A-bomb

The atomic bomb (also called “A-bomb” in obsolete terminology, or sometimes referred to by the misnomer “nuclear bomb“) is the name by which the nuclear fission bomb is commonly referred to. It is an explosive device belonging to the group of nuclear weapons, whose energy is entirely produced by a chain reaction of nuclear fission. The term is also commonly used to refer to thermonuclear weapons, as they make up the majority of today’s nuclear arsenals.

The operation of these devices is based on the nuclear fission reaction, a process of division of the atomic nucleus, which takes place in a heavy element called fissile, into two or more nuclei of lower mass, following the collision with a free neutron. The rupture of the nucleus produces in turn, in addition to lighter elements, also some additional free neutrons, as well as a very significant amount of energy. If the fissile material has a sufficient degree of concentration and is in a sufficiently large mass, called “critical mass”, the free neutrons produced in turn are able to hit new nuclei of a fissile element, producing an uncontrolled chain reaction that propagates throughout the mass of material releasing an immense amount of energy in a very short time.

The atomic bomb is a weapon of mass destruction, the production of which is restricted and sanctioned by the international community through the Nuclear Non-Proliferation Treaty (NPT).

History of the atomic bomb

The theoretical foundation is the principle of mass-energy equivalence, expressed by the equation E = mc² predicted in Albert Einstein’s theory of special relativity. This generic equivalence suggests in principle the possibility of directly transforming matter into energy or vice versa. Einstein saw no practical application to this discovery. He realized, however, that the principle of mass-energy equivalence could explain the phenomenon of radioactivity, or that certain elements emit spontaneous energy.

Subsequently, the hypothesis was advanced that some reactions based on this principle could actually take place inside atomic nuclei. The “decay” of the nuclei causes a release of energy. The idea that a nuclear reaction could also be produced artificially and to a massive extent, that is, in the form of a chain reaction, was developed in the second half of the thirties, following the discovery of the neutron. Some of the main research in this field was conducted in Italy by Enrico Fermi.

A group of European scientists who had taken refuge in the United States (Enrico Fermi, Leó Szilárd, Edward Teller and Eugene Wigner) were concerned about the possible military development of the principle. In 1939 the scientists Fermi and Szilard, based on their theoretical studies, persuaded Einstein to write a letter to President Roosevelt to point out that there was a hypothetical possibility of building a bomb using the principle of fission and it was likely that the German government had already arranged research on the subject. The U.S. government became interested in research.

Enrico Fermi continued in the United States new research on the properties of a rare isotope of uranium, uranium-235, until obtaining the first self-powered artificial chain fission reaction: on December 2, 1942 the group directed by Fermi assembled in Chicago the first “atomic pile” or “nuclear fission reactor” that reached the critical condition, consisting of a mass of natural uranium and graphite arranged in a heterogeneous manner.

A few months earlier, in June 1942, based on calculations performed in a summer physics session at the University of California led by Robert Oppenheimer, it had been concluded that it was theoretically possible to build a bomb that exploited the chain fission reaction. However, its technical implementation required enormous funding.

Much of the investment would be used to produce uranium sufficiently “enriched” with its isotope 235 or sufficient quantities of plutonium-239. The calculations indicated that to produce a critical mass a percentage of enrichment was needed, i.e. a concentration of fissile isotope, much higher than that required for a nuclear reactor.

The first atomic bomb was made with a project secretly developed by the US government. The program took on an industrial scale in 1942 (cf. Manhattan Project). To produce the fissile materials, uranium-235 and plutonium-239, gigantic plants were built at a total cost of two billion dollars at the time. The materials (with the exception of plutonium, produced in the reactors of the Hanford laboratories in the state of Washington, and uranium, produced in the laboratories of Oak Ridge) and technical devices, mainly the implosion detonator, were produced in the laboratories of Los Alamos, a purpose-built center in the New Mexico desert. The project was directed by Robert Oppenheimer and included the world’s leading physicists, many of them refugees from Europe.

The first plutonium bomb (codenamed The Gadget) was detonated in the Trinity test on July 16, 1945 at the Alamogordo firing range in New Mexico. The first uranium bomb (Little Boy) was dropped on the center of Hiroshima City on August 6, 1945. The second plutonium bomb codenamed Fat Man, was dropped on Nagasaki on August 9, 1945. These were the only cases of military use of nuclear weapons, in the form of strategic bombing.

The Soviet Union quickly caught up: Stalin activated the so-called Borodino operation, which, thanks to Soviet research and also to the contribution of Western spies, achieved unexpected successes. The first fission bomb was tested on August 29, 1949, thus ending the U.S. monopoly. The United Kingdom, France and China experimented with fission devices in 1952, 1960 and 1964 respectively. Israel built the first weapon in 1966 and is believed to have carried out a test together with South Africa in 1979, but its arsenal is still undeclared. India carried out its first test in 1974. Pakistan began producing nuclear weapons in 1983 and carried out a test in 1998. North Korea carried out its first test in 2006. Nuclear warheads, based on both the principle of nuclear fission and thermonuclear fusion, can be installed not only on aerial bombs but also on missiles, artillery shells, mines and torpedoes.

In 1955, the Russell-Einstein Manifesto was compiled: Russell and Einstein promoted a statement, inviting scientists from around the world to come together to discuss the risks to humanity posed by nuclear weapons.

South Africa, which began producing atomic bombs in 1977, was the only country to voluntarily cancel its nuclear program in 1989, dismantling under IAEA control all the weapons it had already built.

A-bomb description

The fission chain reaction of nuclei occurs in an uncontrolled form (i.e. very rapidly divergent) in a mass of fissile material, in practice uranium-235 or plutonium-239, with a sufficient degree of purity. The core of these particular isotopes, radioactive and heavy elements, is not entirely stable. This has the property of being able to capture a free neutron that collides with it at a sufficiently low speed. The capture is immediately followed by the very rapid breaking of the nucleus into several fragments, including other neutrons, also sufficiently “slow” to be captured by nearby nuclei.

A metallic mass composed entirely, or almost entirely of these atoms, has the property of generating within it a nuclear chain reaction, that is, a repetition of events in which the breaking of an atomic nucleus caused by a collision with a free neutron, in turn releases free neutrons that produce the splitting of nuclei of neighboring atoms. The process repeats itself giving an exponential progression. This happens only on condition that in the vicinity of each fissile atom, there is a sufficiently high number of other fissile isotope atoms, so that the probability of further collisions is close to 1. That is, the mass of fissile material must be sufficiently large (it must contain a sufficiently large number of atoms) and the atoms must be sufficiently close, that is, the element must be sufficiently “concentrated” (the concentration of fissile isotope is called enrichment of this element).

When a sufficiently pure (enriched) mass of a fissile element is in sufficiently large quantities, a nuclear chain reaction occurs spontaneously: it is then said that the material exceeds a certain threshold called critical mass. Under such conditions, the metallic mass satisfies the statistical properties that produce the very rapid multiplication of collisions and fissions. The exact extent of the critical mass depends on its particular geometric shape. However, its purity must be high, above 90%.

The instant a mass is made supercritical, the chain reaction occurs, releasing an enormous amount of energy in a very short time. The fission of a heavy element is a strongly exothermic nuclear reaction. The explosion is very powerful for the enormous amounts of energy released in nuclear reactions, on the order of millions of times higher than those involved in chemical reactions involving similar masses.

The uncontrolled chain reaction differs from the nuclear chain processes that take place in a nuclear reactor for the production of electricity, by the course of the process with respect to time. In a reactor the nuclear reaction takes place in a mass of fissile material that can be very large, but where the element is much less concentrated. In such conditions there is no very rapid release of energy, the release is slower and can be moderate. The reaction in a reactor is always kept below predetermined temperature and criticality parameters, in a stable state, i.e. controlled, i.e. in which energy is released steadily over time without any possibility of explosion.

In common usage, the name “atomic bomb” is sometimes misused for other nuclear weapons, of similar or higher power, thus also including bombs that use the other type of nuclear reaction, the thermonuclear fusion of light element nuclei.

The term “atomic bomb” in the original classification of “A-bomb” properly referred only to fission bombs. Those that use thermonuclear fusion are called H-bombs or hydrogen bombs, or even grouped in the definition of “thermonuclear weapons”. The nuclear weapons present in contemporary arsenals are practically all of the latter type. The fission bomb, however, is still a fundamental component of the thermonuclear weapons themselves, constituting their heart or trigger, thermonuclear weapons are therefore “two-stage” bombs. This is because the fusion of light nuclei can only be triggered with very high energies, and the fission bomb is the only device capable of producing the very high values of pressure and temperature necessary to trigger the thermonuclear fusion reaction.

Principle of operation

The principle of the atomic bomb is the nuclear fission chain reaction, the physical phenomenon whereby the atomic nucleus of certain elements with atomic mass greater than 230 can split (fission) into two or more nuclei of lighter elements when hit by a free neutron. Fission can be triggered in massive form, i.e. as a chain reaction, if the fissile nuclei are so numerous and close to each other that further collision of the released neutrons with new fissile nuclei is likely. The isotopes that can be used in practice are uranium-235 and plutonium-239. These heavy metals are the fissile materials par excellence.

When a free neutron hits a nucleus of 235 U or ²³⁹Pu, it is captured by the nucleus for a very short time, making the compound nucleus unstable: this breaks within 10⁻¹² seconds into two or more nuclei of lighter elements, simultaneously releasing two to four neutrons. About one percent of its mass is converted into energy in the form mainly of photons and kinetic energy of residual light nuclei and free neutrons, totaling about 200 MeV.

The neutrons released by the process can in turn collide with other fissile nuclei present in the system, which then fission releasing further neutrons and propagating the chain reaction throughout the mass of material. As already mentioned, however, the chain reaction occurs if and only if the probability of neutron capture by the fissile nuclei is sufficiently high, that is, if the nuclei are numerous, very close to each other and the losses due to escape from the system are appropriately reduced. This is obtained, typically, by shaping in a geometry with a low surface/volume ratio a certain amount of highly enriched metallic uranium (or plutonium ), in which the fissile isotope is present in a concentration much higher than the natural one, even higher than 90% of the total, and in such quantity that the final assembly exceeds the so-called critical mass.

The exact value of the “critical mass” depends on the chosen element, the degree of its enrichment and the geometric shape (a shield that surrounds the mass itself preventing the escape of neutrons can also contribute to reducing its value). Indicatively, it is of the order of a few kilograms.

In the warhead of an atomic bomb, the fissile material is kept separate into several subcritical masses, or shaped into a geometric shape with a hollow spherical shell, which makes the mass subcritical thanks to the high surface/volume ratio such as to make the neutron balance unfavorable.

The bomb is detonated by concentrating the fissile material together by means of conventional explosives that instantly bring the various masses into contact or collapse the spherical shell, thus uniting the material into a supercritical mass. At the center of the system is also placed a neutron initiator, a small beryllium device containing a few grams of a substance strongly emissive of alpha particles such as polonium, a system that helps the explosion by irradiating the mass with a wave of neutrons at the right time. The warhead is possibly coated externally with a beryllium screen that partially reflects neutrons that would otherwise be lost outside.

Energy and power of the nuclear device are direct functions of the quantity of fissile material and its percentage of enrichment, as well as of the efficiency of the weapon, i.e. the percentage of material that actually undergoes fission, the latter determined by the quality or calibration of its detonation system.

The mass of fissile material in an atomic bomb is called a core.

Chain reaction

The neutron-induced nuclear chain reaction in a mass of ²³⁵U takes place according to a scheme of this type:

165 MeV for the kinetic energy of new atoms formed as a result of fission

5 MeV for neutron kinetic energy

8 MeV for instantaneous gamma radiation energy

5 MeV for beta decay energy of fission products

6 MeV for gamma decay energy of fission products

11 MeV for neutrino kinetic energy

This significant energy production is linked to the fact that the sum of the resulting masses (fission fragments and neutrons) is slightly lower than the initial mass of the nucleus and the neutron that generated the fission: a very small percentage of this mass is lost, “transformed” into energy. The amount of energy released by nuclear reactions is much greater than that of chemical reactions in relation to the amount of matter involved.

The binding energy inside the nuclei (strong interaction) is much more intense than that which binds the outer electrons of two atoms together. The binding energy within the nuclei is a measure of mass. In the equivalence principle E=mc², since the second term of equality is an enormous quantity (due to the value of the constant “c”, the speed of light in vacuum, equal to 299 792 458 m/s) the energy “E” is enormous compared to a small mass “m”. For comparison, in a water molecule the bond of hydrogen atoms can produce an energy of about 16 eV, ten million times less than that released by the uranium nucleus. One gram of uranium-235 that undergoes full fission produces about 8 x 10¹⁰ joules, which is as much as the combustion of about 3 tons of coal.

Fissile material

The fissile materials used in atomic bombs are plutonium-239 or enriched uranium, which can only be produced in highly industrialized countries, the existence of a uranium enrichment cycle or nuclear reactors or other systems capable of producing plutonium-239 from the isotope uranium-238 through the nuclear fertilization reaction being required upstream.

Naturally occurring uranium is a mixture of about 99.3% of isotope with mass number 238 and approximately 0.7% isotope with mass number 235; Of the two, only the last is fissile. In order to accumulate a sufficient amount, it is, therefore, necessary to “enrich” the uranium with its isotope 235. The core of a uranium bomb must therefore be composed of a mass composed largely of uranium-235, i.e. highly enriched uranium.

This “enrichment” occurs with the separation of the isotope 235 from the isotope 238, to obtain a gradually higher concentration of the first element. The industrial enrichment cycle begins with the conversion of natural uranium into uranium hexafluoride (UF 6), a gaseous substance that allows to subsequently exploit the different diffusion rate that distinguishes 235 UF 6 from ²³⁸UF ₆ to separate the two isotopes. The same process can also be performed with uranium tetrachloride (UCl₄). These substances can be brought to the gaseous state at low temperatures, which allows the two isotopes to be separated mechanically. The substance is centrifuged at very high speed, in special ultra-centrifuges mounted in series (“cascade”). These progressively concentrate the isotope 235 separating it from the chemical homolog 238, exploiting the very small difference in specific gravity between the two. Enriched uranium for atomic warheads is composed of about 97% U-235.

It is also possible to separate the isotope 235 with other methodologies, on a smaller scale or with much more sophisticated technologies (such as a laser).

The waste product of the enrichment process is uranium, in large quantities, composed almost entirely of the isotope 238 therefore useless for nuclear reaction, with a very low percentage of U-235. It is the so-called depleted uranium, that is, uranium with a fraction of U-235 less than 0.2%. It is classified as radioactive waste, but is used to build bullets and bombs in conventional weapon systems. The toxicity of depleted uranium, of chemical and radiological origin, is very high and makes the use of these weapons systems dangerous even for the armies that use them, when uranium is inhaled or ingested.

Within masses lower than the critical one, as long as they are concentrated in small volumes, fissions in uranium and plutonium are more frequent than in natural minerals, where fissile isotopes are less concentrated. After a certain period of time, due to this loss of fissile isotopes, the fissile material is no longer usable due to the presence of a high amount of fission fragments.

Construction features

An atomic bomb is formed by a metal core of a few tens of kilograms of uranium enriched over 93% (“weapon-grade” uranium), or a few kilograms of plutonium containing at least 93% of the isotope 239 (“weapon-grade” plutonium). It is also possible to build a bomb using very few kilograms of uranium, following the construction principles developed for plutonium bombs; It is also possible, today, to build bombs with mini-cores using a few hundred grams of plutonium. The mass of the core is always, however, subcritical (if it were not so, the bomb would explode ahead of time).

The core is inserted into a heavy metal container, such as uranium-238, to form a thick shell called a tamper (“buffer” or “flare”) that limits the escape outside of neutrons, useful for the reaction at the time of the explosion, and above all has the function of retaining, through an inertial reaction to the pressure exerted by its thermal expansion, the core for the time necessary for the reaction, about 1 microsecond. The time available for the reaction greatly increases efficiency, i.e. the percentage of material that undergoes fission.

The explosion is triggered by the use of conventional explosives that bring parts of the core closer together or modify it so as to make the mass supercritical. By means of detonator systems (which can be complex and of different types) the core is modified in shape and concentration so as to bring it to a supercritical state. There are essentially two alternative techniques, from an engineering point of view, to produce this effect. The two solutions are:

1. The system of separate blocks, also called ballistic detonation, “projectile” or “cannon” (gun-triggered fission bomb). In this type of project the core of fissile material is divided into two parts, a “projectile” of subcritical mass and a “target”, more massive but also with subcritical mass. At the time of the explosion, an explosive charge pushes the projectile at high speed into a barrel up to the target, so as to join to form a single supercritical mass;
2. The implosion system. It is much more efficient than the separate block system but also extremely complex to design. It is based on the simultaneous explosion of many detonators placed on the surface of a crown of explosive material surrounding the core in the shape of a hollow sphere of subcritical mass so as to produce a high pressure on the latter. The increase in pressure, compressing the fissile material and eliminating the cavity, changes its shape and increases its density, so as to bring it to a supercritical state. The system is supported by a system of containers (“tamper” and frame) around the core with the functions of reducing neutron leaks, retaining the thermal expansion of the core and making the implosion shock wave uniform.

The two construction models both contain an initiator of the nuclear reaction, i.e. a small spherical device usually made of beryllium and containing an alpha-emitting material such as polonium-209 or 210, which, once activated, acts as a neutron source. The initiator is placed at the center of the core and is activated by the pressure exerted by the latter when it is compressed by the detonation of the conventional explosive surrounding it. The initiator sequentially gives rise to these effects:

1. its beryllium casing is broken through when the mass implodes;
2. the alpha radiation emitted by polonium interacts with beryllium-9 producing beryllium-8 and free neutrons;
3. The neutrons released by this device are in huge quantities and trigger fission in a mass that is now supercritical.

Separate block detonation system

The trigger in separate blocks is called a bomb with “projectile detonation” or “cannon”. It is the easiest to build, requires rudimentary technology. However, it only works well with uranium-235. Plutonium, in fact, due to the non-eliminable traces of the isotope 240, is more unstable and therefore the device would require measures with which it would become too bulky to be used.

The atomic bomb dropped on Hiroshima, Little Boy, was such a device. The principle is that one subcritical mass of uranium is projected (“fired”) against another subcritical mass of uranium. The bomb is formed by a tube at one end of which there is a projectile consisting of a block of uranium-235 of hollow cylindrical shape, at the other end is the “target”, another cylindrical block of uranium-235, equal in size to the cavity of the projectile and of lower mass, where the neutron generator is also located.

Detonation occurs when the projectile is launched by means of an explosive charge and joins the target exceeding the critical mass and creating a supercritical mass. By hitting the target, the projectile also activates the neutron initiator, which, however, in this case is superfluous (in Little Boy four were inserted only for a matter of safety) precisely because of the “cannon” configuration of the device, in which fission occurs spontaneously once the mass and density have been created, Supercritical.

These devices have very poor efficiency. To build a bomb requires a few tens of kilograms of uranium-235, an extremely rare natural isotope, but most of this mass (98.5%) is wasted, that is, it does not give rise to any nuclear reaction. The “Little Boy” device contained 64.13 kg of uranium, of which only 1.5% underwent nuclear fission. The low efficiency is due to the fact that the important concentration effect of the implosion system on the core is missing, and the inertial containment is entrusted only to the masses of the container. The latter (tamper) is also less effective having to contain a very large mass.

The assembly of such a large mass is also quite dangerous. In addition, separate block devices cannot have an explosive power much larger than 20 kilotons because the amount of uranium cannot be increased at will. For all these reasons, in principle weapons based on this system are not built.

A few dozen bombs like this were built after World War II, mainly by Britain and the Soviet Union. These were dismantled in the fifties. In the seventies, South Africa alone built five bombs like this, which were also dismantled.

Implosion detonation system in the A-bomb

This system was used in the bomb that exploded on Nagasaki, Fat Man. The core is a hollow sphere of a few kilograms of plutonium-239. It is placed inside several concentric spheres of different metals and surrounded by a complex system of explosive charges and electronic detonators. At the center of the hollow sphere is placed the polonium-beryllium primer.

When the explosive surrounding the core is detonated, the concentric shock wave produces the perfectly symmetrical implosion of the plutonium mass. The central cavity disappears, the material crushed by the shock wave is concentrated by a factor of 2 or more, the mass becomes supercritical, the central initiator activates. The Nagasaki bomb had an efficiency of around 15%, and contained less than 7 kg of plutonium.

Nuclear explosion of the A-bomb

Much of the energy released by the nuclear explosion consists simply of direct radiant energy, i.e. heat effect of light radiation produced by the nuclear reaction. The light is radiated by the “ball of fire” formed by ionized gases, which expands at the point of explosion, for a time of the order of milliseconds.

Explosion gases at hundreds of millions of degrees emit light radiation of such intensity that objects even hundreds of meters away that are illuminated directly reach temperatures of thousands of degrees in milliseconds.

Another significant portion of energy is discharged in the form of a supersonic shock wave produced by the violent thermal expansion of the air. The wavefront caused by a 20-kiloton explosion has a supersonic velocity within a radius of a few hundred meters, and proceeds at infrasonic speeds with destructive effects up to distances of the order of kilometers. If the bomb explodes in an atmosphere where the air has normal density, shock wave fronts are produced – including the hemispherical one generated by reflection from the ground surface – which produce an overpressure, in the peak area, of the order of 350-750 g/cm². The human body has a high resistance to overpressure. All parts of the human organism, with the exception of the eardrum membrane, are able to withstand overpressures even 5-6 times higher than these.

However, even if the human body is resistant to pressure itself, in practice it can be hit by the very high speed debris contained in the shock front or projected against blunt objects. On the contrary, buildings – especially buildings for civil use – normally have a much lower resistance to overpressure than those of the shock front, and the large surfaces that characterize them (walls, roofs, windows) translate the shock wave into enormously devastating forces. The shock front of a nuclear explosion causes virtually all the buildings exposed in the vicinity to collapse. In an explosion of 20 kilotons the shock wave is able to sweep away buildings hundreds of meters or kilometers away.

The effect of these destructive components (radiation and shock wave) is maximized if the bomb is detonated at a certain height above the ground. If the bomb exploded on the ground, however, much of its energy would be absorbed by the ground and its effects would have a reduced range.

A significant share of energy (5-10%) is emitted in the form of ionizing radiation at high energies.

The explosion of a nuclear weapon over a densely populated area produces, due to the shock wave and temperatures, a carpet of rubble strewn with numerous small fires. When there is a large surface on which numerous points of fire are distributed, the geometry of the convective currents causes a phenomenon called superfire (or Feuersturm) that is the union of all the foci in a single gigantic fire of the entire surface fed by a very violent centripetal convective current. According to some estimates, in the bombings of Hiroshima and Nagasaki the largest share of victims would have been caused by the superfires that developed during the tens of minutes following the explosion.

Effects of nuclear explosions

The effects of a nuclear explosion on an inhabited area can therefore be summarized in these categories:

1. Direct effects from thermal/light radiation: surfaces illuminated directly by the explosion can reach very high temperatures, which however depend a lot on the type of surface and its color, that is, on its properties to reflect or absorb light. A nuclear explosion typically produces two radiant pulses, the first within the first 5-6 milliseconds, and the second after 80-100 milliseconds, up to times of the order of 1 second. The second pulse occurs when the vapor sphere of the explosion has expanded sufficiently to be transparent again. The temperatures reached by exposed surfaces, in the case of body surfaces of living beings, can cause fatal burns, and even destroy organisms. It should be noted that this effect occurs on people who at that time are in outdoor areas directly exposed to the explosion, not shielded by other objects or protective suits. The flash of the explosion, also having a strong component of high frequencies, can destroy the retina causing blindness.
2. Mechanical effects of the shock wave: the overpressure wave instantly destroys buildings and artifacts for civil use, and this corresponds to an immense projection of debris at very high speeds (hundreds of meters / second). In Hiroshima, fragments of glass projected by the explosion penetrated concrete walls even at distances of 2,200 meters from the epicenter of the explosion. The reinforced concrete walls of the Hiroshima Red Cross Hospital show a surface strewn with holes and cuts as if they had been hit by volleys of bullets, caused in reality by the tiny shards of glass projected at very high speed by the explosion.
3. High-energy ionizing radiation: the radiation emitted by a nuclear explosion is mainly gamma radiation, has high intensity, but its emission has a very short duration. It should be noted that damage to living organisms, such as acute disease produced by radiation or other diseases, including genetic damage due to malformations of fetuses, can be caused not only by direct exposure to explosion, but also and above all by contact with contaminated dust and water.
4. Superfires or Feuerstürme: They involve people in the rubble area, and are believed to have been the cause of the relatively largest share of casualties in the Hiroshima and Nagasaki explosions.

References (sources)

Section 8.0 The First Nuclear Weapons, 2002; Nuclear Weapons – UNODA, un.org;Article Bomba atomica, Wikipedia.