How the atomic bomb works
The uranium core contains 92 protons. Natural uranium is basically a mixture of two isotopes: U238 (in whose core there are 146 neutrons) and U235 (143 neutrons), the latter in natural uranium, only 0.7%. The chemical properties of isotopes are absolutely identical, therefore, it is impossible to separate them by chemical methods, but the difference in masses (235 and 238 units) allows this to be done by physical methods: a mixture of uranium is converted into gas (uranium hexafluoride), and then pumped through countless porous partitions. Although uranium isotopes are indistinguishable neither in appearance nor chemically, they are separated by a chasm in the properties of nuclear characters.
The U238 fission process is paid: a neutron arriving from outside must bring with it an energy of 1 MeV or more. And U235 is disinterested: for the excitation and subsequent decay of the incoming neutron, nothing is required, its binding energy in the nucleus is quite enough.
When a neutron enters a fissionable nucleus, an unstable compound is formed, but very quickly (after 10–23–10–22 s), such a nucleus breaks up into two fragments that are not equal in mass and “instantly” (within 10–16−10− 14 c) emitting two or three new neutrons, so that over time the number of fissile nuclei can also multiply (such a reaction is called chain). This is possible only in U235, because the greedy U238 does not want to share from its own neutrons, whose energy is an order of magnitude less than 1 MeV. The kinetic energy of particles - fission products by many orders of magnitude exceeds the energy released during any act of a chemical reaction in which the composition of the nuclei does not change.
Fission products are unstable and for a long time “come to their senses”, emitting various radiation (including neutrons). Neutrons that are emitted after a considerable time (up to tens of seconds) after fission are called delayed, and although their fraction is small compared to instant ones (less than 1%), the role they play in the operation of nuclear facilities is crucial.
Fission products during numerous collisions with surrounding atoms give them their energy, raising the temperature. After neutrons appeared in the assembly with fissile material, the heat release power can increase or decrease, and the assembly parameters, in which the number of fissions per unit time is constant, are called critical. The criticality of the assembly can be maintained both with a large and a small number of neutrons (with a correspondingly greater or lesser heat release power). The thermal power is increased either by pumping additional neutrons from the outside into the critical assembly, or by making the assembly supercritical (then more and more numerous generations of fissile nuclei supply additional neutrons). For example, if it is necessary to increase the thermal power of a reactor, it is brought to such a mode when each generation of instant neutrons is slightly less numerous than the previous one, but thanks to delayed neutrons, the reactor hardly passes a critical state. Then it does not accelerate, but gains power slowly - so that its growth can be stopped at the right time by introducing neutron absorbers (rods containing cadmium or boron).
Neutrons generated during fission often fly past surrounding nuclei without causing fission again. The closer a neutron is born to the surface of a material, the more chance it has of flying out of fissile material and never returning back. Therefore, the shape of the assembly that saves the largest number of neutrons is a ball: for a given mass of matter, it has a minimal surface. A ball of 94% U235 without cavities without cavities inside is not surrounded (secluded) and becomes critical with a mass of 49 kg and a radius of 85 mm. If the assembly of the same uranium is a cylinder with a length equal to the diameter, it becomes critical with a mass of 52 kg. The surface decreases with increasing density. Therefore, explosive compression, without changing the amount of fissile material, can lead the assembly to a critical state. It is this process that underlies the widespread nuclear charge design.
But more often than not, uranium is used in nuclear weapons, but plutonium-239. It is obtained in reactors by irradiating uranium-238 with powerful neutron fluxes. Plutonium costs about six times as much as U235, but in fission, the Pu239 nucleus emits an average of 2, 895 neutrons - more than U235 (2, 452). In addition, the probability of plutonium fission is higher. All this leads to the fact that a secluded ball of Pu239 becomes critical with an almost three times less mass than a ball of uranium, and most importantly - with a smaller radius, which allows to reduce the dimensions of the critical assembly.
Assembly is carried out from two carefully fitted halves in the form of a spherical layer (hollow inside); it is obviously subcritical - even for thermal neutrons and even after being surrounded by a moderator. Around the assembly of very precisely fitted blocks of explosives mounted charge. In order to preserve neutrons, it is necessary to preserve the noble shape of the ball during the explosion - for this, the explosive layer must be detonated simultaneously along its entire outer surface, compressing the assembly evenly. It is widely believed that this requires a lot of electric detonators. But this was only at the dawn of "bombing": for the operation of many tens of detonators, a lot of energy and a considerable size of the initiation system were required. In modern charges, several detonators selected by a special technique are used that are close in characteristics to the detonators that trigger highly stable (in terms of detonation velocity) explosives in grooves milled in a polycarbonate layer (whose shape on a spherical surface is calculated using Riemann geometry methods). A detonation with a speed of about 8 km / s will run absolutely equal distances through the grooves, at the same moment of time it will reach the holes and blow up the main charge - at all the required points at the same time.
An inward blast compresses the assembly with more than a million atmospheres of pressure. The assembly surface decreases, the internal cavity almost disappears in plutonium, the density increases, and very quickly - in a dozen microseconds, the compressible assembly skips a critical state on thermal neutrons and becomes significantly supercritical on fast neutrons.
After a period determined by the insignificant time of an insignificant deceleration of fast neutrons, each of their new, more numerous generation adds 202 MeV of energy produced by the fission into the assembly substance already bursting with monstrous pressure. On the scale of the occurring phenomena, the strength of even the best alloy steels is so scanty that no one even thinks to take it into account when calculating the dynamics of an explosion. The only thing that prevents the assembly from flying apart is inertia: in order to expand the plutonium ball in a dozen nanoseconds by only 1 cm, it is necessary to give the substance an acceleration tens of trillions of times faster than the gravitational acceleration, which is not easy.
In the end, the substance nevertheless scatters, fission ceases, but the process does not end there: energy is redistributed between the ionized fragments of the separated nuclei and other particles emitted during fission. Their energy is on the order of tens or even hundreds of MeV, but only electrically neutral gamma-quanta of high energies and neutrons have a chance to avoid interaction with matter and “slip away”. Charged particles, on the other hand, quickly lose energy in acts of collisions and ionizations. At the same time, radiation is emitted - although it is no longer hard nuclear, but softer, with an energy three orders of magnitude lower, but still more than sufficient to knock electrons from atoms - not only from the outer shells, but in general everything. A mess of bare nuclei, electrons and radiation stripped from them with a density of in grams per cubic centimeter (try to imagine how good it is to light up under light that has acquired the density of aluminum!) - all that a moment ago was a charge - comes in a kind of equilibrium . In a very young fireball, a temperature of the order of tens of millions of degrees is established.
It would seem that even soft, but moving at the speed of light radiation should leave far behind the substance that gave rise to it, but this is not so: in cold air, the range of Kev energy quanta is centimeters, and they move not in a straight line, but changing the direction of motion, re-emitted at each interaction. Quantums ionize the air, spread in it, like cherry juice poured into a glass of water. This phenomenon is called radiation diffusion.
A young 100-kt fireball in a few tens of nanoseconds after the completion of the fission burst has a radius of 3 m and a temperature of almost 8 million Kelvin. But after 30 microseconds, its radius is 18 m, however, the temperature drops below a million degrees. The ball devours space, and the ionized air almost does not move behind its front: radiation cannot transmit a significant momentum during diffusion. But it pumps enormous energy into this air, heating it, and when the radiation energy is depleted, the ball begins to grow due to the expansion of the hot plasma, bursting from the inside by what was previously a charge. Expanding, like an inflated bubble, the plasma membrane becomes thinner. Unlike a bubble, of course, nothing inflates it: there is almost no substance left on the inside, everything flies from the center by inertia, but 30 microseconds after the explosion, the speed of this flight is more than 100 km / s, and the hydrodynamic pressure in the substance - more than 150, 000 atm! To become too thin a shell is not destined, it bursts, forming “blisters”.
Which of the mechanisms of energy transfer of the fireball to the environment prevails depends on the power of the explosion: if it is large, the main role is played by radiation diffusion, if it is small, the expansion of the plasma bubble. It is clear that an intermediate case is possible when both mechanisms are effective.
The process captures new layers of air, energy is already not enough to strip all the electrons from the atoms. The energy of the ionized layer and fragments of the plasma bubble is depleted, they are no longer able to move a huge mass in front of them and noticeably slow down. But what was air before the explosion moves, breaking away from the ball, absorbing all new layers of cold air ... The formation of a shock wave begins.
Shock wave and atomic mushroom
When the shock wave is separated from the fireball, the characteristics of the emitting layer change and the radiation power sharply increases in the optical part of the spectrum (the so-called first maximum). Further, the processes of flashing and changes in the transparency of the surrounding air compete, which leads to the realization of a second maximum, less powerful, but much longer - so much that the output of light energy is greater than in the first maximum.
Near the explosion, everything around it evaporates, further away, it melts, but even further, where the heat flux is already insufficient for melting solids, the soil, rocks, houses flow like liquids under the monstrous, destroying all strength bonds pressure of gas, heated to unbearable for the eyes radiance.
Finally, the shock wave goes far from the point of the explosion, where there remains a loose and weakened, but expanding many times cloud of condensed, converted into the smallest and very radioactive dust of the vapors of what was visited by the plasma of the charge, and what was close in its terrible hour to a place from which you should stay as far as possible. The cloud begins to rise. It cools, changing its color, “puts on” a white cap of condensed moisture, dust stretches behind it from the surface of the earth, forming a “leg” of what is commonly called an “atomic mushroom”.
Attentive readers can estimate the energy release in an explosion with a pencil in their hands. When the assembly is in a supercritical state of the order of microseconds, the neutron age is of the order of picoseconds and the multiplication factor is less than 2, about a gigajoule of energy is released, which is equivalent to ... 250 kg of TNT. And where are the kilo and megatons?
Neutrons - Slow and Fast
In non-fissile matter, “bouncing” away from nuclei, neutrons transfer to them a part of their energy, the larger, the lighter (closer to them in mass) the nucleus. The more neutrons participated in the larger number of collisions, the more they slow down, and finally come into thermal equilibrium with the surrounding matter — they become thermalized (this takes milliseconds). The speed of thermal neutrons is 2200 m / s (energy 0.025 eV). Neutrons can slip away from the moderator and are captured by its nuclei, but with a slowdown, their ability to enter into nuclear reactions increases significantly, therefore neutrons that are “not lost” more than compensate for the decrease in numbers.
So, if you surround a ball of fissile material with a moderator, many neutrons will leave the moderator or be absorbed in it, but there will be those that return to the ball (“reflect”) and, having lost their energy, are much more likely to cause fission events. If the ball is surrounded by a layer of beryllium with a thickness of 25 mm, then you can save 20 kg of U235 and still achieve a critical state of assembly. But they pay time for such savings: each subsequent generation of neutrons, before causing fission, must first slow down. This delay reduces the number of neutron generations generated per unit time, which means that the energy release is delayed. The less fissile material in the assembly, the more a moderator is required for the development of a chain reaction, and fission occurs at increasingly lower-energy neutrons. In the extreme case, when criticality is achieved only on thermal neutrons, for example, in a solution of uranium salts in a good moderator - water, the mass of assemblies is hundreds of grams, but the solution simply boils periodically. The emitted vapor bubbles reduce the average density of the fissile material, the chain reaction stops, and when the bubbles leave the liquid, the flash of divisions is repeated (if the vessel is clogged, the steam will break it - but it will be a thermal explosion devoid of all typical "nuclear" signs).
The fact is that the chain of fissions in the assembly does not begin with a single neutron: they are injected into the supercritical assembly by millions in the required microsecond. В первых ядерных зарядах для этого использовались изотопные источники, расположенные в полости внутри плутониевой сборки: полоний-210 в момент сжатия соединялся с бериллием и своими альфа-частицами вызывал нейтронную эмиссию. Но все изотопные источники слабоваты (в первом американском изделии генерировалось менее миллиона нейтронов за микросекунду), а полоний уж очень скоропортящийся — всего за 138 суток снижает свою активность вдвое. Поэтому на смену изотопам пришли менее опасные (не излучающие в невключенном состоянии), а главное — излучающие более интенсивно нейтронные трубки (см. врезку): за несколько микросекунд (столько длится формируемый трубкой импульс) рождаются сотни миллионов нейтронов. А вот если она не сработает или сработает не вовремя, произойдет так называемый хлопок, или «пшик» — маломощный тепловой взрыв.
Нейтронное инициирование не только увеличивает на много порядков энерговыделение ядерного взрыва, но и дает возможность регулировать его! Понятно, что, получив боевую задачу, при постановке которой обязательно указывается мощность ядерного удара, никто не разбирает заряд, чтобы оснастить его плутониевой сборкой, оптимальной для заданной мощности. В боеприпасе с переключаемым тротиловым эквивалентом достаточно просто изменить напряжение питания нейтронной трубки. Соответственно, изменится выход нейтронов и выделение энергии (разумеется, при снижении мощности таким способом пропадает зря много дорогого плутония).
Но о необходимости регулирования энерговыделения стали задумываться много позже, а в первые послевоенные годы разговоров о снижении мощности и быть не могло. Мощнее, мощнее и еще раз мощнее! Но оказалось, что существуют ядерно-физические и гидродинамические ограничения допустимых размеров докритической сферы. Тротиловый эквивалент взрыва в сотню килотонн близок к физическому пределу для однофазных боеприпасов, в которых происходит только деление. В итоге от деления как основного источника энергии отказались, ставку сделали на реакции другого класса — синтеза.Статья «Дамоклов меч» опубликована в журнале «Популярная механика» (№1, Январь 2009).