Nuclear weapons are the most effective in the history of mankind in terms of cost / efficiency: the annual costs of developing, testing, manufacturing and maintaining these weapons make up from 5 to 10 percent of the military budget of the United States and the Russian Federation - countries with an already formed nuclear production complex, developed atomic power engineering and the presence of a fleet of supercomputers for mathematical modeling of nuclear explosions.
The use of nuclear devices for military purposes is based on the property of atoms of heavy chemical elements to decay into atoms of lighter elements with the release of energy in the form of electromagnetic radiation (gamma and X-ray ranges), as well as in the form of kinetic energy of scattering elementary particles (neutrons, protons and electrons) and nuclei of atoms of lighter elements (cesium, strontium, iodine and others)
The most popular heavy elements are uranium and plutonium. Their isotopes, when fissioning their nucleus, release from 2 to 3 neutrons, which in turn cause the fission of the nuclei of neighboring atoms, etc. A self-propagating (so-called chain) reaction with the release of a large amount of energy occurs in the substance. To start the reaction, a certain critical mass is required, the volume of which will be sufficient for the capture of neutrons by atomic nuclei without the emission of neutrons outside the substance. Critical mass can be reduced with a neutron reflector and an initiating neutron source
The fission reaction is started by combining two subcritical masses into one supercritical one or by compressing a spherical shell of a supercritical mass into a sphere, thereby increasing the concentration of fissile matter in a given volume. Fissile material is combined or compressed by a directed explosion of a chemical explosive.
In addition to the fission reaction of heavy elements, the reaction of synthesis of light elements is used in nuclear charges. Thermonuclear fusion requires heating and compression of matter up to several tens of millions of degrees and atmospheres, which can be provided only due to the energy released during the fission reaction. Therefore, thermonuclear charges are designed according to a two-stage scheme. The isotopes of hydrogen tritium and deuterium (requiring minimum values of temperature and pressure to start the fusion reaction) or a chemical compound - lithium deuteride (the latter, under the action of neutrons from the explosion of the first stage, is divided into tritium and helium) are used as light elements. Energy in the fusion reaction is released in the form of electromagnetic radiation and kinetic energy of neutrons, electrons and helium nuclei (so-called alpha particles). The energy release of the fusion reaction per unit mass is four times higher than that of the fission reaction
Tritium and its self-decay product deuterium are also used as a source of neutrons to initiate the fission reaction. Tritium or a mixture of hydrogen isotopes under the action of the compression of the plutonium shell partially enters into a fusion reaction with the release of neutrons, which transform plutonium into a supercritical state.
The main components of modern nuclear warheads are as follows:
- stable (spontaneously not fissile) isotope of uranium U-238, extracted from uranium ore or (in the form of an impurity) from phosphate ore;
- radioactive (spontaneously fissile) isotope of uranium U-235, extracted from uranium ore or produced from U-238 in nuclear reactors;
- radioactive isotope of plutonium Pu-239, produced from U-238 in nuclear reactors;
- stable isotope of hydrogen deuterium D, extracted from natural water or produced from protium in nuclear reactors;
- radioactive isotope of hydrogen tritium T, produced from deuterium in nuclear reactors;
- stable isotope of lithium Li-6, extracted from ore;
- stable isotope of beryllium Be-9, extracted from ore;
- HMX and triaminotrinitrobenzene, chemical explosives.
The critical mass of a ball made of U-235 with a diameter of 17 cm is 50 kg, the critical mass of a ball made of Pu-239 with a diameter of 10 cm is 11 kg. With a beryllium neutron reflector and a tritium neutron source, the critical mass can be reduced to 35 and 6 kg, respectively.
To eliminate the risk of spontaneous operation of nuclear charges, they use the so-called. weapons-grade Pu-239, purified from other, less stable isotopes of plutonium to the level of 94%. With a periodicity of 30 years, plutonium is purified from the products of spontaneous nuclear decay of its isotopes. In order to increase the mechanical strength, plutonium is alloyed with 1 mass percent gallium and coated with a thin layer of nickel to protect it from oxidation.
The temperature of radiation self-heating of plutonium during storage of nuclear charges does not exceed 100 degrees Celsius, which is lower than the decomposition temperature of a chemical explosive.
As of 2000, the amount of weapons-grade plutonium at the disposal of the Russian Federation is estimated at 170 tons, the United States - at 103 tons, plus several tens of tons accepted for storage from the NATO countries, Japan and South Korea, which do not possess nuclear weapons. The Russian Federation has the largest plutonium production capacity in the world in the form of weapons-grade and power nuclear fast reactors. Together with plutonium at a cost of about $ 100 per gram (5-6 kg per charge), tritium is produced at a cost of about 20 thousand US dollars per gram (4-5 grams per charge).
The earliest designs for nuclear fission charges were the Kid and Fat Man, developed in the United States in the mid-1940s. The latter type of charge differed from the first in the complex equipment for synchronizing the detonation of numerous electric detonators and in its large transverse dimensions.
The "Kid" was made according to a cannon scheme - an artillery barrel was mounted along the longitudinal axis of the air bomb body, at the muffled end of which was one half of the fissile material (uranium U-235), the second half of the fissile material was a projectile accelerated by a powder charge. The uranium utilization factor in the fission reaction was about 1 percent, the rest of the U-235 mass fell out in the form of radioactive fallout with a half-life of 700 million years.
The "Fat Man" was made according to an implosive scheme - a hollow sphere of fissile material (Pu-239 plutonium) was surrounded by a shell made of uranium U-238 (pusher), an aluminum shell (quencher) and a shell (implosion generator), made up of five- and hexagonal segments of a chemical explosive, on the outer surface of which electric detonators were installed. Each segment was a detonation lens of two types of explosives with different detonation velocities, converting the diverging pressure wave into a spherical converging wave, uniformly compressing the aluminum shell, which in turn compressed the uranium shell, and that - the plutonium sphere until its inner cavity closed. An aluminum absorber was used to absorb the recoil of a pressure wave as it passes into a material with a higher density, and a uranium pusher was used to inertly hold plutonium during the fission reaction. In the inner cavity of the plutonium sphere, a neutron source was located, made from the radioactive isotope polonium Po-210 and beryllium, which emitted neutrons under the influence of alpha radiation from polonium. The utilization factor of fissile matter was about 5 percent, the half-life of radioactive fallout was 24 thousand years.
Immediately after the creation of "Kid" and "Fat Man" in the United States, work began to optimize the design of nuclear charges, both cannon and implosion schemes, aimed at reducing the critical mass, increasing the utilization rate of fissile material, simplifying the electric detonation system and reducing the size. In the USSR and other states - owners of nuclear weapons, the charges were initially created according to an implosive scheme. As a result of design optimization, the critical mass of the fissile material was reduced, and its utilization factor was increased several times due to the use of a neutron reflector and a neutron source.
The beryllium neutron reflector is a metal shell up to 40 mm thick, the neutron source is gaseous tritium filling a cavity in plutonium, or iron hydride with titanium impregnated with tritium, stored in a separate cylinder (booster) and releases tritium under the action of heating by electricity immediately before using a nuclear charge, after which tritium is fed through the gas pipeline into the charge. The latter technical solution makes it possible to multiply the power of the nuclear charge depending on the volume of pumped tritium, and also facilitates the replacement of the gas mixture with a new one every 4-5 years, since the half-life of tritium is 12 years. An excess amount of tritium in the booster makes it possible to reduce the critical mass of plutonium to 3 kg and significantly increase the effect of such a damaging factor as neutron radiation (by reducing the effect of other damaging factors - a shock wave and light radiation). As a result of design optimization, the fissile material utilization factor increased to 20%, in the case of an excess of tritium - up to 40%.
The cannon scheme was simplified due to the transition to radial-axial implosion by making an array of fissile material in the form of a hollow cylinder, crushed by the explosion of two end and one axial explosive charge
The implosion scheme was optimized (SWAN) by making the outer shell of the explosive in the form of an ellipsoid, which made it possible to reduce the number of detonation lenses to two units spaced apart from the poles of the ellipsoid - the difference in the velocity of the detonation wave in the cross section of the detonation lens ensures the simultaneous approach of the shock wave to the spherical surface the inner layer of the explosive, the detonation of which uniformly compresses the beryllium shell (combining the functions of a neutron reflector and a pressure wave recoil damper) and a plutonium sphere with an inner cavity filled with tritium or its mixture with deuterium
The most compact implementation of the implosion scheme (used in the Soviet 152-mm projectile) is the execution of an explosive-beryllium-plutonium assembly in the form of a hollow ellipsoid with a variable wall thickness, which provides the calculated deformation of the assembly under the action of a shock wave from an explosive explosion into a final spherical structure
Despite various technical improvements, the power of nuclear fission charges remained limited to the level of 100 Ktn in TNT equivalent due to the unavoidable expansion of the outer layers of fissile matter during the explosion with the exclusion of matter from the fission reaction.
Therefore, a design was proposed for a thermonuclear charge, which includes both heavy fission elements and light fusion elements. The first thermonuclear charge (Ivy Mike) was made in the form of a cryogenic tank filled with a liquid mixture of tritium and deuterium, in which an implosive nuclear charge of plutonium was located. Due to the extremely large dimensions and the need for constant cooling of the cryogenic tank, a different scheme was used in practice - an implosive "puff" (RDS-6s), which includes several alternating layers of uranium, plutonium and lithium deuteride with an external beryllium reflector and an internal tritium source
However, the power of the “puff” was also limited by the level of 1 Mtn due to the beginning of the fission and synthesis reaction in the inner layers and the expansion of unreacted outer layers. In order to overcome this limitation, a scheme was developed for the compression of light elements of the fusion reaction by X-rays (second stage) from the fission reaction of heavy elements (first stage). The enormous pressure of the flux of X-ray photons released in the fission reaction allows lithium deuteride to be compressed 10 times with an increase in density by 1000 times and heated during the compression process, after which lithium is exposed to the neutron flux from the fission reaction, turning into tritium, which enters into fusion reactions with deuterium. The two-stage thermonuclear charge scheme is the cleanest in terms of the radioactivity yield, since secondary neutrons from the fusion reaction burn out unreacted uranium / plutonium to short-lived radioactive elements, and the neutrons themselves are extinguished in the air with a range of about 1.5 km.
For the purpose of uniform crimping of the second stage, the body of the thermonuclear charge is made in the form of a peanut shell, placing the assembly of the first stage in the geometric focus of one part of the shell, and the assembly of the second stage in the geometric focus of the other part of the shell. The assemblies are suspended in the bulk of the body using foam or airgel filler. According to the rules of optics, the X-ray radiation from the explosion of the first stage is concentrated in the narrowing between the two parts of the shell and is evenly distributed over the surface of the second stage. In order to increase the reflectivity in the X-ray range, the inner surface of the charge body and the outer surface of the second stage assembly are covered with a layer of dense material: lead, tungsten, or uranium U-238. In the latter case, the thermonuclear charge becomes three-stage - under the action of neutrons from the fusion reaction, U-238 turns into U-235, whose atoms enter into a fission reaction and increase the explosion power
The three-stage scheme was incorporated in the design of the Soviet AN-602 aerial bomb, the design power of which was 100 Mtn. Before the test, the third stage was excluded from its composition by replacing uranium U-238 with lead due to the risk of expanding the zone of radioactive fallout from the fission of U-238 beyond the test site. The actual capacity of the two-stage modification of the AN-602 was 58 Mtn. A further increase in the power of thermonuclear charges can be made by increasing the number of thermonuclear charges in the combined explosive device. However, this is not necessary due to the lack of adequate targets - the modern analogue of the AN-602, placed on board the Poseidon underwater vehicle, has a radius of destruction of buildings and structures by a shock wave of 72 km and a radius of fires of 150 km, which is quite enough to destroy metropolitan areas such as New York or Tokyo
From the point of view of limiting the consequences of the use of nuclear weapons (territorial localization, minimizing the release of radioactivity, tactical level of use), the so-called precision single-stage charges with a capacity of up to 1 Ktn, which are designed to destroy point targets - missile silos, headquarters, communication centers, radars, air defense missile systems, ships, submarines, strategic bombers, etc.
The design of such a charge can be made in the form of an implosive assembly, which includes two ellipsoidal detonation lenses (chemical explosive from HMX, inert material made of polypropylene), three spherical shells (neutron reflector made of beryllium, piezoelectric generator made of cesium iodide, fissile material from plutonium) and an internal sphere (lithium deuteride fusion fuel)
Under the action of a converging pressure wave, cesium iodide generates a superpowerful electromagnetic pulse, the electron flow generates gamma radiation in plutonium, which knocks out neutrons from nuclei, thereby initiating a self-propagating fission reaction, X-rays compresses and heats lithium deuteride, the neutron flux generates tritium from lithium, which enters into reaction with deuterium. The centripetal direction of fission and fusion reactions ensures 100% use of thermonuclear fuel.
Further development of nuclear charge designs in the direction of minimizing power and radioactivity is possible by replacing plutonium with a device for laser compression of a capsule with a mixture of tritium and deuterium.
