers and tampers, which were moderately effective reflectors (natural uranium tampers, for example). Levitated or hollow pits increase the energy efficiency of implosion. Using highly efficient, lightweight reflectors made of beryllium further increases the mass efficiency of the implosion system. Such pits are only slightly tamped, and will dissassemble very rapidly once the fission reaction reaches high energy levels.

Before fusion boosting, it was arguable whether the most efficient overall system employed dedicated high mass tampers or not. Now that modern weapons typically use fusion boosting, which increases the reaction rate tremendously, lack of tamper material is no longer a drawback. This has helped allow for the miniaturization of nuclear weapon systems.

Neutron trigger / initiator

One of the key elements in the proper operation of a nuclear weapon is initiation of the fission chain reaction at the proper time. To obtain a significant nuclear yield of the nuclear explosive, sufficient neutrons must be present within the supercritical core at just the right time. If the chain reaction starts too soon, the result will be only a 'fizzle yield', much below the design specification; if it occurs too late, there may be no yield whatsoever. Several ways to produce neutrons at the appropriate moment have been developed.

Early neutron triggers consisted of a highly radioactive isotope of Polonium (Po-210), which is a strong alpha emitter combined with beryllium which will absorb alphas and emit neutrons. This isotope of polonium has a half life of almost 140 days. Therefore, a neutron initiator using this material needs to have the polonium replaced frequently. The polonium is produced in a nuclear reactor. The initiator used in the Fat man A-bomb was called the urchin. It was a marble sized hollow sphere made of berylium metal, that had a little bit of polonium 210 inside of it.

The initiation pulse of the neutrons must occurr at the right time. In an initiator the polonium, and the beryllium need to be kept apart until the appropriate moment. The appropriate moment to initiate a neutron pulse is when the spherical implosion wave strikes the pit. The detonation of a gun type atomic weapon does not require a neutron source to initiate it.

Another method of providing source neutrons is through a pulsed neutron emitter, which is a small ion accelerator with a metal hydride target. When the ion source is turned on to create a plasma of deuterium or tritium, a large voltage is applied across the tube which accelerates the ions into tritium rich metal (usually scandium). The ions are accelerated so that there is a high probability of nuclear fusion occurring. The deuterium-tritium fusion reactions emit a short pulse of 14 MeV neutrons which will be sufficient to initiate the fission chain reaction. The timing of the pulse can be precisely controlled, making it better for an implosion weapon design.

An initiator is not strictly necessary for an effective gun design , as long as the design uses "target capture" (in essence, ensuring that the two subcritical masses, once fired together, cannot come apart until they explode). Initiators were only added to Little Boy late in its design. The use of an initiator can guarantee precise control (to the millisecond) over the timing of the explosion.

Comparison of the two methods

The gun type method is essentially obsolete and was abandoned by the United States as soon as the implosion technique was perfected. Other nuclear powers, such as the United Kingdom, never even built an example of this type of weapon. As well as only being possible to produce this weapon using highly enriched U-235, the technique has other severe limitations. The implosion technique is much better suited to the various methods employed to reduce the weight of the weapon and increase the proportion of material which fissions.

There are also safety problems with gun type weapons. For example, it is inherently dangerous to have a weapon containing a quantity and shape of fissile material which can form a critical mass through a relatively simple accident. Furthermore if the weapon is dropped from an aircraft into the sea, then the moderating effect of the light sea water can also cause a criticality accident without the weapon even being physically damaged. Neither can happen with an implosion type weapon since there is normally insufficent fissile material to form a critical mass without the correct detonation of the lenses.

Implosion type weapons normally have the pit physically removed from the centre of the weapon and only inserted during the arming procedure so that a nuclear explosion cannot occur even if a fault in the firing circuits causes them to detonate the explosive lenses simultaneously as would happen during correct operation. It has also been hypothesized, based on open sources, that the Permissive Action Links for some types of nuclear weapons may involve the use of encoded secret timing offsets for the explosives in a given weapon's combination of explosives and lenses, needed to create a unified focus for the shockfront.

Alternatively, the pit can be "safed" by having its normally-hollow core filled with an inert material such as a fine metal chain. While the chain is in the center of the pit, the pit can't be compressed into an appropriate shape to fission; when the weapon is to be armed, the chain is removed. Similarly, a serious fire will detonate the explosives, destroying the pit and spreading plutonium to contaminate the surroundings (as has happened in several weapons accidents) but cannot possibly cause a nuclear explosion.

The South African nuclear program was probably unique in adopting the gun technique to the exclusion of implosion type devices, and built around five of these weapons before they abandoned their program.

Practical limitations of fission weapons

A pure fission bomb is practically limited to a yield of a few hundred kilotons by the large amounts of fissile material needed to make a large weapon. It is technically difficult to keep a large amount of fissile material in a subcritical assembly while waiting for detonation, and it is also difficult to physically transform the subcritical assembly into a supercritical one quickly enough that the device explodes rather than prematurely detonating such that a majority of the fuel is unused (inefficient predetonation). The most efficient pure fission bomb possible would still only consume ~25% of its fissile material before being blown apart, and can often be much less efficient (Fat Man only had an efficiency of 14%, Little Boy only about 1.4% efficient). Large yield, pure fission weapons are also unattractive due to the weight, size, and cost of using large amounts of highly enriched material. The highest yield pure fission bomb design was that detonated by the United States as the Ivy King test in 1952, with a yield of 500 kt. It is likely near the upper limit of a pure fission design, and was itself a highly complex and potentially dangerous device.

Another limitation of some fission bomb designs is the need to keep the electronic circuitry within a certain range of temperatures to prevent malfunction. Some weapons were designed with internal heaters to maintain a stable temperature (a method still used by NASA in its space probes); other, more unusual, methods were contemplated by the United Kingdom (see chicken powered nuclear bomb).

Thermonuclear weapons

The amount of energy released by a weapon can be greatly increased by the addition of nuclear fusion reactions. Fusion releases even more energy per reaction than fission, and can also be used as a source for additional neutrons. The light weight of the elements used as fusion fuel, combined with the larger energy release, means that fusion is a very efficient fuel by weight, making it possible to build extremely high yield weapons which are still portable enough to easily deliver. Fusion is the combination of two light atoms, usually isotopes of hydrogen, to form a more stable heavy atom and release excess energy. The fusion reaction requires the atoms involved to have a high thermal energy, which is why the reaction is called thermonuclear. The extreme temperatures and densities necessary for a fusion reaction in these weapons are generated by the explosion of a fission weapon (a pure fusion weapon is a hypothetical design that does not need a fission primary, but no weapons of this sort have ever been developed). Thermonuclear weapons are also known as fusion weapons or hydrogen bombs.

Fusion boosting

The simplest way to utilize fusion is to put a mixture of deuterium and tritium inside the hollow core of an implosion style plutonium pit (which usually requires an external neutron generator mounted outside of it rather than the initiator in the core as in the earliest weapons). When the imploding fission chain reaction brings the fusion fuel to a sufficient pressure, a deuterium-tritium fusion reaction occurs and releases a large number of energetic neutrons into the surrounding fissile material. This increases the rate of burn of the fissile material and so more is consumed before the pit disintegrates. The efficiency (and therefore yield) of a pure fission bomb can be doubled (from about 20% to about 40% in an efficient design) through the use of a fusion boosted core, with very little increase in the size and weight of the device. The amount of energy released through fusion is only around 1% of the energy from fission, so the fusion chiefly increases the fission efficiency by providing a burst of additional neutrons.

Fusion boosting provides two strategic benefits. The first is that it obviously allows weapons to be made much smaller, lighter, and use less fissile material for a given yield, making them cheaper to build and deliver. The second benefit is that it can be used to render weapons immune to radiation interference (RI). It was discovered in the mid-1950s that plutonium pits would be particularly susceptible to partial pre-detonation if exposed to the intense radiation of a nearby nuclear explosion (electronics might also be damaged, but this was a separate issue). RI was a particular problem before effective early warning radar systems because a first strike attack might make retaliatory weapons useless. Boosting can reduce the amount of plutonium needed in a weapon to below the quantity which would be vulnerable to this effect.

While this technique, sometimes known as "gas boosting," uses fusion — the reaction associated with the so-called “hydrogen bomb” — it is still seen as simply boosting what is still properly a “fission” bomb. In fact, fusion boosting is very common and used in most modern weapons, including the fission primaries in most thermonuclear weapons.

Staged thermonuclear weapons

The basic principles behind modern thermonuclear weapons design were developed independently by scientists in different countries. Edward Teller and Stanislaw Ulam at Los Alamos worked out the idea of staged detonation coupled with radiation implosion in what is known in the United States as the Teller-Ulam design in 1951. Soviet physicist Andrei Sakharov independently arrived at the same answer (which he called his "Third Idea") in 1955.

The full details of staged thermonuclear weapons have never been fully declassified and among different sources outside the wall of classification there is no strict consensus over how exactly a hydrogen bomb works. The basic principles are revealed through two separate declassified lines by the US Department of Energy: "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 "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."

A general interpretation of this, following on the 1979 court case of United States v. The Progressive (which sought to censor an article about the workings of the hydrogen bomb; the government eventually dropped the case and much new information about the weapon was declassified), is as follows:

A fission weapon (the "primary") is placed at one end of the warhead casing. When detonated, it first releases X-rays at the speed of light. These are reflected from the casing walls, which are made of heavy metals and serve as X-ray mirrors. The X-rays travel into a space surrounding the secondary, which is a column or sphere of lithium-deuteride fusion fuel encased by a natural uranium "tamper"/"pusher". Here, the X-rays cause a pentane-impregnated polystyrene foam filling the case to convert into a plasma, and the X-rays cause ablation of the surface of the jacket surrounding the secondary, imploding it with enormous force. Inside the secondary is a "sparkplug" of either enriched uranium or plutonium, which is caused to fission by the compression, and begins its own nuclear explosion. Compression of the fusion fuel and the high temperature caused by the fission explosion cause the deuterium to fuse into helium and emit copious neutrons. The neutrons transmute the lithium to tritium, which then also fuses and emits large amounts of gamma rays and more neutrons. The excess neutrons then cause the natural uranium in the "tamper", "pusher", case and x-ray mirrors to undergo fission as well, adding more power to the yield.

The process by which the “secondary” (fusion) stage is compressed by the x-rays from the “primary” (fission) stage is known as “radiation implosion.” It was widely misunderstood for many years and is incorrectly depicted in many authorities, including Rhodes (1986) and in the Progressive article. The actual mechanism that compresses the secondary stage is neither the “radiation pressure” of the x-rays, nor the physical pressure of the plasmatized foam. It is also not attributable to a neutron flux or hydrodynamic shock moving down from the primary. Instead it is only the x-ray radiation 'burning' the outside surface of the tamper/pusher away. X-rays surround and heat the whole outside of the tamper/pusher until the outside layer of it ablates/explodes away from the secondary in all directions, creating an inward “ablation pressure.” In other words, heated by x-rays, the outside layers of the tamper/pusher explode outward, like a rocket, driving the remaining layers inward in an implosion.

Advanced thermonuclear weapons designs

The largest modern fission-fusion-fission weapons include a fissionable outer shell of U-238, the more inert waste isotope of uranium, or X-ray mirrors constructed of polished U-238. This otherwise inert U-238 would be detonated by the intense fast neutrons from the fusion stage, increasing the yield of the bomb many times. For maximum yield, however, moderately enriched uranium is preferable as a jacket material. For the purposes of miniaturization of weapons (fitting them into the small re-entry vehicles on modern MIRVed missiles), it has also been suggested that many modern thermonuclear weapons use spherical secondary stages, rather than the column shapes of the older hydrogen bombs.

Multi-staged thermonuclear devices have been developed. In such a design, a fission “primary” is used to compress a fusion “secondary,” in the usual fashion, but that explosion is, in turn, used to compress another, much larger fusion stage, the “tertiary.” A three-stage bomb, the Mk. 41, was deployed by the United States and the USSR’s Tsar Bomba, was also a three stage weapon. In theory, there is no limit to how many stages could be added, though there is no foreseeable need for five or six stage weapons with yields which could approach a gigaton.

The cobalt bomb uses cobalt in the shell, and the fusion neutrons convert the cobalt into cobalt-60, a powerful long-term (5 years) emitter of gamma rays, which produces major radioactive contamination. In general this type of weapon is a salted bomb and variable fallout effects can be obtained by using different salting isotopes. Gold has been proposed for short-term fallout (days), tantalum and zinc for fallout of intermediate duration (months). To be useful for salting, the parent isotopes must be abundant in the natural element, and the neutron-bred radioactive product must be a strong emitter of penetrating gamma rays.

The primary purpose of this weapon is to create extremely radioactive fallout to deny a region to an advancing army, a sort of wind-deployed mine-field. No cobalt weapons have have ever been atmospherically tested, and as far as is publicly known none have ever been built. A number of very "dirty" thermonuclear weapons, however, have been developed and detonated, as the final fission stage (usually a jacket of natural or enriched uranium) is itself sometimes analogous to "salting" (for example, the Castle Bravo shot). The British did test a bomb that incorporated cobalt as an experimental radiochemical tracer (Antler/Round 1, 14 September 1957). This 1 kt device was exploded at the Tadje site, Maralinga range, Australia. The experiment was regarded as a failure and not repeated.

The thought of using cobalt, which has the longest half-life of the feasible salting materials, caused Leó Szilárd to refer to the weapon as a potential doomsday device. With a five year half-life, people would have to remain shielded underground for a very long time until it was safe to emerge, effectively wiping out humanity. However, no nation has ever been known to pursue such a strategy. The movie Dr. Strangelove famously incorporated such a doomsday weapon as a major plot device.

A final variant of the thermonuclear weapons is the enhanced radiation weapon, or neutron bomb which are small thermonuclear weapons in which the burst of neutrons generated by the fusion reaction is intentionally not absorbed inside the weapon, but allowed to escape. The X-ray mirrors and shell of the weapon are made of chromium or nickel so that the neutrons are permitted to escape.

This intense burst of high-energy neutrons is the principal destructive mechanism. Neutrons are more penetrating than other types of radiation so many shielding materials that work well against gamma rays do not work nearly as well. The term "enhanced radiation" refers only to the burst of ionizing radiation released at the moment of detonation, not to any enhancement of residual radiation in fallout (as in the salted bombs discussed above).

Other design issues

Miniaturization

The first nuclear weapons developed were large and idiosyncratic devices which weighed many tons and could only be dropped out of especially large aircraft as gravity bombs. In the years immediately after World War II, though, developments in rocketry spurred efforts to reduce the size of nuclear weapons to the point that they could fit on a missile. Over time, miniaturization of weapons has proven a major investment of nations with nuclear weapons, and one of the primary justifications used for programs of nuclear testing, which provide the raw data necessary for pushing the limits of nuclear weapons design. Weapons systems which require relatively small thermonuclear weapons, such as MIRVed missiles, are thought to be obtainable only with large amount of test result data. The physically smallest nuclear warheads deployed by the United States was the W54, used in the Davy Crockett recoilless rifle; the warheads in this weapon weighed about 23 kilograms and had yields ranging between 0.01 and 0.25 kilotons. This is small by comparison to thermonuclear weapons, but remains a very large explosion with lethal acute radiation effects and potential for substantial fallout. It is generally believed that the W54 is close to the smallest possible nuclear weapon.

Stockpile stewardship

In the years after the Cold War, most nuclear powers eventually stopped conducting programs of nuclear testing, primarily for political reasons. In many countries this has led to questions about how much was known about the safety and reliability of aging nuclear weapons systems. In the U.S., a program of stockpile stewardship has attempted to test the reliability of old warheads without full nuclear testing, often by using computer simulation.

References

• Glasstone, Samuel and Dolan, Philip J., The Effects of Nuclear Weapons (third edition), U.S. Government Printing Office, 1977. PDF Version
• Cohen, Sam. The Truth About the Neutron Bomb: The Inventor of the Bomb Speaks Out, William Morrow & Co., 1983.
• Militarily Critical Technologies List (MCTL) from the US Government's Defense Technical Information Center
• Grace, S. Charles, Nuclear Weapons: Principles, Effects and Survivability (Land Warfare: Brassey's New Battlefield Weapons Systems and Technology, vol 10)
• Smyth, Henry DeWolf. Atomic Energy for Military Purposes. Princeton, N.J.: Princeton University Press, 1945. (see: Smyth Report)
• The Effects of Nuclear War, Office of Technology Assessment (May 1979).
• Rhodes, Richard. Dark Sun: The Making of the Hydrogen Bomb. New York: Simon and Schuster, 1995. ISBN 0684824140
• Rhodes, Richard. The Making of the Atomic Bomb. New York: Simon and Schuster, 1986. ISBN 0684813785
• More information on the design of two-stage fusion bombs

External links

• Carey Sublette's Nuclear Weapon Archive is a reliable source of information and has links to other resources.
• The Federation of American Scientists provide solid information on weapons of mass destruction, including nuclear weapons and their effects

• Nuclear technology
• Nuclear weapons