Was the neutron bomb ever developed?

Nuclear weapons, also as Nuclear weapons or Nuclear weapons are bombs that draw the energy for an explosion from nuclear reactions, i.e. nuclear fission or fusion, see also atomic bomb explosion. The technical development of nuclear weapons since the 1940s 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 expand suddenly through chemical conversion of the solid or liquid explosive into hot gases, nuclear weapons release enormous amounts of energy in a fraction of a second, which heat the bomb and the surrounding area to temperatures in the million Kelvin range. This means that every solid is evaporated into a particularly hot gas. This results in an order of magnitude greater volume expansion. The pressure wave generated in this way has enough force to kill all living beings over a distance of kilometers.

Nuclear weapons draw their enormous energy from the differences in the binding energies of atomic nuclei, which lead to energy releases in the reactions of nuclear fission and nuclear fusion. The energy releases are millions of times greater than the chemical binding energies in molecules. However, the energy released in the form of very rapid heating also drives the nuclear explosives apart. Therefore, the underlying chain reaction has to cover the entire fissile material as quickly as possible, otherwise this would only release a small part of the energy and the bomb explode with far below the intended destructive effect. Therefore, unlike nuclear reactors for civilian energy generation, nuclear fission weapons are used as pure, easily fissionable nuclides as possible, such as highly enriched uranium or almost pure plutonium-239, and rapid over-criticality is sought during construction.

Fission bomb or fission bomb (atomic bomb)

A classic nuclear fission bomb (atomic bomb) is constructed in such a way that several parts of the fissile material come together at the intended point in time, so that together they exceed the critical mass, but each part alone falls below the critical mass. As soon as the critical mass is reached, a neutron source begins to emit neutrons, which then trigger a chain reaction in the fissile material. The number of neutrons newly generated by nuclear fission (nuclear fission) is then greater in each fission generation than the number of neutrons that have escaped from the material and are absorbed in the material without fission. Polonium-beryllium, which has to mix at the right time, is often used as a neutron source. With polonium-beryllium sources, alpha particles emitted by polonium react with beryllium (see neutron).

A chemical explosive used in a nuclear weapon is called octol. It consists of HMX and TNT, which are mixed in a ratio of 70 to 30.

Gun design

In this way, a subcritical, hollow uranium cylinder can be shot at a subcritical uranium mandrel that lacks precisely this cylinder inside (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 its rather elongated design, the gun design is suitable for elongated nuclear weapons such as Bunker Buster (see below) and nuclear grenades that are fired from tubular weapons. As a chemical explosive z. B. propellants used for artillery shells, such as cordite.

The Little Boy uranium bomb dropped on Hiroshima was constructed similarly. The construction was considered so safe that a previous test ignition was dispensed with. The bomb contained 64 kilograms of uranium enriched to 80 percent uranium-235. 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.

The actual fission set usually has an approximate spherical shape. The fissile material bullet is shot at a rigid fissile material target. In another construction, two floors are shot against each other. An additional, rigid and centrally placed third fissile material part or an imploding reaction aid are probably fictitious.

Implosion bomb

Basic design


Another design shows the implosion bomb Fat Man, which was dropped over Nagasaki. The fissile material (usually plutonium, uranium 235 or an alloy of both metals) is located in the middle as a non-critical mass, either as a solid or as a hollow 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 gun-design bomb and a very large amount of fissile material can be used. In addition, the exploitation of atomic explosives is higher because the fissile material stays together longer and in a more favorable form during the detonation.

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

Build up of explosives around the core

Simply building a shell of an explosive around the core would not be enough, as the explosive begins to burn off spherically around the detonator. So you would need a gigantic number of detonators to achieve a reasonably acceptable compression and not to press the hollow sphere into a crescent moon or star. So you are faced with the task of transforming a spherically diverging shock wave into a spherically converging one. For this you need two explosives with different burning rates. At the transition between the explosives, the shock wave is refracted like light on a lens, which is why the term "explosive lens" is also used. In order to achieve the desired effect for an implosion bomb, such a lens must have a hyperboloid of revolution made of slowly expanding explosive in the center and around it rapidly expanding explosive. Analogous to optics, the refractive index of the lens is greater, the more the expansion speeds of the explosives used differ.

The arrangement in the Trinity gadget consisted of 32 polygonal lenses arranged like a soccer ball. But later 40, 60, 72 and finally 92 lenses were used. However, all these lens systems have the disadvantage that they are polygonal and therefore the compression is not optimal at the corners. In principle, it would be possible to condense a core with just a single, complexly shaped lens. However, this lens would be very large and ultimately heavier and more unwieldy than the configurations used, even if it is easier to ignite.


Modern nuclear weapons have an additional layer of mostly beryllium or uranium U-238 between the conventional, highly explosive explosives and the actual nuclear fuel. Since this layer reflects neutrons, the critical mass can be reduced according to the following table:

Share of U-235 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, due to its inertia, this layer delays the expansion of the fissile material after the chain reaction has started. 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.

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 fissile material 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, 6 aluminum bolts are usually installed as spacers. The advantage of this design is that the entire hollow sphere does not have to be compressed at once. Instead, initially only the small mass of the shell is 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; only the hollow sphere has 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 studs can be replaced with foam (polyurethane foam, expanded polystyrene, etc.).


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 for the precise determination of optimal parameters and thus a high numerical calculation effort. This is one reason why modern nuclear weapons can only be constructed with high-performance computers and these computers are purchased by the respective arms authorities on the one hand and are subject to export restrictions on the other. The calculation results are mostly classified as secret and are only published in very few cases - the published numerical values ​​can certainly be questioned. 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 or 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 (mission 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.

France built and deployed the warhead from 1966 to 1980 MR-31 the largest plutonium bombs built to date 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 is cannon-based (Gun design) worked.



It is crucial for all construction principles that the chain reaction only takes place as long as the arrangement is supercritical. So that as many nuclear fission as possible take place, it should be kept supercritical for as long as possible. But 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. So it 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 (time of the first criticality). With 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. Without a chain reaction, the arrangement would eventually expand again only because of its own inertia. The chain reaction breaks off when it becomes subcritical (time of the second criticality). The expansion is accelerated when additional energy is released from nuclear fission, but only when it exceeds a certain value, namely when the material evaporates. One calls this value Bethe Tait Energy. Only when this minimum energy is reached does the expansion accelerate. But by then, many generations of the chain reaction had already taken place. The chain reaction continues during the expansion, until the point in time of the 2nd criticality 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 2nd criticality point, and the more nuclear fission can still take place.

With an optimal utilization of the fuel, the beginning of the chain reaction should be placed in such a way that the time of the Bethe-Tait energy coincides with the maximum supercriticality. If it were reached earlier, fewer neutrons would be formed and only smaller amounts of the nuclear fuel would be converted.

The worst possible point in time would be the onset of the chain reaction at the time of the first criticality. Because then the point in 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.

But if it is very overcritical, some time will still pass before it becomes subcritical again. And 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 increase in supercriticality continues until the Bethe Tait energy is reached. And in the accelerated expansion that followed, further nuclear divisions are taking 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.

Ignition before the optimal point in time is known as Pre-ignition, an ignition after the optimal time as Delayed ignition. In order to obtain 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 neutrons can come from the fissile material itself through spontaneous nuclear decay or through an additional neutron source. In highly enriched U-235, around 80 million atomic nuclei decay per second and kilogram, emitting alpha particles, but only around two neutrons are produced. In the 64 kg of the Hiroshima bomb, statistically speaking, 0.17 neutrons were released between the criticality point and the complete assembly into a ball (1.38 ms).

For the Hiroshima bomb, a probability of 12% for a pre-ignition was given in 1945, corresponding to the probability for a neutron within the 1.38 ms given above. In order to prevent pre-ignition of bombs according to the gun design, the nuclear bomb design must also be free of other neutron emitters. For example, U-238 (with 20 neutrons per kilogram per second) should be avoided in the envelope, but 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 6 guns based on the cannon barrel principle, 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 plutonium-240.The compacting of 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 assembly can therefore only be used 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.

There 240Pu through neutron capture 239Pu is formed, which in turn is made up by neutron capture 238U arises is the proportion of 240The higher the burn-up of the nuclear fuel, the greater the Pu. Reactors that are supposed to produce weapons-grade plutonium are therefore operated with little burnup. For reasons of economy, a high burn-up rate 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%. 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 uranium bomb in Hiroshima also had such a neutron source as a bomb or fission 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 polonium 210, spatially separated from each other. The two substances merged 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 235U releases the explosion strength of 1 kT. The Hiroshima bomb thus weighed around 650 g 235U 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 materials and the “secondary” radioactivity excited 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 consequently limited by the maximum subcritical mass of two or three pieces of fissile material.

Of course you can combine 2 or more gun barrels, then more parts of the load can be shot at each other. However, this is associated with significantly increased effort in the simultaneous ignition of the propellant charges and other problems, since the unification of all parts of the load at the intended location must take place precisely in terms of time.

With the implosion principle, the gap material is also compacted. This reduces the critical mass and higher supercriticalities and thus better efficiencies are possible. In addition, the spherical arrangement is also 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 in terms of "blasting technology" to carry out the compaction as spherically as possible (or as planned, since there are also other shapes, e.g. hollow cylinders).

Ultimately, this is also a considerable safety advantage of the implosion principle. In order to trigger a core detonation, the chemical detonating explosive on its outer shell has to be detonated in a defined time at a large number of points so that the explosion front runs from the outside inwards 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 chemical explosion and environmental contamination will take place from the fissile material released.

Since the explosion front usually moves convexly away from the ignition point, 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 introduced into the weapon electronics by means of appropriate codes when the deployment is intended. This significantly reduces the risk of theft or loss of a warhead, since attempts to detonate it improperly will usually be unsuccessful.

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

238U-fission through a U-238 reflector / cladding

In addition to the actual fissile material, a reflector made of inexpensive natural uranium or depleted uranium (238U) can 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 U-238 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) were caused by the implosion of 235U released approx. 425 kT of energy and an additional 75 kT through the partially split 238U out of the envelope. An increase in performance through U-238 in the reflector is only possible with bombs according to the implosion design, since that 238U releases a large number of neutrons through spontaneous fission and would therefore lead to a pre-ignition with a high probability in the case of the gun design.

If a small atomic bomb, e.g. B. a bunker buster after the gun design and 235U conceived as fissile material, the theoretical problem arises that at the location of the atomic explosion 235U would win. To prevent this from happening, such a nuclear weapon can be made of a jacket or ballast 238U must be given. In the atomic explosion, both uranium are mixed and the degree of purity is reduced. To avoid pre-ignition this is 238U to be installed spatially separated from the explosive device.

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

Hydrogen bomb


The hydrogen bomb is detonated by an atomic bomb.


Both nuclear fission and nuclear fusion achieve their energy turnover due to the mass defect according to the theory of the equivalence of mass and energy according to Albert Einstein. However, the fusion of small atomic nuclei enables significantly greater energy yield, in relation to the total mass, than the splitting of large nuclei (however, more energy is released per individual splitting of large atomic nuclei, e.g. uranium than, e.g., with a single fusion of two hydrogen atoms). This created the motivation to use this effect with a fusion bomb.

Problems with the first hydrogen bomb

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

In US usage as Super and later than Classical super A large amount of the hydrogen isotopes tritium or deuterium is placed next to or around a fission explosive device that acts 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 also referred to as "alarm clock design" due to its geometric appearance.

This arrangement does not work with deuterium, since most of the energy from the fission bomb is emitted as X-rays and penetrates the deuterium. To solve the problem, tritium could be used instead of deuterium, which has a much lower ignition temperature. However, tritium is comparatively expensive - instead of a hydrogen bomb of this type, a very large fission bomb could be built at a lower cost. A mixture of tritium and deuterium appears as a way out, whereby the fusion of the tritium generates the necessary energy for the ignition of the inexpensive deuterium. However, calculations led to a high required tritium content of 50% and thus only low cost savings.

Another problem with the Classical super is the low efficiency of hydrogen combustion - the fusion extinguishes very quickly, ignition largely does not take place, a large part of the hydrogen does not react. The second stage of the construction fizzles out like a subcritical fission explosive device with the high energy of the fission detonator.

The design of a simple fusion mass next to or around a fission core is therefore unsuitable for hydrogen bombs, 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 low.



With Teller-Ulam-Design the difficulties of the Classical super solved. The solution, found on the Soviet side by Sakharov, also became known as "Sakharov's third idea".

The primary fission explosive device and the secondary fusion explosive device are located in a housing (physically "cavity"). The explosion of the fission explosive device, which is converted into plasma, creates such high temperatures that the fission plasma radiates in the X-ray range.

The housing is filled with the X-rays and the spherical or tubular jacket of the second stage is heated to the same temperatures. This process is called "thermalization". The heating causes the outer layers of the secondary explosive device to explode, causing compression and an inwardly directed shock wave.

The shock wave collapses in the center of the second stage. The geometry of the secondary part is as symmetrical as possible (spherical or cylindrical) so that this collapse of the shock waves occurs (in one point or in a straight line). At the place where the shock waves collapse, temperatures are so high that fission and fusion conditions, i.e. sufficiently high temperature and pressure, are created and the first reactions of the second stage take place. The second stage of the bomb, the fusion, is detonated by the compression and simultaneous radiant heating.

Due to the high-energy alpha particles that are created during the deuterium fusion, the deuterium in the environment is heated so that it can fuse; a focal wave thus runs from the inside out.


Before Teller and Ulam invented the construction principle of the radiation implosion or the surrounding mantle, there was the problem that a large part of the deuterium would be swept apart by the explosion of the primary fission explosive before it could fuse. With this arrangement, however, the thermalization occurs much faster than the expansion of the primary fission plasma. Before the expanding fission plasma reaches the secondary part, the focal wave has already run from the inside to the outside.

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 critical state by the shock wave, thus triggering a fission explosion. 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 in the reflective cavity, the configuration can be further reduced in size.

A similar, albeit civil, fusion-implosion principle also pursues the inertial confinement fusion (ICF - Inertial Confinement Fusion) [1].

Fusion explosives

As a fusion device in the first bomb of this type Ivy Mike Frozen liquid deuterium was used. This is unsuitable for military atomic bombs, since the cooling effort is very large and therefore very expensive. The high-pressure storage of the deuterium gas at normal temperature is also heavy and voluminous and therefore 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 one could also breed plutonium from uranium, which would have a higher energy yield. For these reasons, the deuterium is chemically bound in a solid. Of all solid chemical hydrogen compounds, lithium deuteride, 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% by mass of deuterium. The lithium also takes part in the core processes and produces 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 helium-3 produced can also react further:

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

There is also a considerable number of other nuclear reactions, but these take place comparatively rarely and therefore contribute little to the overall reaction. When building thermonuclear weapons, both the lithium isotope 6Li as well as the isotope 7Li can be used. The total reactions with deuterium are:

If neutrons are required for the fission of a U-238 jacket in a three-stage hydrogen bomb, then 7Li used. If you are interested in a higher energy yield, use 6Li. These isotopes are isolated from natural lithium by enrichment.

In addition to the above equations of important conversions, there are a number of less important reactions. Overall remains of the reactions 4Hey left, unreacted deuterium and a large number of neutrons. The reactive tritium is almost completely used up in the reactions. Per MT explosive force must be arithmetically - when using pure 6Li and if each atom reacts - 15.6 kg lithium deuteride 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 purely 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 also called thermonuclear bomb called.

Nuclear weapons according to the Teller-Ulam design are also euphemistically called clean atom bombs when they obtain a high proportion of their explosive power from nuclear fusion.Since nuclear fusion produces fewer and more short-lived radioactive substances than nuclear fission, such nuclear weapons produce comparatively few radioactive fissile substances in their second stage. What remains, however, are the fissile substances from the first “ignition stage”, i.e. the fission bomb, as well as the fissile substances from the surrounding radioactivity induced by neutrons, which together form the fallout. The bomb is “clean” insofar as if the same explosive effect were achieved with pure nuclear fission bombs, much more radioactive substances would be produced.

Three-stage hydrogen bomb

The ratio of the explosive forces of the first and second stages is limited to a maximum of approx. 200, a factor of 20 to 50 is common. Since fission bombs as the first stages are limited to several hundred kT, the maximum explosive force of the second stage is approx. 10 to 25 MT. There are several ways to increase the explosive power of a thermonuclear bomb:

  • One could try 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 “Sparkplug” of the second stage.
  • Theoretically, a geometric arrangement of several detonator bombs could detonate a large second or 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.
  • One can add another Teller-Ulam stage, i. H. 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 of uranium238U, 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 size, provides a large proportion of the total energy. In a simple atom bomb, a few kilograms of uranium or plutonium are fissioned. There can be several tons of uranium in a tertiary hydrogen bomb. 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. 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 a dirty bomb. The American test bomb Redwing Tewa, for example, was built according to this principle, with a total explosive force of approx. 5 MT and 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 50–60 megatons TNT equivalent. On a 238U - sheathing was dispensed with in order to keep the large fallout caused by the force of the explosion to a minimum. With uranium coating as fourth stage had this bomb had an estimated explosive power of at least 100 megatons of TNT, the radiation 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

In order to increase neutron production, a small amount of the gases deuterium and tritium can be placed in the center of the hollow sphere with nuclear explosives, in contrast to the neutron bomb, in which the deuterium-tritium mixture Next the fission explosive device is arranged. Typical amounts of a deuterium-tritium mixture are 2 to 3 g. Due to the pressure and the heat that arise during the beginning of the chain reaction, nuclear fusion of these substances occurs, with many high-energy neutrons being generated:

The fusion of the deuterium or tritium makes only a small contribution to energy production, 1 g of tritium releases less than 0.2 kT of explosive force. However, the neutrons released from the fusion split a larger proportion of the fission fuel and release a comparatively high amount of energy. The neutrons from 1 g of tritium can split 80 g of plutonium. Since the neutrons released from nuclear fusion are very fast, the splitting of the plutonium releases a particularly large number of fast neutrons, which in turn split other other plutonium nuclei. In total, approx. 450 g of plutonium are split by 1 g of tritium (compared to a structurally identical fission bomb without boosting) and release approx. 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, 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 - whether or not used as detonators for a hydrogen bomb - are boosted today.

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 can have an explosive force of up to around 700 kT even after the Sloika design be built. 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, so it has a high level 238U portion.

The onion skin construction principle (“Sloika” or “layer cake”) is very similar to the original “Classical Super”, which only surrounds an atomic 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 the fission explosive device has been ignited, 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, one slow neutron, one lithium-6 and one deuterium atom are transformed into two helium nuclei, energy and one fast neutron. 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 die 238U split nuclei in the outer layer, thereby releasing energy in turn. Atomic bombs of this type were developed and tested in particular by Great Britain, for example in the Grapple 2 test explosion 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. Many neutrons are also shot back into the fusion layer from the nuclear fission in the outer uranium layer, where they generate a second generation of tritium. Due to the retroactive effect of the 238U-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 235U or 239Pu in the first stage and is therefore inexpensive. This design was chosen in the Soviet Joe-4 nuclear test on August 12, 1953. In this atomic test were made through the inner fission stage 235U generated 40 kT, from the nuclear fusion of the second layer approx. 70 kT and from the nuclear fission in the third layer 290 kT.

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 serves to multiply neutrons, 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 238U can only be cleaved with neutrons with a minimum energy of 1.5 MeV.

Other variants

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

  • In all two-stage bombs, the first stage can be used as a boosted mission bomb what is generally used today.
  • The two-stage mission 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 made at Castle Nectar Test ignited on May 13, 1954. As in the first stage, the conditions relating to the critical mass apply.
  • In all H-bombs (partly also A-bombs) with an outer uranium layer this can also be used 235U or 239Pu run. The American test bomb Cherokee 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.
  • Moderated nuclear weapons consist of a normal fission bomb, in which the fission material does not consist of enriched uranium or plutonium, but of a metal hydride of these substances such as UH3. The hydrogen contained in the material acts as a moderator on the neutrons, i. H. 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 1 kg. However, the density of the fissile material is considerably lower, which is why the bomb loses its criticality very quickly after the chain reaction begins. 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 had an explosive force of 0.2 kT and did not even destroy the 100-meter-high mast on which it was mounted. The experiment was similar Ray on April 11, 1953, in which uranium hydride was also used, but together with deuterium.

Nuclear weapons with special effects

Neutron weapon

A neutron weapon ("enhanced radiation weapon") is a hydrogen bomb with deuterium-tritium fuel, the construction of which is essentially the Teller-ulam Design is similar. 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. 700 neutron warheads were built and destroyed again under Reagan's successor, George H. W. Bush. In June 1980, the French President Giscard d'Estaing also announced that France would develop a neutron bomb. In 1999 the People's Republic of China announced that it was technically capable of building neutron bombs.

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 bomb, but outside it. As a result, only a small part of the neutrons emitted by the nuclear fusion hit the fissure material and the greater 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 could be used to build a bomb that, with a given explosive power, 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 (or presumably some 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, kills people and other living things through radiation, but can 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, about 30% of the energy is given off as a pressure wave and a further 20% as thermal radiation (otherwise about 50% and 35% [2]). A neutron weapon would be conceivable with the explosive power of the Hiroshima or Nagasaki bomb, but with much higher radiation doses.

In the case of tactical neutron weapons with usually low explosive power, it can be assumed that most civil (non-reinforced) buildings will also be 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.

For the tactical and political aspects of neutron bombs, see also nuclear weapon.

Cobalt bomb

The cobalt bomb (also known as “salted bomb”) is intended to radioactively contaminate an area as strongly as possible in order to permanently exclude survival in bunkers or after leaving them. For this purpose, large amounts of cobalt are built into the jacket of a fission or fusion bomb.

The naturally occurring 59Co is converted into 60Co converted. This isotope has a half-life of 5.26 years and emits two gamma quanta of high penetrability per nuclear decay, so it radiates an area very strongly and damages all life. However, it is not known that such a bomb was built.

In addition to cobalt, the naturally occurring tantalum 181 was also discussed, which is converted by neutron bombardment into tantalum 182 with a half-life of 115 days; it would thus also generate extremely strong radiation for a few years. In addition, zinc-64 through neutron bombardment leads to radioactive zinc-65 with a half-life of 244 days and gold-197 to radioactive gold-198 with a half-life of only 2.69 days. A gold bomb would therefore only cause a comparatively short-term contamination.

Dirty bombs

Dirty bombs are weapons that either do not contain enough fissile material for the critical mass or do not contain an ignition mechanism, but whose effect is based on distributing radioactive material at the target using conventional explosives in order to prevent the Contaminate the environment.

A “dirty” plutonium bomb would theoretically be able to make tens of thousands of people seriously ill and make the target area uninhabitable. It would probably be of interest to terrorists who can obtain plutonium, but only in quantities below the critical mass and from a technical point of view would not be able to build the complicated ignition mechanism.

It is partly disputed whether plutonium-based “dirty bombs” would be really effective in practice, since the activity of plutonium-239 is low due to its long half-life; short-lived isotopes such as Cs-137 or Ir-192 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.


  • Smyth, Henry DeWolf. Atomic Energy for Military Purposes, Princeton University Press, 1945. (The US Government's first open report on nuclear weapons) (Smyth Report)
  • Egbert Kankeleit, Christian Küppers, Ulrich Imkeller: Report on the suitability of reactor plutonium for weapons Report IANUS-1/1989.
  • Carson Mark, Explosive Properties of Reactor-Grade Plutonium, Science & Global Security, Vol. 4, p.111, 1993


  1. A. Schaper, Arms Control at the Stage of Research and Development? - The Case of Inertial Confinement Fusion Science & Global Security, Vol. 2, pp. 1-22, 1991
  2. [1]
  • http://nuclearweaponarchive.org Nuclear Weapons Archive, the former High Energy Weapons Archive. A site about nuclear weapons on the internet.
  • Nuclear Weapons Frequently Asked Questions See in particular Section 4.0 'Engineering and Design of Nuclear Weapons'
  • http://www.cddc.vt.edu/host/atomic/ Trinity Site, sister project of HEWA on the history, technology and consequences of nuclear armament
  • The effects of a nuclear explosion
  • [2] German documentation on the atomic bomb
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