Nuclear weapon design

The first nuclear weapons, though large, cumbersome and inefficient, provided the basic design building blocks of all future weapons. Here the Gadget device is prepared for the first nuclear test: Trinity.

Nuclear weapon designs are physical, chemical, and engineering arrangements that contribute to the detonation of a nuclear weapon. They are divided into two classes, fission type and fusion type. Each class is based on the dominant energy source used at detonation.

• Fission weapons derive their power from nuclear fission. Heavy nuclei, typically uranium or plutonium, are bombarded by neutrons and split into lighter elements, more neutrons and energy. These newly liberated neutrons bombard other nuclei, causing them to split and subsequently bombard other nuclei, and so on. The result is called a nuclear chain reaction that releases immense amounts of energy. These fission weapons are historically called atomic bombs, atom bombs, or A-bombs. These names however, are not precise because most chemical reactions release energy from atomic bonds (excluding bonds between nuclei) and fusion is no less atomic than fission. Despite this confusion, nuclear weapons, specifically fission-types, are referred to as atomic bombs.

• Fusion weapons rely on nuclear fusion. Light nuclei such as deuterium and tritium (isotopes of hydrogen) combine together into heavier elements to release an immense amount of energy. Weapons that have a fusion stage are referred to as hydrogen bombs or H-bombs because of their primary fuel. Hydrogen bombs are also called thermonuclear weapons because the fusion reactions within the weapons require extremely high temperatures to occur.

Similarities between weapon types

The distinction between these two types of weapon is blurred by the fact that fission and fusion reactions are combined in nearly all modern nuclear weapons. In a fusion weapon, an initial small fission reaction is used to reach the necessary conditions of high temperature and pressure required for fusion. Conversely, in a fission weapon, Fusion elements may be present in the core of the fission device because they generate additional neutrons, increasing 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 both fission and fusion weapons release energy from transformations of the atomic nucleus, the general term for all types of nuclear explosive devices is nuclear weapon.

Other nuclear weapons

Other specific types of nuclear weapons include: neutron bomb (enhanced radiation) and cobalt bomb (increased fallout).

Fission weapons

The simplest nuclear weapons are pure fission bombs. Fission devices were the first type of nuclear weapon built during the Manhattan Project. They are the basic building block for all advanced nuclear weapons.

Fissile materials

A fissile material is one that can support a fission chain reaction. Uranium-235 and plutonium-239 are the fissile materials most often used in nuclear bombs. Producing or procuring them is usually the most difficult part of a weapons development program. During the Manhattan Project for example, around 90% of the total program budget was devoted to the production of U-235 and Pu-239. Modern weapon cores known as pits (see below), often combine the two isotopes.

The most desirable isotopes used 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 have a low spontaneous fission rate. The primary isotopes which fit these criteria are U-235, Pu-239 and U-233.

Several other isotopes have been considered as potentially usable in fission weapons, though no country has been known to produce them for this purpose. Bombs using uranium-233 are possible, however it is not known if any have been tested. The fact that neptunium-237 "can be used for a nuclear explosive device" was declassified by the U.S. Department of Energy in 1992. Research by the Los Alamos National Laboratory in 2002 determined that the critical mass for Np-237 is about 60 kg, 20% higher than that of U-235.

Uranium-235

The Calutron was an early form of electromagnetic isotope separation used to produce enriched uranium, and operated on the principle that U-235 is slightly lighter than U-238.

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. The most difficult part of any nuclear weapons production program, is the considerable technological investment needed for isotope enrichment. Uranium of more than 20% 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. This is a sphere with a diameter of 17 cm. The critical size can be reduced to about 15 cm with the use of a neutron reflector surrounding the sphere.

When compressed using the implosion method, the critical mass is reduced, so these values do not indicate the amounts needed for a weapon.

Plutonium-239

A "button" of reactor-bred plutonium.

Plutonium (atomic number 94), 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, such as in a breeder reactor. When U-238 absorbs a neutron, the result is U-239. The isotope beta decays twice into Pu-239. Pu-239 has a higher probability of 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 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 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. Because of this limitation plutonium-based weapons must be implosion-type, rather than gun-type (see below). Weapons-grade plutonium contains 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, resulting in a sphere with a diameter of 10 cm.

Whether plutonium with higher Pu-240 content is usable in a nuclear weapon is not certain from the unclassified literature, but some sources claim that it is possible . Even if technically feasible, such a weapon's yield would be unpredictable and probably much lower than with weapons-grade plutonium given a similarly sophisticated bomb design, and the radioactivity of the Pu-240 would considerably complicate handling.

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. Therefore, the supercritical mass has to be kept together for 0.8 µs.

Uranium-233

U-233 is an artificially produced isotope, bred from thorium-232 in a nuclear reactor. The fissile properties of U-233 are generally somewhere between those of U-235 and Pu-239. In practice, uranium 233 is relatively difficult to use in weapons because its high rate of alpha emission acts on impurities producing unwanted neutrons. After a few years its decay products produce harmful radiation, dangerous to anyone refurbishing the weapons. However, using U-233 as fissile material is considered viable, especially for countries such as India, that have thorium but not uranium deposits.

Efficiency

Main article: Nuclear weapon yield

The efficiency of a fission weapon is the fraction of the fissile material that actually fissions. For a pure fission weapon, the maximum is approximately 25%. For Fat Man it was 14%, for Little Boy only 1.4%. Fusion boosting increases the fission efficiency to 40%.

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, resulting in a chain reaction producing 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 advancing the chain reaction quickly.

The main challenge in producing an efficient explosion using nuclear fission is keeping the bomb together long enough for the release of a substantial fraction of the available nuclear energy.

Prior to detonation, a nuclear weapon consists of one or more pieces of weapons-grade fissionable materials which are in a subcritical state. In designs where two subcritical masses are physically separated from each other, they are rapidly brought together at detonation. The main method employed is the implosion method, where one sphere of material is compressed and becomes supercritical.

A simpler method is the gun method, this is 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, however, the two pieces are shaped favorably. i.e. The first piece fits into a hole in the second piece when brought together. Alternatively, two subcritical hemispheres are brought together to create a critical sphere. More than two pieces of subcritical material may also be used to produce a spherical critical mass.

In order to avoid predetonation, whereby the material heats up and expands rapidly before being in its optimal state, and to produce an efficient nuclear detonation, the fissile material must be brought into its optimal supercritical state very rapidly. Consequently, only a small—sometimes very small—part of the fissionable material actually undergoes fission and the yield is correspondingly low.

To start the chain reaction at the right moment, a neutron trigger / initiator is used.

The main technical difficulty in design and manufacture of a fission weapon is the need to both reduce the time of assembly of a supercritical mass to a minimum and to keep the number of stray (pre-detonation) neutrons to a minimum.

Supercritical mass combination methods

There are two techniques for assembling a supercritical mass. One brings two sub-critical masses together, known as the Gun method. The other compresses a sub-critical mass into a supercritical one, known as the Implosion method.

Gun method

The "gun" assembly method.

The "gun" assembly method.

Main article: Gun-type fission weapon

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. This is called the gun-type method. The Little Boy weapon detonated over Hiroshima worked this way.

Because of the relatively long amount of time it takes to combine the materials, this method of combination can only be used for U-235 as predetonation is more likely for Pu-239 which has a higher spontaneous neutron release due to Pu-240 contamination. Because of this disadvantage, a plutonium gun-type weapon would have to be impractically long to accelerate the plutonium "bullet" to the required extreme velocity.

Although in Little Boy 60 kg of 80% grade U-235 was used (hence 48 kg), the minimum is approximately 20 to 25 kg, versus 15 kg for the implosion method.

For technologically advanced nations the gun method is essentially obsolete. With regard to the risk of proliferation and use by terrorists, this 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, nations or groups with relatively low levels of technological sophistication could create an inefficient, though still quite powerful, nuclear weapon.

The scientists who designed the Little Boy weapon were confident enough of its likely success that they did not test a design first before using it in war. In any event, it could not be tested before being deployed as there was only sufficient U-235 available for one device.

Implosion method

Illustration of the implosion method

The more difficult, but in many ways superior, method of combination is referred to as the implosion method. It uses conventional explosives surrounding the material to rapidly compress the mass to a supercritical state. This compression reduces the volume by a factor of 2 to 3.

For Pu-239 assemblies, a contamination of only 1% of Pu-240 produces so many spontaneous neutrons that a gun-type device would begin fissioning before full assembly, leading to very low efficiency. For this reason the more technically difficult implosion method must be used for plutonium bombs such as the test bomb used in the Trinity shot and the subsequent Fat Man weapon detonated over Nagasaki.

Weapons assembled with this method are generally more efficient than the weapons employing the gun method of combination, even ignoring the problem of spontaneous neutrons. The reason that the implosion method is more efficient is because it not only combines the masses, but also increases the density of the mass, and thereby increases the neutron multiplication factor k of the fissionable assembly. Most modern weapons use a hollow plutonium core, or pit, with an implosion mechanism for detonation. Also, low density Pu also called delta-plutonium is used due to its high compressibility.

This schematic animation shows the basics of the implosion process.

A twofold increase in the density of the pit will tend to result in a 10-20 kiloton nuclear explosion. A 3-fold compression may produce a 40-45 kiloton nuclear yield, a fourfold compression may produce a 60-80 kiloton nuclear yield, and a fivefold compression of the pit, which is very hard to achieve, may produce an 80-100 kiloton nuclear yield. Getting a 5-fold compression of the pit requires a very strong, massive and very efficient lens implosion system.

This precision compression of the pit creates a need for very precise design and machining of the pit and explosive lenses. To convert spherically expanding shock waves into spherically converging shock waves requires a hyperbolic boundary between fast and slow explosives. This in turn requires accurate milling/turning of the surface of pre-molded explosives. 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 referred to as watermelons.

The primary of a thermonuclear weapon is thought to be a standard implosion method fission bomb, though probably built with a core boosted by small amounts of fusion fuel for extra efficiency (see below). Also see, Teller-Ulam design.

Pit

The core of a nuclear weapon, the fissile material and any reflector or tamper bonded to it, is known as the pit.

Casting and then machining plutonium is difficult not only because of its toxicity, but also because plutonium has many different metallic phases. As plutonium cools, changes in phase result in distortion. This distortion is normally overcome by alloying it with 3–3.5 molar% (0.9–1.0% by weight) gallium which causes it to take up its delta phase over a wide temperature range. When cooling from molten it then suffers only a single phase change, from epsilon to delta, instead of the four changes it would otherwise pass through. 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 dioxide for power reactors, there is the difficulty of removing the gallium. Modern pits may be composites of plutonium and uranium-235.

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

Pits made of alloys containing a mixture of both plutonium and uranium have been used in the past, and may possibly continue in currently stockpiled designs.

Explosive lens

Modern high explosive lenses. The colored areas are the fast explosive, while the white areas are the slow explosives.

It is not sufficient to pack explosive into a spherical shell around the tamper and detonate it simultaneously at several places because the tamper and plutonium pit will simply squeeze out between the gaps in the detonation front. Instead the shock wave must be carefully shaped into a perfect sphere centered on the pit and travelling inwards. This is achieved by using a spherical shell of closely fitting and accurately shaped bodies of explosives of different propagation speeds to form explosive lenses (see also shaped charge). A spherical high explosive shell like the one described above means a shell composed of more then two high explosive lenses . A spherical high explosive shell containing only 1 or two high explosive lenses ,however can achieve a smooth & successful spherical implosion wave.This requires the use of multi-point electrical detonation in which the entire explosive spherical shell is simultaneously detonated at up to 100 exactly symmetrical, & equidistant points on its outer surface .

The lenses must be accurately shaped, chemically pure and homogeneous 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 detonate at exactly the same moment within the lenses simultaneously. i.e. within 100 nanoseconds. Once the shock wave has been shaped, there may also be an inner homogeneous spherical shell of explosive to give it greater force, known as a supercharge.

The Fat Man device dropped on Nagasaki used 32 lenses in a pattern of a truncated icosahedron. Later, more efficient bombs used 40, 60, 72, and 92 lenses. The arrangement of lenses is similar to a soccer ball.

It is speculated that modern designs may use a prolate spheroidal pit and two-point detonation, i.e. just a single explosive lens at each end of the weapon. The end result is formation of a supercritical sphere, but with a vastly superior level of reliability when compared to a weapon requiring dozens of simultaneous detonations.

Nowadays, exploding-bridgewire detonators have been replaced by slapper detonators, an improved design in both modern nuclear and conventional weapons.

Pusher

The explosion shock wave might be of such short duration that only a fraction of the pit is compressed at any instant 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 and beryllium for its highly neutron reflective capability) may be needed. A pusher is located between the explosive lens and the tamper. It works by reflecting some of the shockwave backwards thereby having the effect of lengthening its duration.

A tamper or reflector may be used as the pusher as well, although a low density material is best used for the pusher but a high density one for the tamper. To maximize efficiency of energy transfer, the density difference between layers is minimized.

Most 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, allowing 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 placed inside of a hollow sphere of beryllium, tungsten carbide, or U-238 serving as the tamper. This may also be placed inside of a hollow pusher sphere made of aluminium, steel, or other metallic material. Additionally, 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 by a small amount of nuclear fusion.

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 short time the material holds together under the extreme pressures of the explosion, thereby increasing the efficiency of the weapon. i.e. increases the fraction of the fissile material that actually fissions. High density (hence high inertia) is important; tensile strength has a negligible effect. Materials of high density also tend to be good reflectors of neutrons.

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, beryllium metal is the best reflector material but makes an extremely poor tamper.

The materials used as reflectors, from highest to lowest efficiency are;

• Beryllium metal,
• Beryllium oxide or Tungsten Carbide,
• Uranium,
• Tungsten,
• Copper,
• Water,
• Graphite or Iron.[1]

There is a design tradeoff in choosing to employ a tamper, reflector, or combined material. The weight of the combined pit assembly (the pusher, tamper, reflector, and the fissile material), has to be accelerated inwards by the implosion assembly explosives. The larger the pit assembly, the more explosive energy is required to compress it at the correct 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 increase the mass efficiency of the implosion system. Such pits are only slightly tamped, and will dissassemble 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 in miniaturization of nuclear weapons.

Neutron trigger or, 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, 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', well 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 138 days. Therefore, a neutron initiator using this material needs to have the polonium replaced frequently. The polonium is produced in a nuclear reactor. To supply the initiation pulse of neutrons at the right time, the polonium and the beryllium need to be kept apart until the appropriate moment and then thoroughly and rapidly mixed by the implosion of the weapon. This method of neutron initiation is sufficient for weapons utilizing the slower gun combination method, but the timing is not precise enough for an implosion weapon design.

The Fat Man weapon used a finely tooled initiator known as an "urchin", made of alternating concentric layers of beryllium and polonium separated by thin gold foil. When these layers are mixed by the Munroe effect, the high-energy alpha particles produced by the polonium collide with beryllium nuclei, expelling neutrons thereby initiating the fission process.

Another method of providing source neutrons is through a pulsed neutron emitter, which is a small ion accelerator with a metal hydride target. Ion acceleration increases the probability of nuclear fusion. When the ion source is turned on it creates a plasma of deuterium or tritium, a large voltage is then applied across the tube accelerating the ions into a tritium-rich metal (usually scandium). The deuterium-tritium fusion reactions emit a short pulse of 14 MeV neutrons sufficient to initiate the fission chain reaction. Using this method, the timing of the pulse can be precisely controlled (to the millisecond). Because of this, ion acceleration is particularly suited to an implosion weapon.

An initiator is not strictly necessary for an effective gun design, as long as the design uses "target capture", which is a method that ensures that the two subcritical masses once together, cannot come apart until they explode. Initiators were added to Little Boy later in its design.

Comparison between methods

The gun-type method is essentially obsolete and completely abandoned by the United States by the early 1960s. There was some use of the design, or possibly a variant "double gun" design (in which two subcritical masses were accelerated towards each other), in nuclear artillery [2]. Other nuclear powers, such as the United Kingdom, never built any 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.It is known however, that the republic of South Africa did manufacture up to 7 gun type atomic weapons in the 1980s. * These were later disassembled during the 1990s , and their Heu pits were placed in storage .

Safety issues associated with Arming a weapon

• Gun-method Weapon issues

Gun-type weapons have some important safety problems. 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. Also, if the weapon falls into water, then the moderating effect of the light water can also cause a criticality accident, even without the weapon being physically damaged.

• Implosion-method weapon issues

Neither of these effects are possible with implosion-type weapons since there is normally insufficient fissile material to form a critical mass without the correct detonation of the lenses.

Some older implosion-type weapons, such as the US Mark 4 and Mark 5, have the pit physically removed from the center of the weapon and only inserted during the arming procedure, a procedure known as in-flight insertion. This way a nuclear explosion cannot occur even if a fault in the firing circuits causes the explosive lenses to detonate simultaneously. More modern weapons do not use this technique.

Methods used to prevent accidental detonation

• Encryption of firing circuits

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 and lenses needed to create a unified focus for the shockfront as would occur during functional detonation.

A diagram of the Green Grass warhead's steel ball-bearing safety device, shown left, filled (safe) and right, empty (live). The steel balls were emptied into a hopper underneath the aircraft before flight, the steel balls could be re-inserted using a funnel by rotating the bomb on its trolley and raising the hopper.

• Steel ball safety method

As shown in the diagram, one method used to decrease the likelihood of accidental detonation used metal balls. The balls were emptied into the pit. this would prevent detonation by increasing density of the hollowed pit. This design was used in the Green Grass weapon, also known as the Interim Megaton Weapon and was also used in Violet Club and the Yellow Sun Mk.1 bombs.

• Chain safety method

Alternatively, the pit can be "safed" by having its normally-hollow core filled with an inert material such as a fine metal chain, possibly made of cadmium to absorb neutrons. 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, although a serious fire could detonate the explosives, destroying the pit and spreading plutonium to contaminate the surroundings as has happened in several weapons accidents, it could not however, cause a nuclear explosion.

• Wire safety method

The US W47 warhead used in Polaris A1 and Polaris A2 had a safety device consisting of a boron-coated-wire inserted into the hollow pit at manufacture. The warhead was armed by withdrawing the wire onto a spool driven by an electric motor. However, once withdrawn the wire could not be re-inserted. Source: Hansen: Swords of Armageddon.

The South African nuclear program was probably unique in adopting the gun technique to the exclusion of implosion type devices, and disclosed to the IAEA upon joining that organization that they had built six of these weapons before they abandoned their program and dismantled the weapons.

Practical limitations of the fission bomb

The most powerful fission bomb ever tested was probably the American Ivy King with a yield of 500 kt, close to the limit of what is possible (the British Orange Herald design used a type of fusion boosting, although the boosting is believed to have failed).

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 (Little Boy, for instance, was 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.

Note that for nuclear weapons in general the energy just from fission is not limited: e.g., the Castle Bravo was a fission-fusion-fission weapon with a yield from fission alone of 10 Mt, and an additional 5 Mt from fusion. Thus, the direct effect of fusion on the yield is, in this case, smaller than the fusion's effect of enabling greater fission.

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).

Linear implosion

Normal implosion weapons take a subcritical mass of fissile material and compress it into supercriticality.

Linear implosion uses a mass of nuclear material which is more than one critical mass at normal pressure and a spherical configuration. The mass is configured in a lower density non-spherical configuration prior to firing the weapon, and then small to moderate amounts of explosive collapse and slightly reshape the nuclear material into a supercritical mass which then undergoes chain reaction and explodes. The material does not attain higher densities than standard.

Three methods are known to compress and reshape the nuclear material:

• Collapsing hollow spaces inside the nuclear material
• Using plutonium which is stabilized in the low density delta phase at a density of 16.4, which changes phase to stable denser alpha-phase (density 19.8) under moderate explosive compression from shockwaves. Alpha phase plutonium is stable at a density of 19.8 at standard conditions, so this phase change does not count as explosive compression per se.
• Shaping an explosive and nuclear material so that the explosive pressure changes a stretched-out, elliptical or football shape to collapse towards a spherical or more spherical end shape.

Linear implosion is utilized for weapons where a very small diameter, only slightly larger than a critical mass' diameter, is needed in the weapon. 6 inch (152 mm) diameter linear implosion weapons were deployed by the US, the W48 and designed and tested but never deployed W74 and W82, and smaller ones are allegedly possible.

A bare critical mass of plutonium at normal density and without additional neutron reflector material is roughly 10 kilograms. To achieve a large explosive yield (tens or hundreds of tons), a linear implosion weapon needs somewhat more material, on the order of 13 kilograms. 13 kilograms of (highest density) alpha-phase Plutonium at a density of 19.8 g/cm³ is 657 cubic centimeters, a sphere with a diameter of 10.8 cm (4.25 inches).

Linear implosion weapons could use tampers or reflectors, but the overall diameter of the fissile material plus tamper/reflector increases compared to the volume required for an untamped unreflected pit. To fit weapons into small artillery shells (155 mm and 152 mm are known; 105 mm has been alleged to be possible by nuclear weapon designer Ted Taylor), bare pits may be required.

Linear implosion weapons have much lower efficiency due to low pressure, and require 2-3 times more nuclear material than conventional implosion weapons. They are also considerably heavier, and much smaller than conventional implosion weapons. The W54 nuclear warhead used for special purposes and the Davy Crockett nuclear artillery unit was about 11 inches diameter and weighs 51 pounds. The W48 artillery shell is 6 inches in diameter and weighs over twice as much, and probably requires at least twice as much plutonium. Independent researchers have determined that one model of US conventional implosion fission weapon cost $1.25 million per unit produced, of which $0.25 million was the total cost for all non nuclear components and $1 million the cost of the plutonium. Linear implosion weapons, requiring 2-3 times more plutonium, are extremely expensive.

Fusion weapons

Fusion is the combination of two light nuclei, usually isotopes of hydrogen, to form a more stable heavy nucleus and release excess energy. Nuclear weapons which utilize nuclear fusion can have far greater yields than weapons which use only fission, as fusion releases more energy per kilogram and can also be used as a source of fast neutrons to cause fission in depleted uranium. The light weight of the elements used in fusion make it possible to build extremely high yield weapons which are still portable enough to deliver. Compared with large fission weapons, fusion weapons are cheaper and much less at risk of accidental nuclear explosion. The fusion reaction requires the nuclei 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 are generated with energy from a fission explosion. 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.

Boosting

Main article: Boosted fission weapon

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.

Boosting is typically done with a deuterium/tritium mixture in gas form which is pumped into the core during the arming sequence from an exterior reservoir. Tritium has a short half life (12.3 years), is very expensive and is very chemically reactive with uranium and plutonium. Having the tritium reservoir outside of the bomb allows easy replenishment and removal of waste Helium-3 without having to take the bomb core apart. (Theoretically, there are ways a solid hydride or a deuterium/tritium liquid could be used instead, but the use of gas is almost universal.)

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 a "fission" bomb. In fact, fusion boosting is very common and used in most modern weapons, including the fission primaries in most thermonuclear weapons.

See also: Layer cake, Joe 4, ("Alarm Clock").

Staged thermonuclear weapons

A simplified Teller-Ulam thermonuclear weapon design.

Main article: Teller-Ulam design

Staged thermonuclear weapons, also known as hydrogen bombs, utilize the power of a fission bomb to ignite fusion fuel. They are considered "staged" in that they chain together different weapon components for increasingly large explosions.

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 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 United States 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."

The process by which the “secondary” (fusion) stage is compressed by the x-rays from the “primary” (fission) stage is known as radiation implosion. Its exact operation has never been declassified though a number of theories have been put forward by people outside the classified domain, based on speculation, declassified information, interviews with former weapons scientists, and independent theoretical calculations.

One approach often cited, 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. This last effect can greatly increase both the yield of the device and the amount of fission-related fallout. Non-fissionable materials (lead, tungsten, etc.) can be used in the tamper/pusher and case instead of fissionable ones (uranium or thorium), reducing the yield and the fallout accordingly.

Firing sequence for a Teller-Ulam design thermonuclear weapon, according to the "foam" scheme.

Others have questioned the "foam" mechanism of radiation implosion described above, and instead indicated that 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, but suggest rather that 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.

Modern nuclear weapons probably use fusion stages that are spherical, rather than cylindrical. [3]

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 example, in the Castle Bravo test, the largest US test ever, of the total 15 megaton yield, 10 megatons were from fission of the natural uranium tamper. For even higher yield, however, moderately enriched uranium can be used as a jacket material.

In 1999, information came out implying that in some U.S. designs, the primary (top) is oblate, while the secondary (bottom) is spherical.

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. In 1999, it also became known that certain advanced U.S. thermonuclear designs used non-spherical (oblate) primaries.

Three-stage thermonuclear devices have also been developed, whereby a third, larger fusion stage (a "tertiary") is compressed by the energy of the fusion "secondary" (described above). 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 currently no need for five- or six-stage weapons with yields that could approach a gigaton, they could possibly be of use in deflecting Near-Earth Objects such as Asteroids and Comets which are in danger of colliding with the Earth and large enough to do significant damage (have high Torino Scale values).

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), and 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 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 test). 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 was 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.

Another variant of the thermonuclear weapon 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. The 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).

Nuclear bombs are capable of creating a destructive electromagnetic pulse (EMP) on a wide scale. The Soviet Union conducted significant research into producing nuclear weapons specially designed for upper atmospheric detonations, a decision that was later followed by the U.S. and the UK. Only the Soviets ultimately produced any significant quantity of such warheads, most of which were disarmed following the Reagan-era arms talks. EMPs can also be created by non-nuclear electromagnetic bombs.

Miniaturization

Comparing the relative size of a number of U.S. nuclear weapons.

The Fat Man weapon (60 inch (1.5 meters) diameter) compared to the W54 weapon (11 inch (28 centimeter) diameter), put into service only 16 years later. The two images are to the same scale.

The first nuclear weapons were large, idiosyncratic devices weighing many tons that could be dropped only out of especially large aircraft as gravity bombs. In the years following World War II though, developments in rocketry spurred efforts to reduce the size of nuclear weapons so that they could fit on a missile. Soon, miniaturization of weapons proved a major investment of nations with nuclear weapons and was one of the primary justifications for programs of nuclear testing, which provide the data necessary for advancing nuclear-weapons design. Weapons systems that require relatively small thermonuclear weapons, such as MIRVed missiles, are thought to be achievable only with many tests. The smallest nuclear warhead deployed by the United States was the W54, which was used in the Davy Crockett recoilless rifle; warheads in this weapon weighed about 23 kg and had yields of 0.01 to 0.25 kilotons. This is small in 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 may be nearly the smallest possible nuclear weapon, though this may be only smallest by weight or volume, not simply smallest diameter.

Certainly nuclear warheads of much smaller diameter have been made. Nuclear artillery was available to NATO during the cold war era, the nuclear warheads could be fired from standard 155mm (just over 6 inches) artillery pieces.

In 16 years, the US went from the Mark III nuclear weapon (Fat Man design), which was roughly 60 inches (1.5 meters) in diameter and weighed roughly 10,300 pounds (4,700 kilograms), to the W54 design, less than approximately 11 inches (28 centimeters) diameter and weighing about 51 pounds (23 kilograms), a factor of 162 times smaller volume and over 200 times lighter. The Mark III had a yield of 21 kilotons, while the W54 design was tested up to 6 kilotons (test Hardtack II Socorro); the W54's yield is a factor of 46 times greater per unit volume and 56 times greater per unit mass. The W54 warhead could fit inside the explosive lens assembly of the Mark III.

It has been suggested that the Soviet Union's lack of miniaturization technology in the 1950s was directly responsible for their early advances in the space race. Since they were unable to reduce the size of their weapons to the same degree as the U.S., they were forced to build larger missiles to deliver them. It was these large missiles that allowed the USSR to put satellites and humans into orbit before the U.S.

Stockpile stewardship

After the Cold War, most nuclear powers have stopped conducting programs of nuclear testing, primarily for political reasons. In many countries, inability to test has led to questions about the safety and reliability of aging nuclear weapons systems. In the U.S., a program of "stockpile stewardship"—managed by the Los Alamos National Laboratory and Lawrence Livermore National Laboratory—has attempted to gauge the reliability of old warheads without full nuclear testing, often by using computer simulation.

References

General

• 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
• 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 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. Simon and Schuster, New York, (1995 ISBN 0-684-82414-0)
• Rhodes, Richard. The Making of the Atomic Bomb. Simon and Schuster, New York, (1986 ISBN 0-684-81378-5)

External links

• Carey Sublette's Nuclear Weapon Archive is a reliable source of information and has links to other sources.
  • Nuclear Weapons Frequently Asked Questions: Section 4.0 Engineering and Design of Nuclear Weapons
• The Federation of American Scientists provide solid information on weapons of mass destruction, including nuclear weapons and their effects
• Globalsecurity.org provides a well-written primer in nuclear weapons design concepts (site navigation on righthand side).
• More information on the design of two-stage fusion bombs
• Militarily Critical Technologies List (MCTL) from the US Government's Defense Technical Information Center
• "Restricted Data Declassification Decisions from 1946 until Present", Department of Energy report series published from 1994 until January 2001 which lists all known declassification actions and their dates. Hosted by Federation of American Scientists.