Overview
The goal of this section is to accurately and concisely describe the history of nuclear weapons design. By establishing the historical narrative of weapons design, I believe that we can better understand contemporary warhead design.
The first nuclear weapons ever detonated in an application of warfare were the Little Boy and the Fat Man. The Little Boy was a gun-type fission device. In a gun-type detonation, a conventional explosive is used to propel a hollow cylinder of sub-critical mass into a cylinder of similarly sub-critical mass. The cylinders combine into a critical mass of fissile material. The nuclear chain reaction is started via a neutron from a source near the mass, such as Beryllium. A tamper, such as Uranium-238, reflects neutrons and provides additional inertia but does not participate in the reaction. For the case of Uranium-238, this is because U238 is not as fissile as Uranium-235. Uranium-238 requires a high-energy neutron to fission (>1 MeV), which can not be practically produced in a pure fission bomb (Uranium-238 was used as an example because it is a commonly used tamper in modern nuclear weapons due to its ability to increase the yield in fission-fusion-fission weapons. Little Boy (pictured) used Tungsten as its tamper). Due to the crude nature of the gun-type fission device, it is not very efficient; in fact, only 1.38% of the Uranium in Little Boy actually fissioned.
Fat Man, however, was not a gun-type device. Although the design of Fat Man was still relatively crude, it used a more nuanced and efficient design. The Fat Man was a Plutonium-based implosion device. In an implosion-type weapon detonation, conventional explosive is detonated around a sphere of sub-critical nuclear material, which produces a shock wave that drastically increases the pressure surrounding the fissile material, so much so that the sub-critical mass is compressed into a smaller, denser mass, causing it to go critical. Neutrons are then injected into the critical mass, starting the nuclear chain reaction. Fat Man (pictured) is an example of an implosion-type weapon. The Fat Man was about 10 times as efficient as the Little Boy.
The first nuclear weapons ever detonated in an application of warfare were the Little Boy and the Fat Man. The Little Boy was a gun-type fission device. In a gun-type detonation, a conventional explosive is used to propel a hollow cylinder of sub-critical mass into a cylinder of similarly sub-critical mass. The cylinders combine into a critical mass of fissile material. The nuclear chain reaction is started via a neutron from a source near the mass, such as Beryllium. A tamper, such as Uranium-238, reflects neutrons and provides additional inertia but does not participate in the reaction. For the case of Uranium-238, this is because U238 is not as fissile as Uranium-235. Uranium-238 requires a high-energy neutron to fission (>1 MeV), which can not be practically produced in a pure fission bomb (Uranium-238 was used as an example because it is a commonly used tamper in modern nuclear weapons due to its ability to increase the yield in fission-fusion-fission weapons. Little Boy (pictured) used Tungsten as its tamper). Due to the crude nature of the gun-type fission device, it is not very efficient; in fact, only 1.38% of the Uranium in Little Boy actually fissioned.
Fat Man, however, was not a gun-type device. Although the design of Fat Man was still relatively crude, it used a more nuanced and efficient design. The Fat Man was a Plutonium-based implosion device. In an implosion-type weapon detonation, conventional explosive is detonated around a sphere of sub-critical nuclear material, which produces a shock wave that drastically increases the pressure surrounding the fissile material, so much so that the sub-critical mass is compressed into a smaller, denser mass, causing it to go critical. Neutrons are then injected into the critical mass, starting the nuclear chain reaction. Fat Man (pictured) is an example of an implosion-type weapon. The Fat Man was about 10 times as efficient as the Little Boy.
The Thermonuclear Warhead
In a thermonuclear detonation fissile material in the core explodes, producing heat and neutrons. These neutrons convert some of the Lithium 6 surrounding the core to tritium, and the resulting heat allows for deuterium-deuterium and deuterium-tritium reactions to take place. These reactions produce neutrons with high enough energy that allows Uranium-238 to fission, which releases even further amounts of energy and neutrons. Lithium-6 is used because it has a neutron cross-section of 940 barns (9.4×10-22 cm2), and therefore readily produces tritium in reactions like:
63Li + 10n → 42He + 32H + 4.8 MeV
In addition to its military applications, this form of fusion has been recommended for future controlled fusion in nuclear reactors.
63Li + 10n → 42He + 32H + 4.8 MeV
In addition to its military applications, this form of fusion has been recommended for future controlled fusion in nuclear reactors.
Physical Effects of Nuclear Weapons
Thermal Effects
Large amounts of electromagnetic radiation in the visible, infrared, and ultraviolet regions of the electromagnetic spectrum are emitted from the surface of the fireball within the first minute or less after detonation. The chief hazard of thermal radiation is the production of burns and eye injuries in exposed individuals. Such thermal injuries may occur even at distances where blast and initial nuclear radiation effects are minimal. Absorption of thermal radiation will also cause the ignition of combustible materials and may lead to fires which then spread rapidly among the debris left by the blast.
The fireball from a nuclear explosion reaches blackbody temperatures greater than 107 degrees Kelvin, so that the energy at which most photons are emitted corresponds to the x-ray region of the electromagnetic spectrum. For detonations occurring below 30,000 meters (~100,000 feet) these X-rays are quickly absorbed in the atmosphere, and the energy is reradiated at blackbody temperatures below 10,000 degrees Kelvin. Both of these temperatures are well above that reached in conventional chemical explosions, about 5,000 degrees Kelvin. For detonations below 100,000 feet, 35% to 45% percent of the nuclear yield is effectively radiated as thermal energy.
In addition to the high temperature of the nuclear fireball, the blackbody radiation is emitted in a characteristic two-peaked pulse with the first peak being due to the radiating surface of the outrunning shock. As the fireball expands and its energy is deposited in an ever-increasing volume its temperature decreases and the transfer of energy by thermal radiation becomes less rapid. At this point, the blast wave front begins to catch up with the surface of the fireball and then moves ahead of it, a process called hydrodynamic separation. Due to the tremendous compression of the atmosphere by the blast wave, the air in front of the fireball is heated to incandescence. Thus, after hydrodynamic separation, the fireball actually consists of two concentric regions: the hot inner core known as the isothermal sphere; and an outer layer of luminous shock-heated air (Nuclear Weapon Thermal Effects, n.d.).
Large amounts of electromagnetic radiation in the visible, infrared, and ultraviolet regions of the electromagnetic spectrum are emitted from the surface of the fireball within the first minute or less after detonation. The chief hazard of thermal radiation is the production of burns and eye injuries in exposed individuals. Such thermal injuries may occur even at distances where blast and initial nuclear radiation effects are minimal. Absorption of thermal radiation will also cause the ignition of combustible materials and may lead to fires which then spread rapidly among the debris left by the blast.
The fireball from a nuclear explosion reaches blackbody temperatures greater than 107 degrees Kelvin, so that the energy at which most photons are emitted corresponds to the x-ray region of the electromagnetic spectrum. For detonations occurring below 30,000 meters (~100,000 feet) these X-rays are quickly absorbed in the atmosphere, and the energy is reradiated at blackbody temperatures below 10,000 degrees Kelvin. Both of these temperatures are well above that reached in conventional chemical explosions, about 5,000 degrees Kelvin. For detonations below 100,000 feet, 35% to 45% percent of the nuclear yield is effectively radiated as thermal energy.
In addition to the high temperature of the nuclear fireball, the blackbody radiation is emitted in a characteristic two-peaked pulse with the first peak being due to the radiating surface of the outrunning shock. As the fireball expands and its energy is deposited in an ever-increasing volume its temperature decreases and the transfer of energy by thermal radiation becomes less rapid. At this point, the blast wave front begins to catch up with the surface of the fireball and then moves ahead of it, a process called hydrodynamic separation. Due to the tremendous compression of the atmosphere by the blast wave, the air in front of the fireball is heated to incandescence. Thus, after hydrodynamic separation, the fireball actually consists of two concentric regions: the hot inner core known as the isothermal sphere; and an outer layer of luminous shock-heated air (Nuclear Weapon Thermal Effects, n.d.).
Residual Radiation
Blast and thermal effects occur to some extent in all types of explosions, whether conventional or nuclear. The release of ionizing radiation, however, is a phenomenon unique to nuclear explosions and is an additional casualty producing mechanism superimposed on blast and thermal effects. This radiation is basically of two kinds, electromagnetic and particulate, and is emitted not only at the time of detonation (initial radiation) but also for long periods of time afterward (residual radiation). Initial or prompt nuclear radiation is that ionizing radiation emitted within the first minute after detonation and results almost entirely from the nuclear processes occurring at detonation. Residual radiation is defined as that radiation which is emitted later than 1 minute after detonation and arises principally from the decay of radioisotopes produced during the explosion (NATO Handbook on the Medical Aspects of NBC Defensive Operations AMedP-6(B)).
In a land or water surface burst, large amounts of earth or water will be vaporized by the heat of the fireball and drawn up into the radioactive cloud. This material will become radioactive when it condenses with fission products and other radio-contaminants or has become neutron-activated. There will be large amounts of particles of less than 0.1 micrometer to several millimeters in diameter generated in a surface burst in addition to the very fine particles which contribute to worldwide fallout. The larger particles will not rise into the stratosphere and consequently will settle to earth within about 24 hours as local fallout. Severe local fallout contamination can extend far beyond the blast and thermal effects, particularly in the case of high yield surface detonations (ibid.)
Blast and thermal effects occur to some extent in all types of explosions, whether conventional or nuclear. The release of ionizing radiation, however, is a phenomenon unique to nuclear explosions and is an additional casualty producing mechanism superimposed on blast and thermal effects. This radiation is basically of two kinds, electromagnetic and particulate, and is emitted not only at the time of detonation (initial radiation) but also for long periods of time afterward (residual radiation). Initial or prompt nuclear radiation is that ionizing radiation emitted within the first minute after detonation and results almost entirely from the nuclear processes occurring at detonation. Residual radiation is defined as that radiation which is emitted later than 1 minute after detonation and arises principally from the decay of radioisotopes produced during the explosion (NATO Handbook on the Medical Aspects of NBC Defensive Operations AMedP-6(B)).
In a land or water surface burst, large amounts of earth or water will be vaporized by the heat of the fireball and drawn up into the radioactive cloud. This material will become radioactive when it condenses with fission products and other radio-contaminants or has become neutron-activated. There will be large amounts of particles of less than 0.1 micrometer to several millimeters in diameter generated in a surface burst in addition to the very fine particles which contribute to worldwide fallout. The larger particles will not rise into the stratosphere and consequently will settle to earth within about 24 hours as local fallout. Severe local fallout contamination can extend far beyond the blast and thermal effects, particularly in the case of high yield surface detonations (ibid.)
Appendix A
Little Boy device at bottom, covered with tarp for security reasons
Little Boy device at bottom, covered with tarp for security reasons
Appendix B
An Internal Schematic of the Fat Man Implosion Device
An Internal Schematic of the Fat Man Implosion Device
Appendix C
Internal Schematic of the American Thermonuclear W88 Warhead
Internal Schematic of the American Thermonuclear W88 Warhead