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. This thermal radiation travels outward from the fireball at the speed of light, 300,000 km/sec. The chief hazard of thermal radiation is the production of burns and eye injuries in exposed personnel. 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 10 7 K, 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 m (100,000 ft) these X-rays are quickly absorbed in the atmosphere, and the energy is reradiated at blackbody temperatures below 10,000° K. Both of these temperatures are well above that reached in conventional chemical explosions, about 5,000° K. For detonations below 100,000 feet, 35 percent 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.
The outer layer initially absorbs much of the radiation from theisothermal sphere and hence the apparent surface temperature of the fireball and the amount of radiation emitted from it decreases after separation. But, as the shock front advances still farther, the temperature of the shocked air diminishes and it becomes increasingly transparent. As the shock front temperature drops below 6,000° K, thermal radiation decreases when the shock front becomes transparent to radiation from the interior. This occurs between 10 –5 and 10 –2 seconds after detonation. At about 0.1 second after detonation, the shock front becomes sufficiently transparent that radiation from the innermost, hottest regions becomes visible, producing a second thermal peak. This results in an unmasking of the still incandescent isothermal region and an increase in the apparent surface temperature of the fireball. This phenomena is referred to as breakaway. Before the second peak begins the fireball has radiated only about one quarter of its total energy. About 99 percent of the total thermal energy is contained in the second pulse. The duration of this pulse depends on the yield of the weapon and the height of burst (HOB); it ranges from only about 0.4 s for a 1 kT airburst to more than 20 s for a 10 MT explosion.The rate of thermal emission from the fireball is governed by its apparent surface temperature. The thermal output of a nuclear air burst will then occur in two pulses), an initial pulse, consisting primarily of ultraviolet radiation, which contains only about 1% of the total radiant energy of the explosion and is terminated as the shock front moves ahead of the fireball, and a second pulse which occurs after breakaway.
Both theory and experiment indicate that the dominant thermal pulse can be adequately represented by a blackbody at a temperature between 6,000° and 7,000° K, which places the peak of the spectrum near the boundary between the ultraviolet and the visible regions of the spectrum. The shape of the Planck spectrum is such that most of the radiation is contained in the visible and infrared regions. \ The response of any given system to the thermal pulse depends on the absorption properties of the test subject but also to the distance from the burst and the atmospheric conditions between fireball and target such as clouds, snow, aerosols, and dust. The atmosphere is not equally transparent at all wavelengths, so the spectrum of the radiation incident on a target must be correctly calculated and then simulated. By the same token, known atmospheric absorption effects can be used by a system incorporating sensors at different distances from a nuclear explosion to establish the characteristics of the explosion itself and, therefore, the weapon type. Such information would be very useful in selecting appropriate responses. Sensors used to deliver information on which decision makers can rely, however, must be calibrated against simulated nuclear fireballs under a wide range of atmospheric conditions. The range of thermal effects increases markedly with weapon yield. Thermal radiation decays only as the inverse square of the distance from the detonation. Thus, weapons in the megaton class and above are primarily incendiary weapons, able to start fires and do other thermal damage at distances well beyond the radius at which they can topple buildings or overturn armored vehicles. The effect of thermal radiation on unprotected human beings is likely to be very serious, producing flash burns over large areas of the body. However, the Hiroshima and Nagasaki bombings demonstrated that once the victim is beyond the radius at which light-colored fabrics are directly ignited, even simple precautions can greatly reduce the extent and seriousness of thermal injuries. Many examples exist of people severely burned on their faces and arms, but unburned beneath even a thin shirt or blouse. Thermal effects on structures are equally complex. The response of a structure to the thermal pulse from a nuclear weapon depends upon its composition (wood, masonry, concrete); the type and albedo of any exterior paint; the transparency of any windows facing the burst; the type, texture, and composition of roofing; and even the presence or absence of awnings and shades. For weapons in the 1 to 200-kiloton region used against structures commonly found in the West, blast effects are likely to predominate; larger weapons will have the ability to start fires at distances far greater than they can inflict significant blast damage. Films of tests conducted in Nevada in the 1950’s confirm that at the extreme distance at which wood-frame houses can be ignited by lower yield weapons, the buildings are blown apart seconds later by the blast wave, while structures which survive the blast do not ignite after the blast. Tests conducted in the Pacific using megaton-class weapons show the opposite effect. Secondary fires started by broken gas mains, electrical short circuits, etc., can considerably amplify the overall effects of a nuclear detonation.