A bomb, commonly known as an atomic bomb, fission bomb or nuclear bomb, is an explosive device where energy is obtained by the nuclear fission of a critical mass of fissile elements such as uranium 235 or plutonium 239. Its process was covered by French patent 971-324 from 1939 to 1959.
Fission bombs were the first nuclear weapons developed; it is also the explosion of a critical fissile mass which allows the ignition of an H bomb in modern devices.
In the history of nuclear weapons, it is to this day the only type of bomb that has been used in a conflict.
For obvious safety reasons, the fissile elements of an atomic bomb are held in a subcritical configuration to prevent accidental nuclear explosion. It is just before the triggering of the bomb that the various safety measures set are lifted to prevent the critical form from being reached; the bomb is then said to be armed.
In an atomic bomb, it is important that the fissile elements are brought together as quickly as possible. Indeed, the fissile elements used are also radioactive, and naturally give off neutrons. Because of this, a nuclear fission reaction can start before all the fissile material has the best configuration. The power of the explosion is then reduced, because the small explosion which would result would dispel the remainder of the fissionable material before it could take part in the reaction.
There are several techniques for reuniting the fissile material and thus achieving the over-critical configuration, which triggers the nuclear explosion. We can cite two techniques: by insertion, and by implosion
The simplest technique to trigger an explosion is to throw a block of fissile material against another block, made of the same material, or better, a cylindrical block inside a hollow block. This is the technique of insertion, also called the technique of the pistol - or the barrel. Thus, the critical conditions reached are and the chain reaction is initiated.
The block of fissile material is projected using a very powerful explosive, to allow the shape to be reached quickly. The disadvantage of this technique is that although this form is reached quickly (on the order of a millisecond), it is not achieved enough for plutonium 239, which still contains isotopes, in particular plutonium 240, releasing spontaneously neutrons, which initiates the explosion prematurely, just when conditions become critical. It is for this reason that the insertion technique is only used for uranium 235 bombs.
The bomb dropped on Hiroshima, Little Boy, used this technique. The fact that this technique was used without prior testing (unlike the implosion type used on Nagasaki) shows how reliable this mode of operation is, and relatively easy to master.
The implosion technique is more complex to implement. It consisted in gathering the fissile arranged in a hollow ball, then in compressing so as to increase its density and thus reach a supercritical configuration, which will trigger the nuclear fission reaction and therefore the explosion.
Its implementation is very delicate: the compression of the fissile material is carried out using very powerful explosives arranged all around. But the detonation of these explosives is linked to a set of detonators which must be rigorously synchronized. In addition, each explosion tends to create a spherical shock wave, centered on the detonator. Or on must obtain a shock wave simultaneously at all the external points of the fissile material, which one can imagine as a hollow ball. These shock waves must deform to pass from spheres centered on the outside to a sphere of common center. This is achieved by using explosives whose shock wave travels at different speeds, which leads to its deformation. The machining of the shapes of these explosives must therefore be done with precision.
A similar problem arises with plutonium, which can take on several states (phases) with different mechanical characteristics, and which therefore tends to become heterogeneous, which would lead to a deformation of the shock wave. This is remedied, as in iron metallurgy - where a common additive is carbon - by adding small amounts of another element, often gallium.
The technique of implosion makes it possible to reach the supercritical arrangement much more quickly than by that of the insertion. By implosion, the delay is of the order of two to three microseconds, which is about a hundred times faster than by insertion. This technique makes it possible to use plutonium 239 as fissile material. The implosion technique is also safer since the critical configuration can only be reached in the event of use of the conventional explosive and not by simply moving a piece of metal as in the insertion system.
We can further improve the efficiency and / or reduce the critical mass by between the explosive and the fissile material various canapes which can have a mechanical effect by their inertia or by spreading the shock wave over time (thus prolonging the explosion. ), or slow down the loss of neutrons (neutron reflector reducing the critical mass).
If the presence of a critical mass is enough to trigger a chain reaction, it is not necessarily explosive: it is not in a nuclear power plant, or during a criticality accident.
To obtain an atomic explosion, a chain reaction must be set in motion in a fissile material, so that the free neutrons can multiply exponentially, causing it to pass rapidly from a subcritical configuration (k = 0.9) to a distinct configuration. supercritical (typically, k = 3). We then speak of over-critical mass.
For this, you must have a sufficient quantity of fissile material, this is the critical mass, and in the most compact form possible, a ball, to prevent too many neutrons from escaping through the surface. In atomic bombs, the quantity of fissile material must even be greater than the critical mass, in the order of three times in general.
Each fission releases of the order of 2.93x10^-11 J. To produce by fission an energy equivalent to 20 kt of TNT, i.e. 8.367x10^13 J, the chain reaction must relate to approximately 2.856x10^24 fissions, i.e. therefore of the order of 2^81.2. If the doubling time is of the order of 0.002 66 µs, the entire energy release will take of the order of 0.216 µs.
To ensure an effective explosion, the fissile material must be maintained long enough in a supercritical configuration. But the energy released by the chain reaction tends to heat and disperse the critical mass, reducing its criticality. It is therefore necessary that the transition to criticality is sufficiently sudden so that the criticality reached is high, and that the inertia of the fissile mass is sufficient so that it can remain critical as long as possible before being finally dispersed by the. explosion.
As the system progresses towards the target state, it is first sub-critical, then goes through a just critical state. As soon as criticality is reached, nuclear reactions can develop exponentially, initiated by neutrons from spontaneous fission of the material used, and detonate the assembly before it has reached its optimal state. This is called a "pre-detonation".
In order for the probability of such a pre-detonation to remain low, the probability that a single neutron can be emitted between the transition to the critical state and the optimal state must be negligible. For this, the design of the machine must be such that the time of transition to the state of maximum reactivity is as short as possible, and one uses fissile materials which have only a low rate of emissions. spontaneous neutrons. To achieve a nuclear explosion, the fissile material must be brought to its optimum supercritical state very quickly.
The more spontaneous fission the fissile material used will have, the more quickly it will be necessary to switch to supercritical mode, so that the probability of spontaneous fission before the optimum is as low as possible, or that the chain reaction caused by spontaneous fission did not have time to develop significantly.
Per kilogram of fissile material, uranium 235 produces 0.3 neutrons per second, plutonium 239 produces 22, almost a hundred times more; but above all Pu-239 always contains a fraction of Pu-240 which produces 920 neutrons per gram. It is because of Pu-240 that it is not possible to produce a weapon by reconciliation using plutonium as fissile material: the time required for the reconciliation is too long for the weapon to be reliable. It is also for this reason that the plutonium 240 level must be as low as possible for so-called “military-grade” plutonium.
Conversely, to have a weapon whose power is predictable, it is not possible at the same time to avoid a pre-detonation, and to wait for a spontaneous priming: the assembly reaches its optimal state only for a time. very short ; and the likelihood that initiation will be done by spontaneous fission precisely when it is needed is even lower than that of pre-initiation.
For this reason, nuclear weapons feature a source of neutrons, which send out a burst of neutrons at the optimum time, as determined by the design of the weapon. The quantity of neutrons being of molar order represents the equivalent of ~ 80 doubling of the neutron population, which represents the maximum margin between the passage to criticality and the initiation for a reliable weapon.
Once the critical mass is reached, the chain reaction is triggered. In a complete reaction, each nucleus of fissile material splits into two lighter nuclei (fission products) and additionally releases neutrons. These will then strike other atoms of fissile material, which in turn will release neutrons and so on. The chain reaction is triggered, and the material gives off colossal energy compared to the amount of fissile material involved. However, in an atomic bomb, only a small (sometimes very small) fraction of the fissile material is actually consumed before it dies. 'be dissipated by the explosion, which correspondingly reduces the power of the explosion with regard to the potential energy of the fissionable mass.
For an equal amount of reactants, the energy released during a fission reaction can be of the order of a hundred million times greater than that released by a chemical reaction. This energy is transformed very quickly into heat, by braking these fission products in the surrounding material.
A doped fission weapon is a type of nuclear weapon that uses a small amount of fuel to fuse, in order to increase its fission rate and therefore power. In an H-bomb, the power of the primary stage, and its ability to cause the secondary to explode, is increased (spiked) by a mixture of tritium, which undergoes a nuclear fusion reaction with deuterium.
This reaction (deuterium-tritium fusion) is relatively easy to start, the temperature and compression conditions are within the reach of a fission reaction. The rate of fusion reaction generally becomes significant from 20 to 30 mega-kelvins. This temperature is reached at very low efficiency levels, while less than 1% of the fissile material has cracked (corresponding to a power of the order of a few hundred tonnes of TNT). It is by itself insufficient to start a thermonuclear explosion, but can be used to boost the reaction.
A few grams of deuterium and tritium are placed in the center of the fissionable core, where the explosion of the fissionable mass creates conditions of temperature and pressure sufficient to trigger fusion. The fusion process itself only adds a small amount of energy to the process, maybe 1%. Above all, fusion creates a large flow of very energetic neutrons.
The neutrons released by the fusion reactions added to the neutrons released by the fission cause an increased runaway of the fission reactions, insofar as this flow of neutrons arrives at a time when the core is still very over-critical. Neutrons substantially increase the burn rate of the fissionable material present, plutonium or highly enriched uranium4. The neutrons produced have an energy of 14.1 MeV, which is sufficient to cause the fission of U-238. The number of fission reactions thus increases sharply before the heart actually explodes.
To give an idea of the efficiency of doping, the fusion (supposedly complete) of one mole of tritium (3 g) and one mole of deuterium (2 g) can be triggered with less than 1% of the energy of fission and produces around 1% of the fission energy. But above all, it produces one mole of neutrons (1 g), which, neglecting the losses, could crack one mole (239 g) of plutonium directly, producing 4.6 moles of secondary neutrons, which in turn would crack 4.6 others. moles of plutonium (1099 g). In total, the fission of 1.338 kg of plutonium in two generations adds 23 kilotons of TNT5 equivalent to the core explosion.
This approach is used in modern weapons to ensure sufficient power at the primary stage, while allowing a significant reduction in size and weight6 and immunity to radiation. In addition, doped fission bombs can more easily be immune to parasitic neutron radiation from nearby nuclear explosions.