Nuclear weapons technology

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First atomic bomb explosion Trinity , 16 ms after ignition.

The nuclear weapon design deals with weapons that the energy for an explosion of nuclear reactions -  nuclear fission or -verschmelzungen  refer -. The technical development of nuclear weapons since 1940 has produced a great variety of different variants.

History, classification and other non-technical aspects are covered in the article nuclear weapon .

Mode of action

While conventional explosives obtain their energy from the chemical conversion of the explosive, nuclear weapons release large amounts of energy in a shorter time from nuclear processes that reach temperatures in the million Kelvin range. As a result, any solid in close proximity to a hot gas is evaporated. The heating of the surrounding air and the evaporating solids lead to a sudden volume expansion, which in addition to the emitted heat radiation leads to a strong pressure wave.

Both nuclear fission and nuclear fusion achieve their energy turnover from the difference in the binding energy of the nucleons of the atomic nuclei involved before and after the nuclear reaction. While energies of up to 14  MeV (cf. fusion reactor ) and per nuclear fission even approx. 200 MeV (cf. fission heat ) are released per nuclear fusion , chemical reactions only produce energy in the range of a few eV, in the case of TNT approx. 38.6 eV (see explosion parameters ) per molecule.

Fission bomb or mission bomb

A classic nuclear fission bomb (atomic bomb) is constructed in such a way that at the intended point in time several parts of the fissile material , each individually below the critical mass , are brought together and thus exceed the critical mass. At the same time as the critical mass is reached, a neutron source begins to emit neutrons, which trigger the fission chain reaction. The number of neutrons newly generated by nuclear fission (nuclear fission) is consequently greater in each fission generation than the number of neutrons that have escaped from the material and absorbed in the material without fission, so that the nuclear reaction rate increases rapidly. The critical mass can be reduced by using a neutron reflector .

The energy released in the form of very rapid heating drives the nuclear explosives apart. Therefore, the underlying chain reaction has to cover the entire fissile material as quickly as possible, otherwise only a small part of the possible energy is released. Therefore, for fission weapons - unlike for nuclear reactors for civilian energy production - as pure as possible, readily cleavable nuclides such as highly enriched uranium or almost pure 239 plutonium used in the construction and the rapid onset of prompt Überkritikalität sought.

Polonium - beryllium is often used as a neutron source , which has to mix at the right time. In this mixture, alpha particles emitted by polonium react with beryllium.

One of the chemical explosives used to quickly merge the subcritical pieces is Octol . It consists of HMX and TNT mixed in a ratio of 7 to 3.

Cannon principle

Schematic representation of a nuclear fission
bomb according to the gun design: 1. Conventional explosives (cordite) to accelerate the "projectile"
Barrel 3. Hollow uranium projectile
4. Cylindrical "target"

A subcritical hollow uranium cylinder can be shot at a subcritical uranium mandrel that is missing from the inside of this cylinder (gun design; cannon principle). The completed cylinder exceeds the necessary critical mass and starts the nuclear chain reaction. Due to the design, the total amount of uranium in this arrangement is limited to a few multiples of a critical mass. Because of the rather elongated design, the cannon principle is suitable for elongated nuclear weapons such as "Bunker Buster" (see below) and nuclear projectiles that are fired from tubular weapons. For example, propellants for artillery projectiles, such as cordite, are used as a chemical explosive .

The uranium bomb Little Boy , which was dropped over Hiroshima , was constructed according to this construction method. It was considered so reliable that a prior test ignition was dispensed with. The bomb contained 64 kilograms of uranium, which was enriched to 80 percent 235 U content . The critical mass of the nuclear warhead was reached 25 centimeters or 1.35 milliseconds before the uranium mandrel completely penetrated the uranium cylinder, at a final speed of 300 m / s.

In other constructions, the actual fission set has an approximately spherical shape. The fissile material bullet is shot at a rigid fissile material target, or two bullets are shot against each other. An additional, rigid and centrally placed third fissile material part or an imploding reaction aid are partially discussed.

The cannon principle is not suitable for plutonium that has been incubated in the reactor of a conventional nuclear power plant. Its content of 240 Pu (less fissile and at the same time relatively strong spontaneously fission ) would lead to a pre-ignition and thus to a deflagration with the cannon principle. In contrast, so-called weapons plutonium, produced in specially operated reactors, contains very little 240 Pu.

Implosion bomb

Basic design

Schematic representation of the implosion method

Another design shows the implosion bomb. The Fat Man bomb dropped on Nagasaki was built on this principle. It is in the middle

Implosion bomb animated.gif
Scheme of the explosion of an implosion bomb with explosive lenses
Image taken by an X-ray flash unit shows the spherically converging detonation front

the fissile material (such as plutonium , 235 U or an alloy of both metals) as a non- critical mass , either as a full (subcritical mass) or as a hollow sphere (subcritical in terms of geometry, since there is no full sphere). Around the fissile material there are several layers of high-explosive material such as TNT . During ignition, the explosion energy is directed towards the center of the sphere and compresses the fissile material so strongly that the mass becomes critical. The implosion bomb is believed to be more effective because it detonates faster than a cannon-based bomb and it can use a very large amount of fissile material. In addition, the exploitation of atomic explosives is higher because the fissile material stays together longer and in a more favorable form during the explosion.

Plutonium weapons are only conceivable as implosion weapons due to the higher spontaneous fission rate of the various Pu isotopes and the resulting premature ignition. The construction itself is much more demanding in terms of explosives and ignition technology. Since the scientists involved in the development were not entirely sure, in contrast to the uranium bomb “Little Boy”, the implosion arrangement was tested in advance as part of the “ Trinity Test ” (New Mexico).

Build up of explosives around the core

"Trinity Gadget" with 32 polygonal explosive lenses around the core

Just building a shell from an explosive around the core did not lead to the desired result, as the explosive detonates spherically around the detonator. A very high number of detonators would then be required to achieve acceptable compression and not to press the hollow sphere into a crescent moon or star.

The task was therefore to transform several spherically diverging detonation fronts into a single spherically converging one. Two explosives with different detonation speeds are suitable for this purpose. At the junction of explosives, the detonation front as light is refracted at a lens, which is why in English by an "explosive lens" ( explosives lens is spoken). In order to achieve the desired effect for an implosion bomb, such a lens must have a rotational hyperboloid of slowly detonating explosive in the center, surrounded by a rapidly detonating explosive. Analogous to optics, the refractive index of the lens is greater, the more the detonation speeds of the explosives used differ. The explosive lenses are polygonal so that they can be joined together in a spherical shape.

The arrangement in the "Trinity Gadget" consisted of 32 explosive lenses; later 40, 60, 72 and finally 92 lenses were used. In principle, it would be possible to condense a core with just a single, complex-shaped lens. However, this lens would be larger and heavier than the above. Configurations, even if it is easier to ignite.


Modern nuclear weapons have an additional layer between the conventional, highly explosive explosives and the actual nuclear fuel, usually made of beryllium or pure uranium 238 U (depleted uranium). This layer reflects neutrons ( 9 beryllium even acts as a neutron emitter); earlier, tungsten carbide was used as a reflector . In this way, the critical mass can be reduced according to the following table:

Schematic representation of a nuclear fission bomb according to the implosion design with reflector
Share of 235 U Without reflector Natural uranium (10 cm) Beryllium (10 cm)
93.5% 48.0 kg 18.4 kg 14.1 kg
90.0% 53.8 kg 20.8 kg 15.5 kg
80.0% 68.0 kg 26.5 kg 19.3 kg
70.0% 86.0 kg 33.0 kg 24.1 kg

On the other hand, especially when uranium is used, this layer delays the expansion of the fissile material due to its inertia after the start of the chain reaction. The fissile material thus stays together longer, the chain reaction itself becomes hotter due to the neutron density and the energy efficiency of the bomb increases.

When using 238 U as a jacket, the explosive power can be increased by 10 to 20 percent.

Density adjustment

Schematic representation of a nuclear fission bomb according to the implosion design with reflector and density adjustment

Another layer of aluminum between the explosive and the reflector is used to better transmit the impact of conventional explosives to the heavy metal. Since the explosive has a much lower density than the reflector and fissile material, part of the explosion shock wave of the conventional explosive is reflected at the interface. This part of the energy is not used to compress the fissure material. If a layer of medium density such as aluminum is inserted between the conventional explosive and the reflector, this improves the energy transfer to the gap material and thus its compression.

Floating core

Modern implosion constructions use arrangements in which the fissile material is divided into a shell and a hollow sphere. The gap is filled with gas. In order to hold the hollow sphere in the center of the shell, six aluminum bolts are usually installed as spacers. This design has the advantage that the entire hollow sphere does not have to be compressed at once. Instead, only the small mass of the shell is initially accelerated. It receives a high kinetic energy and hits the hollow ball at high speed. The completion of the critical mass then takes place in a very short time; all that is required is for the hollow sphere to implode under the pressure of the accelerated shell. This design has a large number of different variants. The air gap can thus also be arranged between the reflector and the gap material. The inner ball can be designed as a hollow ball or made of solid material. There may be designs with two spaces. The aluminum bolts can be replaced by foam (polyurethane foam, expanded polystyrene or similar materials).

Schematic representation of a nuclear fission bomb based on the implosion design with a floating core

The picture opposite shows the essential features of a modern design, which has density adjustment, reflector and a floating core. Such constructions require complex mathematical calculations, which can only be carried out with special computer programs, for the precise determination of optimal parameters. The calculation methods and results as well as the programs used are classified as secret by the armaments authorities and details are only published in very few cases - the numerical values ​​that have become known can therefore be questioned. This is also the reason why, in the past, high-performance computers were subject to export restrictions (for example from the USA). However, the basic design of modern nuclear weapons with the features shown is plausible and has been confirmed by various sources.

The construction method is assigned to the German nuclear spy Klaus Fuchs . In addition to the advantages listed above during the subsequent explosion, it was used to remove and add the actual fissile material. In some British and American bomb designs, the actual fissile material was stored outside the bomb in such a way that none of it would have been released in the event of a subcritical accident. The weapon and transport security for these weapons was consequently further improved.


The largest nuclear fission bomb (Fission weapon) ever detonated was built by the USA with an explosive force of 500  kT . It worked on the implosion design and had uranium as a nuclear explosive.

From 1966 to 1980, France built and stationed the MR-31 warhead, the largest plutonium bombs ever built with an explosive force of around 120 kT.

The best-known nuclear weapon based on the implosion design is certainly the Fat Man bomb dropped on Nagasaki , while the Little Boy uranium bomb was based on the gun design .



It is crucial for all construction principles that the chain reaction only takes place as long as the arrangement is supercritical. In order for as many nuclear fission as possible to take place, it should be kept supercritical for as long as possible. As soon as sufficient energy has formed as a result of a large number of nuclear fission, the fission material evaporates, expands and the chain reaction breaks off. It therefore depends on the ignition point in order to make optimal use of the fissile material.

The cannon barrel arrangement becomes critical when the two subcritical uranium halves have approached a certain distance, the time of the first criticality (transition to the supercritical state). In the case of the implosion arrangement, the material is also compacted. As the halves come closer together in the case of the cannon barrel arrangement, or compression in the case of the implosion arrangement, the arrangement becomes supercritical. Even without a chain reaction, the arrangement would eventually expand again only because of the inertia of the conventionally accelerated masses. The chain reaction breaks off when the arrangement becomes subcritical (time of the second criticality). The expansion is accelerated when the fissile material evaporates. This is the case when additional energy is released from nuclear fission, but only when it exceeds a certain value known as Bethe Tait energy . When this minimum energy is reached, the expansion of the nuclear fuel accelerates and the arrangement becomes subcritical again more quickly. Up to this point in time, many generations of the chain reaction had already taken place. The chain reaction also continues during the expansion until the point in time of the second criticality (transition to the subcritical state) is reached. Most of the energy is released during the last few generations of neutrons.

The greater the supercriticality, the longer the phase between reaching the Bethe Tait energy and reaching the second criticality point, and the more nuclear fission can still take place.

In order to achieve an optimal utilization of the fuel, the beginning of the chain reaction should therefore be set in such a way that Bethe-Tait energy and maximum supercriticality are achieved at the same time. If the Bethe Tait energy is reached earlier, however, fewer neutrons would be formed and only smaller amounts of the nuclear fuel would be converted.

The worst case would be the onset of the chain reaction at the time of the first criticality, since then the time of the Bethe Tait energy is reached before the maximum supercriticality and the arrangement expands prematurely. If the arrangement is only slightly supercritical at this point in time, the explosive energy of such a bomb would hardly exceed that of the chemical explosive used. If it is severely overcritical, it will still take some time before it becomes subcritical again. During this time, so many nuclear fission can still take place that the energy released exceeds that of the chemical explosive many times over. First of all, the rise in supercriticality continues until the Bethe Tait energy is reached. In the accelerated expansion that followed, further nuclear divisions took place.

According to Robert Oppenheimer, the first explosion of a plutonium-based implosion bomb (July 16, 1945, test in New Mexico), even in the worst case, would have had an explosion energy that would hardly have been less than 1000 tons of TNT.

An ignition before the optimum time is a spark, an ignition after the optimum time as late ignition referred. In order to get the optimal ignition point, one does not rely on the neutrons from the spontaneous fission, but starts a special neutron generator at the right moment .


After the critical mass has been reached, the bomb has to be detonated by initial neutrons. These can come from the fissile material itself through spontaneous nuclear decay or can be supplied by an additional neutron source. In highly enriched 235 U, around 80 million atomic nuclei decay per second and kilogram, emitting alpha particles, but statistically only about two neutrons per second and kilogram are generated. In the 64 kg of the Hiroshima bomb, statistically speaking, 0.17 neutrons were released between the criticality point and complete assembly (1.38 ms).

For the Hiroshima bomb, a probability of 12 percent for a pre-ignition was given in 1945, corresponding to the probability of an initial neutron within the 1.38 ms specified above. In order to prevent pre-ignition of bombs according to the gun design, the nuclear bomb design must be free of other neutron emitters. So 238 U (with 20 neutrons per kilogram and second) should be avoided in the envelope; Even nuclear weapons that have already exploded in the same target area and their residual neutron radiation can prevent the use of such an atomic bomb.

The cannon barrel principle is no longer used in today's arsenals. The warheads would be far too heavy for modern delivery systems. South Africa had built six cannon barrel guns, but scrapped them again after the change in policy in the early 1990s. It is the first country to have completely disarmed nuclear weapons.

In contrast to uranium, the neutron production of plutonium is high because of the unavoidable proportion of 240 Pu. The assembly of the individual fissile material components in a cannon barrel arrangement takes place so slowly (on the order of milliseconds) that the chain reaction would start at the first criticality. When the Bethe Tait point in time was reached, it would hardly be overly critical and there would only be a deflagration. The cannon barrel arrangement therefore only works with highly enriched uranium, which has a low neutron background, but not with plutonium.

With the implosion arrangement, on the other hand, compaction takes place much faster, on the order of microseconds. It is therefore also suitable for plutonium. Depending on the purity of the plutonium, between around 50,000 (weapon grade plutonium) and 500,000 (reactor plutonium) neutrons per second are produced as a result of spontaneous decay.

Since 240 Pu is formed by neutron capture from 239 Pu, which in turn is formed by neutron capture from 238 U , the higher the burn-up of the nuclear fuel, the greater the proportion of 240 Pu. Reactors that are supposed to produce weapons-grade plutonium are therefore operated with little burnup. For reasons of economy, a high burn-up is used in nuclear power plants. Nevertheless, even plutonium produced in nuclear power plants is suitable for the construction of nuclear weapons to a limited extent. The probability of pre-ignition is higher, but the lower explosive energy by far exceeds that of conventional weapons. However, technical problems are caused by the increased radioactivity and the warming as a result of the radioactive decay.

Late ignition and neutron source

In addition to the advance ignition, a nuclear weapon according to the gun design can also ignite comparatively late if - purely statistically - the initial neutron triggers the chain reaction late. After all, the probability of the Hiroshima bomb not igniting until after 200 ms was 0.15 percent. If an atom bomb is shot at its target at high speed, this delay can significantly change the desired location of the explosion and the projected energy released. For this reason, nuclear weapons have been equipped with neutron sources that start the chain reaction with a larger quantity of neutrons at the precise time as soon as the critical mass has been formed.

The Hiroshima uranium bomb also had such a neutron source as a bomb detonator when it was planned. It could not be determined whether it was installed in the end; the natural radioactivity of the fissile material would probably have been sufficient for the explosion.

The neutron source consisted of two components, beryllium and 210 polonium , housed separately from each other. The two substances were brought together when the uranium projectile hit, and neutron production began. Similar two-component sources were later found in the core of the early implosion bombs, separated by a thin membrane that ruptured during implosion. In modern weapons, an external source is used instead.

Efficiency, size, safety and gun weight

The ratio of split nuclear explosive to total nuclear explosive is called efficiency.

The cleavage of 50 g of 235 U releases the explosive strength of 1 kT. In the case of the Hiroshima bomb, around 650 g of 235 U were split, only a small fraction of the total of 64 kg of uranium. The remaining nuclear explosives are released into the atmosphere and, together with the fission products and the “secondary” radioactivity generated by neutrons, form the fallout .

Fission bombs therefore contain more than the critical mass to be cleaved in order to generate sufficient, desired explosion energy. A mass immediately above the critical mass would result in a marginal explosion strength, with a 1.05-fold mass an explosive force of around 100 t can be expected.

With the simple cannon barrel principle, the maximum possible mass is slightly below double (triple) the critical mass. Both halves of the critical mass must remain subcritical before the explosion in order to prevent radiation accidents and a premature subcritical explosion, a so-called deflagration. The maximum size of pure fission bombs based on the simple cannon principle (uranium bombs) is therefore limited by the maximum subcritical mass of two or three pieces of fissile material.

More than three cannon barrels could also be combined in order to shoot more parts of the charge at each other. However, this is associated with significantly increased effort for the simultaneous ignition of the propellant charges and other synchronization problems, since the unification of all parts of the charge must take place very precisely in order to actually contribute to increasing the explosive force.

With the implosion principle, the gap material is additionally compacted. This reduces the critical mass and thus higher supercriticalities and better efficiencies are possible. In addition, the spherical arrangement is geometrically optimized. But here too there are limits, as chemical explosives cannot be used to compress at will and the mass must be subcritical beforehand. In addition, it is a demanding task “from a blasting point of view” to carry out the compaction as spherically as possible. In addition to the spherical shape, hollow cylinders and other shapes are technically known.

Ultimately, this is a considerable safety advantage of the implosion principle. In order to trigger a nuclear explosion, the chemical detonating explosive on its outer shell must be detonated at a number of points in a defined manner so that the explosion front runs from the outside in towards the nuclear charge in order to compress it. If, as a result of an accident, the explosive device is detonated at only one point, the only thing that takes place is the chemical explosion and contamination of the environment from the fissile material that is then released.

Since the explosion front usually moves away from the ignition point in a convex manner, the explosion front is often formed by layers of different explosives with different explosion speeds in such a way that the desired compression of the fissile material is achieved. While earlier systems were based on the simultaneous ignition at all points provided, in modern systems specific deviations are built in, which have to be compensated for by slightly different times of ignition of the individual detonators. These time differences are only incorporated into the weapon electronics by means of appropriate codes when the deployment is authorized (so-called “ Permissive Action Link ”). This considerably reduces the risk of theft or loss of a warhead or the use of weapons contrary to the instructions, since attempts to detonate it improperly are unsuccessful.

The maximum size of a weapon is further determined by practical handling and handling safety. In practice, boosters are used in fission weapons and hydrogen bomb detonators, small amounts of fusion material within the critical fission mass. The neutrons released during the fusion cause a “hotter” explosion, so the efficiency of the weapon is increased through better utilization of the fissile material. Even higher explosive energies can be achieved with multi-stage weapons such as hydrogen bombs.

238 U fission through a 238 U reflector or jacket

In addition to the actual fission material, a reflector made of inexpensive natural uranium or depleted uranium ( 238 U) can also be used. This material is also split from the nuclear process by the neutrons and releases energy. Released neutrons also heat up the primary fission process similar to a booster. The efficiency of the 238 U in the reflector or bomb jacket is below the critical mass actually used in the bomb.

In one of the strongest pure fission bombs of the Americans ( Ivy King ) , around 425 kT of energy were released by implosion of 235 U and an additional 75 kT by the partially split 238 U from the shell. An increase in performance through 238 U in the reflector is only possible with bombs according to the implosion design, since the 238 U releases a large number of neutrons through spontaneous fission and would therefore lead to a pre-ignition with a high probability in the gun design.

If a small atomic bomb with 235 U is designed as a fissile material (for example a "bunker buster" based on the gun design), the theoretical problem arises that the 235 U is not completely implemented when it explodes in enemy territory, and is therefore used for construction another atomic bomb is available. To prevent this, such a nuclear weapon can be given a jacket or ballast made of 238 U. In the atomic explosion, both uranium are mixed and the degree of purity is reduced. To avoid pre-ignition, the 238 U must be mounted separately from the explosive device.

Bombs with a jacket made of 238 U (when using a booster or a hydrogen bomb) are classified as three-stage weapons and, due to the large amount of fissile material released, they are among the so-called "dirty" bombs.

Hydrogen bomb

Hydrogen bomb Castle Bravo

In nuclear fusion weapons (hydrogen bombs), a conventional atomic explosive device ( fission explosive device) is used to bring about the nuclear fusion of the hydrogen isotopes deuterium and tritium .

The first, unrealizable design

Schematic representation of a hydrogen bomb according to the Classical Super Design

In the basic idea of ​​the hydrogen bomb, which is referred to as super and later as classical super , a large amount of the hydrogen isotopes tritium or deuterium is arranged next to or around a fission explosive device that functions as a detonator. The explosion of the fission explosive device is supposed to heat the hydrogen to ignition temperature so that the fusion explosive ignites. The fictitious configuration was called "alarm clock design" due to its geometric appearance.

This arrangement would not work with pure deuterium, because most of the energy in the fission bomb is generated as thermal X-rays that penetrate the deuterium. The temperature would be sufficient for the deuterium-tritium reaction, but tritium is comparatively expensive - instead of a hydrogen bomb of this type, a very large fission bomb could have been built at lower cost.

Another problem with the Classical Super is the low density of the fuel, because the hydrogen isotopes are gaseous under normal conditions. Before enough fuel had been used, the explosion of the primary fission explosive device would have blown everything apart.

The design of a “fusion mass” of deuterium and tritium next to or around a fission core is therefore unsuitable; a bomb of this type was never built. However, a similar design is used for the neutron bomb , since only a very small amount of tritium-deuterium is required there and therefore the costs remain small.


Schematic representation of a hydrogen bomb according to the Teller-Ulam design:
A - primary nuclear fission explosive device
B - secondary fusion explosive device
1 - chemical explosive
2 - 238 U-jacket
3 - cavity
4 - tritium gas enclosed in plutonium or uranium ball
5 - polystyrene
6 - 238 U- Sheath
7 - lithium 6-deuteride
8 - plutonium
9 - reflective sheath
Representation of the individual steps in the explosion of a Teller-Ulam bomb:
A - bomb before ignition; above the primary nuclear fission bomb; below the secondary fusion charge; both embedded in polystyrene foam.
B - The conventional explosive compresses the plutonium core into a supercritical mass, thus initiating a nuclear fission reaction.
C - The nuclear fission bomb emits X-rays which are reflected off the inside of the housing. This thermalises the polystyrene.
D - The polystyrene foam is turned into plasma and compresses the fusion stage. The nuclear fission chain reaction takes place in the plutonium rod.
E - As a result of the compression and heating, the lithium-6-deuteride begins to fuse. In the second stage, the neutron radiation splits the 238 U. A ball of fire begins to form.

With the Teller-Ulam-Design, named after Edward Teller and Stanisław Ulam , the difficulties of the Classical Super are solved. The solution, found on the Soviet side by Andrei Dmitrijewitsch Sakharov , also became known as "Sakharov's third idea". In the case of independent development in France, the idea is attributed to Michel Carayol , for Great Britain the question of the originator is less clear (see John Clive Ward ).

The primary fission explosive device and the secondary fusion explosive device are located in a housing filled with foam (mostly foamed polystyrene ). The radiation from the fission explosive device is absorbed by the housing wall and creates a thin layer of highly ionized plasma that not only absorbs the primary radiation more efficiently, but also radiates in the X-ray range. The same thing happens with the outer surface of the secondary explosive device. The radiation exchange between the three surfaces - the thin plasma formed from the foam hardly absorbs - is proportional to T 4 and therefore quickly compensates for temperature differences; it is said that the " cavity " (also called in English) thermalises .

Now not only the plasma of the fission stage spreads, but also the superficial plasma layers. Their immense pressure causes an inwardly directed shock front , behind which the material also changes into the plasma state and moves inward. This is also known as a radiation implosion.

The geometry of the secondary part is spherical or cylindrical so that the shock wave converges concentrically on a point or a straight line. Extreme conditions (pressure and temperature) then arise there, which detonate the second stage of the bomb, fusion. The high-energy alpha particles produced during the deuterium fusion increase the temperature further, so that the nuclear reaction propagates outwards like a flame front.

In the center of the secondary part there is usually a “spark plug” called a hollow cylinder or spherical core made of plutonium or enriched uranium, which is also and simultaneously brought into a supercritical state by the compression. The fission serves as an additional ignition source and regulator of the second stage, the efficiency and uniformity of the explosion are increased. With the incorporation of radiation-enhancing material on the surfaces of the cavity, the configuration can be further reduced in size.

A similar fusion implosion principle also follows the inertial confinement fusion (ICF - inertial confinement fusion).

Fusion explosives

Frozen liquid deuterium was used as the fusion explosive device in the first and only bomb to use pure deuterium ( Ivy Mike ) . This is unsuitable for military atomic bombs, since the cooling effort is very large and therefore very expensive. In addition, the high-pressure storage of the deuterium gas at normal temperature is difficult and voluminous and therefore also unsuitable for nuclear weapons. The same considerations apply to a mixture of deuterium and tritium. In addition, tritium is unstable with a half-life of 12.3 years and must therefore be replaced regularly. For the production of tritium in nuclear reactors, neutrons are also required, with which plutonium could also be produced from uranium, which has a higher energy yield.

For these reasons, deuterium is now used in chemically bound form in a solid that also generates the necessary tritium when irradiated with neutrons. Of all solid hydrogen compounds, lithium deuteride (LiD) , which is solid at normal temperature, turned out to be the best solution. It contains more deuterium per unit volume than liquid deuterium and at the same time more than 20 percent by mass of deuterium. The lithium also takes part in the nuclear reactions and produces additional energy. The first attempt by the USA with such a "dry" bomb was the Castle Bravo test on February 28, 1954 with a total explosive force of 15 MT. As early as August 12, 1953, the USSR ignited a transportable LiD construction in its first test. The possible reactions of the deuterium are:

The resulting tritium can generate fast neutrons in a further reaction:

Finally, the 3 helium produced can also continue to react:

The neutrons produced in the above reactions can react with the lithium:

In addition, other nuclear reactions take place, but they contribute comparatively little to the overall reaction. Both lithium isotopes, 6 Li and 7 Li, can be used for thermonuclear weapons . The total reactions with deuterium are:

If many (slower) neutrons are required in a three-stage hydrogen bomb for fission in a 238 U-mantle, 7 Li is cheaper. On the other hand, 6 Li is advantageous for a higher energy yield . Natural lithium consists of 92.5% 7 Li and 7.5% 6 Li. Lithium enriched in 6 Li is obtained by isotope separation processes.

All in all, 4 He remains from the reactions , unreacted deuterium and many neutrons. The reactive tritium is almost completely used up in the reactions. For every megatonne of explosive power - using pure 6 Li and assuming that every atom reacts - 15.6 kg of lithium deuteride must react. Since in practice only about half of the material is used, 36 kg are required.

Since the hydrogen fusion in the Teller-Ulam design is triggered by high pressure and high temperature and not - as in the older Sloika design  - first neutron bombardment from the fission stage is necessary, this type of atomic bomb is referred to as a thermonuclear bomb.

Nuclear weapons based on the Teller-Ulam design are euphemistically referred to as clean atomic bombs because they derive a large proportion of their explosive power from nuclear fusion. Nuclear fusion, i.e. the second stage, produces much less and more short-lived radioactivity than nuclear fission, namely only tritium (see formulas above). What remains, however, are the fission products of the first stage, the fission bomb used to ignite the fusion, and the surrounding materials converted into radioactive isotopes through neutron capture , which together form the fallout. The bomb is only “clean” in comparison with a pure nuclear fission bomb with the same explosive effect.

Three-stage hydrogen bomb

The ratio of the explosive forces of the first to the second stage is limited to a maximum of about 1: 200, a ratio of 1:20 to 1:50 is common. Since fission bombs as the first stage are limited to several hundred kT, the maximum explosive force of the second stage is up to about 100 MT, but usually no more than about 10 to 25 MT. There are several ways to increase the explosive power of a thermonuclear bomb:

  • It would be possible to increase the mass of the second or third stage at the expense of the efficiency and ignitability of this stage. This could be achieved by a conical implosion arrangement of this stage and a linear ignition transmission. The principle was not applied, but can be found remotely in the “Spark Plug” of the second stage.
  • Theoretically, a geometric arrangement of several detonator bombs could detonate a large second and third stage. One of the first hydrogen bombs probably had such a configuration, the efficiency of the second stage was comparatively low due to the “unbalance” of the detonators. The problems and expense of such an arrangement outweigh the problems.
  • Another Teller-Ulam stage could be added to an existing one, that is, the energy released by the first fusion stage is used to detonate the next, even larger, explosive device (the third stage). In the case of an extended Teller-Ulam configuration, the third stage, like the second stage, can consist of a fusion or fission stage.
  • The surrounding metal cylinder can be made from uranium 238 U, a waste product of uranium enrichment . This uranium is split by the fast neutrons (14  MeV ) of the fusion explosive device and, due to its quantity, provides a large proportion of the total energy. In a simple atomic bomb, a few kilograms of uranium or plutonium are fissioned. In a so-called "tertiary hydrogen bomb" there can be several tons of uranium. So there are three stages: the fission explosive charge for igniting the fusion charge, which in turn produces the neutrons for the fission of the uranium in the third stage. The design is therefore also referred to as a fission-fusion-fission design or "three-phase bomb" (FFF bomb). The fission products of the uranium in the third stage are responsible for a large part of the radioactive contamination in such a bomb, it is an exceptionally dirty bomb. The American test bomb " Redwing Tewa " , for example, was built according to this principle. With a total explosive force of about 5 MT, it obtained an explosive force of 4.35 MT from nuclear fission of the first and third stages (test on July 20, 1956).

The term “three-stage hydrogen bomb” or “tertiary hydrogen bomb” is used for these design principles, which can easily lead to confusion. The largest nuclear weapon detonated to date, the Tsar bomb , had two fusion explosive devices and an explosive force of around 50 to 60 megatons of TNT equivalent . A 238 U-coating has been omitted in order to limit given by the explosive force already strong fallout. With uranium jacketing as the fourth stage , this bomb would have had an estimated explosive power of at least 100 megatons of TNT, and the contamination would have been devastating.

Hybrid atomic bombs

Hybrid atomic bombs get a large part of their explosion energy from nuclear fission, but require a fusion component to intensify the nuclear fission. There are different construction methods for this fusion part.

Boosted fissure bombs

To increase neutron production, a small amount of the hydrogen isotopes deuterium and tritium can be introduced as a fusion fuel in the center of the hollow sphere made of nuclear explosives; in contrast to the neutron bomb, in which the fuel is arranged outside the fission explosive device. Typical amounts of a deuterium-tritium mixture are two to three grams. The nuclear fission chain reaction causes the nuclear fusion of these substances to ignite through pressure and heat, with many high-energy neutrons being generated:

Schematic representation of a boosted nuclear fission bomb based on the implosion design

The fusion of the deuterium or tritium only makes a small contribution to energy production, because one gram of tritium releases less than 0.2 kT of explosive force. However, due to the neutrons released from the fusion, a larger proportion of the fission fuel is split and thus the efficiency is multiplied compared to a pure fission explosion. The neutrons from one gram of tritium can split 80 grams of plutonium. Since the neutrons released from nuclear fusion are very fast, a particularly large number of fast neutrons are released when plutonium is split, which in turn split other plutonium nuclei. In total, around 450 grams of plutonium are split by one gram of tritium - compared to a structurally identical fission bomb without boosting - and they release around 7.5 kT of additional energy. By boosting, the explosive power of fission bombs can be roughly doubled.

Technically, the mixture of tritium and deuterium can be present as a compressed gas, at low temperatures as a liquid or as a chemical compound. The US Greenhouse Item's first boosted nuclear weapon (detonated on May 25, 1951, Eniwetok Atoll) used a frozen, liquid mixture of tritium and deuterium to increase the explosive power of a mission bomb from the predicted value (20 kT) to 45.5 kT more than double. In order to avoid the technically complex cooling, the compression of the gases is presumably chosen today. The boosting makes the storage of nuclear weapons more difficult because tritium is radioactive and decays with a half-life of 12.32 years. That is why it has to be continuously produced in nuclear reactors and replaced in nuclear weapons. Despite this difficulty, most mission bombs today - whether as detonators for a hydrogen bomb or not - are boosted. The explosive power of some weapon types can be adjusted by adding different amounts of boosting material; English "dial-a-yield".

It is unclear whether lithium deuteride is also suitable as a booster material, as this initially has a neutron-absorbing effect.

Sloika design (onion skin)

In addition to the Teller-Ulam design, a fusion bomb with an explosive force of up to around 700 kT can also be built according to the Sloika design . Here a fission explosive device is surrounded by a lithium deuteride layer, which in turn is surrounded by a layer of uranium (onion skin principle). In contrast to the primary fission explosive device, the outer uranium layer consists of natural uranium or depleted uranium, i.e. it has a high 238 U content.

The onion skin construction principle (“sloika” or “puff pastry”) is similar in construction to the original “Classical Super”, which only surrounds an atom bomb. It ultimately acts like a booster bomb, with the additional uranium jacket acting like a dirty third stage. Depending on the thickness of the second and third layers, these layers “glow” together and with different degrees of efficiency. The comparatively complex construction, similar to the American “Super”, can be seen as a Russian preliminary stage or development stage to the Teller-Ulam configuration.

There are two different versions of the Sloika design:

Variant I (thin coat)

After igniting the fission explosive device, neutrons are generated in the fission stage, which result in the following nuclear reaction in the lithium deuteride layer:

The resulting tritium T reacts with the deuterium in a further reaction:

As a result, slow neutron, are each a 6 lithium and a deuterium atom releasing energy to two helium nuclei and a fast neutron transformed. The overall reaction therefore consumes and produces one neutron at a time. Since some of the neutrons escape to the outside, the reaction cannot sustain itself and goes out after a short time. For the other reactions described in the Teller-Ulam design, the pressure and temperature in the Sloika design are too low. However, the escaped fast neutrons can split the 238 U nuclei in the outer layer and in turn release energy. Atomic bombs of this type were developed and tested in particular by Great Britain, for example in the test explosion "Grapple 2" on May 31, 1957. A primary fission stage with an explosive force of 300 kT led through the additional layers to an explosion with a total strength of 720 kT.

Variant II (thick coat)

If the fusion and outer uranium layers are made comparatively thick, another mechanism comes into play. From the nuclear fission in the outer uranium layer, many neutrons are shot back into the fusion layer, where they generate a second generation of tritium. The reaction of the 238 U-layer in the fusion layer creates a combined burning of both layers. Since with this variant neutrons from the outer uranium layer also contribute to the bombardment of the lithium deuteride layer, the first fission stage can be made much smaller. This variant therefore requires less gap material 235 U or 239 Pu in the first stage and is therefore cheaper to manufacture. This design was chosen in the Soviet nuclear test "Joe-4" on August 12, 1953. In this atomic test, the inner fission stage produced 40 kT from 235 U, about 70 kT from the nuclear fusion of the second layer and 290 kT from the nuclear fission in the third layer.

This construction is not a pure thermonuclear second stage, there is no independent hydrogen burning. This combined fission-fusion reaction resembles the igniting “spark plug” of a Teller-Ulam configuration: the nuclear fission of the uranium in the outer layer is used for neutron multiplication, the fusion serves to accelerate neutrons. However, it is not an individual neutron that is accelerated; rather, in the course of the fusion process, a slow neutron is consumed and a fast one is generated. The neutron acceleration is necessary because 238 U can only be split with neutrons with a minimum energy of 1.5  MeV .

Other variants

In addition to the basic types outlined above, there are other variants that have only been partially implemented:

  • In all two-stage bombs, the first stage can be performed as a boosted mission bomb , which is commonly used today.
  • The two-stage fission bomb has a similar structure to the Teller-Ulam hydrogen bomb, but instead of the hydrogen explosive device, a second fission stage based on the implosion design is used. So this second stage is not imploded by chemical explosives, but by the first stage. This atomic bomb design was probably never implemented militarily. The design was developed by Ulam for atomic bombs with great explosive strength; It was only subsequently recognized that hydrogen bombs could also be constructed with it. Such a two-stage mission bomb was detonated in the "Nectar" test ( Operation Castle ) on May 13, 1954. As in the first stage, the conditions relating to the critical mass apply.
  • In all H-bombs (partly also uranium or plutonium bombs) with an outer uranium layer, this can also be carried out with 235 U or 239 Pu. The US test bomb "Cherokee" ( Operation Redwing ) from May 20, 1956 was a thermonuclear bomb according to the Teller-Ulam design, but the envelope of the lithium deuteride was made of highly enriched uranium.
  • A cylindrical uranium implosion design appears possible and was briefly tested by the American side during the H-bomb development.
  • The damaged tower from Test Ruth
    Moderated nuclear weapons consist of a normal fission bomb in which, however, the fission material does not consist of enriched uranium or plutonium, but of a metal hydride of these substances such as UH 3 . The hydrogen contained in the material acts as a moderator on the neutrons; it slows them down, increasing the likelihood that they will split other atoms of the fuel. This reduces the critical mass considerably, for uranium to less than one kilogram. However, the density of the fissile material is considerably lower, which is why the bomb loses its criticality very quickly once the chain reaction has started. Several American attempts with this construction method were unsuccessful: In the test "Ruth" ( Operation Upshot-Knothole ) on March 31, 1953, an atomic bomb estimated at 1.5 to 3 kT only achieved an explosive force of 0.2 kT and did not even destroy that 100 m high mast on which it was mounted. The experiment “Ray” on April 11, 1953, in which uranium hydride was also used, but together with deuterium , ran similarly .

Nuclear weapons with special effects

Neutron weapon

Schematic representation of a neutron bomb

A neutron weapon (enhanced radiation weapon) is a hydrogen bomb with deuterium-tritium fuel, the construction of which is essentially similar to the Teller-Ulam design. The design of the weapon is optimized for maximum neutron emission and a comparatively low fallout. The American Samuel T. Cohen developed this weapon back in 1958 and campaigned heavily for its manufacture. So he could not prevail until 1981 under President Ronald Reagan . A total of 700 neutron warheads were built. In June 1980, the French President Giscard d'Estaing announced that France would develop a neutron bomb, and on June 21 the first weapon was tested on the Moruroa Atoll. In 1988 the People's Republic of China tested its first neutron weapon with an explosive force of 1–5 kT. The US neutron bombs were dismantled from 1992 to 2003 under the governments of George HW Bush , Bill Clinton and George W. Bush . France also dismantled its neutron bombs after the end of the Cold War.

Neutron weapons are usually built with a very small primary explosive device. For example, the American Mk79 warhead had an explosive force of 1 kT, with 0.25 kT being released by nuclear fission of plutonium and 0.75 kT by nuclear fusion. Such a bomb is also comparatively small. The warhead contains only about 10 kg of fissile material and a few grams of deuterium-tritium gas.

Compared to a boosted atomic bomb, the deuterium-tritium gas is not inside the fission arrangement, but outside it. As a result, only a small part of the neutrons emitted by the nuclear fusion hit the fissure material and a larger part can escape unhindered. In order to absorb as little neutron radiation as possible, no uranium is used as a cladding for the fusion explosive, but tungsten . Other components are also preferably made of materials that do not absorb fast neutrons, such as chromium or nickel . Secondary neutron sources can also be used.

Since nuclear fusion releases a particularly large number of neutrons compared to nuclear fission, this arrangement can be used to build a bomb which, with a given explosive force, releases many more neutrons than a normal fusion bomb - hence the name. Technically, the deuterium-tritium gas would be stored under high pressure in a small capsule - a few centimeters in diameter. Due to the high pressure storage, the gas does not have to be frozen.

Various, including some possible (and some presumably impossible) designs for neutron weapons are discussed in the literature. The actual design used by neutron bombs is still a secret.

The neutron weapon is considered a tactical weapon that kills people and other living things through radiation, but is supposed to leave buildings largely intact. The higher lethality with lower structural damage can only be understood in relation to other nuclear weapons. Even with a neutron bomb, around 30 percent of the energy is emitted as pressure waves and a further 20 percent as thermal radiation (with conventional nuclear weapons, these values ​​are around 50 percent and 35 percent). A neutron weapon would be conceivable with the explosive power of the Hiroshima or Nagasaki bomb, but with much higher radiation doses. The biological effects of strong neutron radiation are still scarcely researched.

In the case of tactical neutron weapons with usually low explosive power, it can be assumed that most civil (non-reinforced) buildings are destroyed in the area of ​​lethal radiation. The effectiveness of larger neutron weapons is controversial because the neutron radiation (especially in humid climates) is strongly attenuated by the water vapor contained in the air.

Another application of the neutron weapon was as anti-ballistic missile . The Sprint missile was equipped with a W66 neutron weapon and was intended to destroy approaching nuclear warheads in the atmosphere. The principle behind it was that the neutron flux generated in this way should rapidly heat up the fissile material in the target warhead and thereby deform it until it was unusable in order to prevent ignition.

For the tactical and political aspects of neutron bombs, see also nuclear weapon . For a stationing place in Germany in the 1980s see special ammunition depot Gießen .

Cobalt bomb

A cobalt bomb is a form of the salted bomb . Large amounts of a stable isotope (in this case 59 Co ) are built into the shell of a fission or fusion bomb. The neutrons released during the explosion convert the 59 Co into the radioactive 60 Co. This has a half-life of 5.26 years, its radioactivity accordingly decreases over the course of 50 years to about a thousandth of the initial value. 60 Co emits two gamma quanta of high permeability per nuclear decay . For example, an area should be radioactively contaminated as strongly and for a long time as possible in order to exclude human survival outside of bunkers . It is not known whether such a bomb was ever built.

Dirty bombs

The term " dirty bomb " (English. Dirty bomb ) or "radiological bomb" refers to weapons whose effect is based on distributing radioactive material by conventional explosives in the attack target to contaminate the environment without having held a nuclear reaction. These weapons either do not have enough fissile material for the critical mass , do not have a suitable ignition mechanism, or use radioactive isotopes which are easier to obtain and which are fundamentally unsuitable for nuclear reactions.

A “dirty” bomb filled with plutonium would theoretically be able to make a target area uninhabitable for a long time due to the contamination. It might be of interest to terrorists who could indeed procure plutonium, but only in an amount below the critical mass, or who, for technical reasons, would not be able to build the complex ignition mechanism.

However, it is disputed whether plutonium-based dirty bombs would really be effective in practice, since the activity of 239 plutonium is low due to its long half-life (around 24,000 years); short-lived isotopes such as 137 cesium or 192 iridium show a significantly higher activity with the same amount.

The term “dirty bomb” was also used earlier for cobalt bombs, bombs with a “dirty” second or third stage, and bombs detonated near the ground.


Web links

Commons : Nuclear Weapons and Technology  - Collection of images, videos and audio files
Wiktionary: nuclear weapon  - explanations of meanings, word origins, synonyms, translations
Wiktionary: Atomwaffe  - explanations of meanings, word origins, synonyms, translations
Wiktionary: Nuclear weapon  - explanations of meanings, word origins, synonyms, translations


  1. Nuclear Weapon Archive , FAQ, Elements of Fission Weapon Design, Figure, Carey Sublette
  2. ^ A. Schaper: Arms Control at the Stage of Research and Development? - The Case of Inertial Confinement Fusion. ( Memento from May 19, 2005 in the Internet Archive ). Science & Global Security, Vol. 2, pp. 1-22, 1991.
  3. China - Nuclear Weapons. On:
  4. Christopher Ruddy: Interview with neutron bomb inventor Sam Cohen. Bomb inventor says US defenses suffer because of politics. In: June 15, 1997, accessed September 27, 2020 .
  5. Cold War: What Happened to the Neutron Bomb? In: SPIEGEL ONLINE. Retrieved January 17, 2016 .
  6. ^ List of All US Nuclear Weapons. (List of all US nuclear weapons), Nuclear Weapons Archive.
This version was added to the list of articles worth reading on August 29, 2005 .