If the flash is focused through the lens of the eye, a permanent retinal burn will result. At Hiroshima and Nagasaki, there were many cases of flashblindness, but only one case of retinal burn, among the survivors. On the other hand, anyone flashblinded while driving a car could easiIy cause permanent injury to himself and to others. Skin burns result from higher intensities of light, and therefore take place closer to the point of explosion.
First-degree, second-degree and third-degree burns can occur at distances of five miles away from the blast or more. Third-degree burns over 24 percent of the body, or second-degree burns over 30 percent of the body, will result in serious shock, and will probably prove fatal unless prompt, specialized medical care is available. The entire United States has facilities to treat 1, or 2, severe burn cases. A single nuclear weapon could produce more than 10, The thermal radiation from a nuclear explosion can directly ignite kindling materials.
In general, ignitable materials outside the house, such as leaves or newspapers, are not surrounded by enough combustible material to generate a self-sustaining fire.
Fires more likely to spread are those caused by thermal radiation passing through windows to ignite beds and overstuffed furniture inside houses. Another possible source of fires, which might be more damaging in urban areas, is indirect.
Blast damage to Stores, water heaters, furnaces, electrical circuits or gas lines would ignite fires where fuel is plentiful. Direct radiation occurs at the time of the explosion. It can be very intense, but its range is limited. Nuclear explosions are also accompanied by various forms of radiation, lasting a few seconds to remaining dangerous over an extended period of time. Approximately 85 percent of the energy of a nuclear weapon produces air blast and shock , thermal energy heat.
The remaining 15 percent of the energy is released as various type of nuclear radiation. Of this, 5 percent constitutes the initial nuclear radiation, defined as that produced within a minute or so of the explosion, are mostly gamma rays and neutrons. The final 10 percent of the total fission energy represents that of the residual or delayed nuclear radiation, which is emitted over a period of time.
This is largely due to the radioactivity of the fission products present in the weapon residues, or debris, and fallout after the explosion. Little Boy was dropped on Hiroshima. The second weapon, dropped on Nagasaki , was called Fat Man and was an implosion-type device with a plutonium core. The isotopes uranium and plutonium were selected by the atomic scientists because they readily undergo fission. Fission occurs when a neutron strikes the nucleus of either isotope, splitting the nucleus into fragments and releasing a tremendous amount of energy.
The fission process becomes self-sustaining as neutrons produced by the splitting of atom strike nearby nuclei and produce more fission. This is known as a chain reaction and is what causes an atomic explosion. When a uranium atom absorbs a neutron and fissions into two new atoms, it releases three new neutrons and some binding energy. Two neutrons do not continue the reaction because they are lost or absorbed by a uranium atom.
However, one neutron does collide with an atom of uranium, which then fissions and releases two neutrons and some binding energy. Both of those neutrons collide with uranium atoms, each of which fission and release between one and three neutrons, and so on. This causes a nuclear chain reaction. For more on this topic, see Nuclear Fission. In order to detonate an atomic weapon, you need a critical mass of fissionable material.
The uranium atom can split any one of dozens of different ways, as long as the atomic weights add up to uranium plus the extra neutron. The following equation shows one possible split, namely into strontium 95 Sr , xenon Xe , and two neutrons n , plus energy:.
The immediate energy release per atom is about million electron volts Me. Of the energy produced, 93 percent is the kinetic energy of the charged fission fragments flying away from each other, mutually repelled by the positive charge of their protons. This initial kinetic energy imparts an initial speed of about 12, kilometers per second. Here, their motion is converted into X-ray heat, a process which takes about a millionth of a second.
By this time, the material in the core and tamper of the bomb is several meters in diameter and has been converted to plasma at a temperature of tens of millions of degrees. This X-ray energy produces the blast and fire which are normally the purpose of a nuclear explosion.
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