The first thing bomb victims experience is the intense flux of photons from the blast, which releases 70-80% of the bomb's energy. The effects go up to third degree burns. Initial deaths are often due to this effect. The next phenomenon is the supersonic blast front. The pressure front has the effect of blowing away anything in its path. After the front comes the overpressure phase. The effect is similar to being a few hundred meters underwater. The pressure gradually diminishes and there is a negative overpressure phase with a reversed blast wind. This reversal is due to the air rushing back in to fill the void left by the explosion. The air gradually returns to normal atmospheric pressure. At this stage, fires caused by electrical destruction often ignite debris turning much of the blast area into a firestorm.
Electromagnetic Pulse (EMP)
The heat from fusion and fission instantaneously raises the surrounding air to 10 million degrees celsius. This superheated air gives off so much light that it is brighter than the sun and is visible for hundreds of kilometers. The resultant fireball quickly expands, rising at the rate of a few hundred meters per second. After approximately a minute, the fireball has risen several kilometers and has cooled to the extent that it no longer radiates. During expansion, the surrounding cooler air exerts some drag on the rising air which slows the outer edges of the cloud. The unimpeded inner portion rises quicker than the surrounding edges. A vacuum effect occurs when the outer portion occupies the vacuum left by the higher inner portion. The result is a smoke ring. The inner material gradually expands out into a mushroom cloud due to convection. If the explosion occurs close enough to the ground, debris can be sucked up the stem.
Collisions and ionization of the cloud particles result in lightning bolts flickering to the ground. Initially the cloud is orange-red due to a chemical reaction when the air is superheated. As the cloud cools to the surrounding air temperature, water vapor starts to condense turning the clound from red to white. In the final stages, the cloud can reach 100 kilometers wide and 40 kilometers high (for a megaton class explosion).
A nuclear explosion gives off radiation at all wavelengths of light. Some is in the radio / radar portion of the spectrum. This is known as the EMP effect. The strength of the EMP effect is in part related to the altitude of the initial explosion. The higher the explosion occurs in the atmosphere, the stronger the effect. High altitude explosions can damage electronics by inducing a current surge in closed circuit metallic objects. The damage range can extend to over 1,000 kilometers
The three categories of immediate effects are: blast, thermal radiation (heat), and prompt ionizing of nuclear radiation. Their relative importance varies with the yield of the bomb. At low yields, all three can be significant sources of injury. Which an explosive yeild of about 2.5 kilotons (kT), the three effects are roughly equal. All are capable of inflicting fatal injuries at a range of 1km.Thermal Radiation
The fraction of a bomb's yield emitted as thermal radiation blast and ionizing radiation if essentially constant for all yields, but the way the different forms of energy interact with air and objects vary dramatically. Air is essentially transparent to thermal radiation. The thermal radiation affects exposed surfaces, producing damage by rapid heating. A bomb that is 100 times larger can produce equal thermal radiation intensities over areas 100 times larger. The area of an (imaginary) sphere centered on the explosion increases with the square of the radius. Thus the destructive radius increases with the square root of the yield (Actually the rate of increase is somewhat less partly due to the fact that larger bombs emit heat more slowly which reduces the damage produced by each calorie of heat). It is important to note that the area subjected to damage by thermal radiation increases almost linearly which yield.
Blast effect is a volume effect. The blast wave deposits energy in the material it passes through, including air. When the blast wave passes through solid material, the energy left behind causes damage. The more matter the energy travels through, the smaller the effect. The amount of matter increases with the volume of the imaginary sphere centered on the explosion. Blast effects thus scale with the inverse cube law which relates radius to volume.
The intensity of nuclear radiation decreases with the inverse square law much like thermal radiation. However, nuclear radiation is also strongly absorbed by teh air it travels though, which causes the intensity to drop off much more rapidly.
These scaling laws show the effects of thermal radiation grow rapidly with yield (relative to blast), while those of radiation rapidly decline. In a small nuclear attack (bomb yield approximately 15kT) causualties would be seen from all three causes. Burns would be the most prevelant serious injury (two thirds of those who die the first day would be burn victims) and would occur at the greatest range. Blast and burn injuries would be found in 60-70% of survivors. People close enough to suffer significant radiation illness would be well inside the lethal effects radius for blast and flash burns. As a result, only 30% of injured survivors would show radiation illness. Many of these people would have been sheltered from burns and blast effects. Even so, most victims with radiation illness would also have blast injuries and burns.
Which yields in the range of hundreds of kilotons or greater (typical for strategic warheads), immediate radiation injury becomes insignificant. Dangerous radiation levels only exist so close to the explosion that surviving the blast is impossible. On the other hand, fatal burns can be inflicted well beyond the range of substantial blast. A 20 megaton bomb can cause potentially fatal third degree burns at a range of 40km, where the blast can do little more than break windows and cause superficial cuts. A conventially rule of thumb for estimating the short-term fatalities from all causes due to nuclear attack is to count everyone inside the 5 psi blast overpressure contour around the hypocenter as a fatality. In reality, substantial numbers of people inside the contour will survive and substantial numbers of people outside the contour will die, but the assumption is that these two groups will be roughly equal in size. This completely ignores any possible fallout effects.
The chief delayed effect is the creation of huge amounts of radioactive material with long lifetimes (half-lifes ranging from days to millennia). The primary source of these products is the debris left from fission reactions. A potentially significant secondary source is neutron capture by non-radioactive isotopes both within the bomb and in the outside environment. When atoms fission they can split in some 40 different ways, producing a mix of about 80 different isotopes. These isotopes vary widely in stability; some are completely stable while others undergo radioactive decay with half-lifes of fractions of a second. The decaying isotopes may themselves form stable or unstable daughter isotopes. The mixture quickly becomes even more complex. Some 300 different isotopes of 36 elements have been identified in fission products.
Short-lived isotopes release their decay energy rapidly, creating intense radiation fields that also decline quickly. Long-lived isotopes release energy over long periods of time, creating radiation that is much less intense but more persistent. Fission products thus initially have a very high level of radiation that declines quickly, but as the intensity of radiation drops, so does the rate of decline. A useful rule-of-thumb is the "rule of sevens". This rule states that for every seven-fold increase in time following a fission detonation (starting at or after 1 hour), the radiation intensity decreases by a factor of 10. Thus after 7 hours, the residual fission radioactivity declines 90% -- to one-tenth its level of 1 hour. After 49 hours, the level drops again by 90%. After approximately 2 weeks it drops a further 90%; and so on for 14 weeks. The rule is accurate to 25% for the first two weeks, and is accurate to a factor of two for the first six months. After 6 months, the rate of decline becomes much more rapid.
These radioactive products are most hazardous when they settle to the ground as "fallout". The rate at which fallout settles depends very strongly on the altitude at which the explosion occurs (and to a lesser extent on the size of the explosion). If the explosion is a true air-burst (the fireball does not touch the ground), when the vaporized radioactive products cool enough to condense and solidify, they will do so to form microscopic particles. These particles are mostly lifted high into the atmosphere by the rising fireball, although significant amounts are deposited in the lower atmosphere by the mixing that occurs due to convective circulation within the fireball. The larger the explosion, the higher and faster the fallout is lofted and the smaller the proportion that is deposited in the lower atmosphere. For explosions with yields of 100kT or less, the fireball does not rise above the troposphere where precipitation occurs. All of this fallout will thus be brought to the ground by weather processes within months at most (usually much faster). In the megaton range, the fireball rises so high that it enters the stratosphere. The stratosphere is dry, and no weather processes exist there to bring fallout down quickly. Small fallout particles will descend over a period of months or years. Such long-delayed fallout has lost most of its hazard by the time it comes down, and will be distributed on a global scale. As yields increase above lOOkT, progressively more and more of the total fallout is injected into the stratosphere.
An explosion closer to the ground (close enough for the fireball to touch) sucks large amounts of dirt into the fireball. The dirt usually does not vaporize, and if it does, there is so much of it that it forms large particles. The radioactive isotopes are deposited on soil particles, which can fall quickly to earth. Fallout is deposited over a time span of minutes to days, creating downwind contamination both nearby and thousands of kilometers away. The most intense radiation is created by nearby fallout, because it is more densely deposited, and because short-lived isotopes haven't decayed yet. Weather.conditions can affect this considerably of course. In particular, rainfall can "rain out" fallout to create very intense localized concentrations. Both external exposure to penetrating radiation, and internal exposure (ingestion of radioactive material) pose serious health risks. Explosions close to the ground that do not touch it can still generate substantial hazards immediately below the burst point by neutron-activation. Neutrons absorbed by the soil can generate considerable radiation for several hours.
The megaton class weapons have been largely retired, being replaced with much smaller yield warheads. The yield of a modern strategic warhead is, with few exceptions, now typically in the range of 200-750 kT. Recent work with sophisticated climate models has shown that this reduction in yield results in a much larger proportion of the fallout being deposited in the lower atmosphere, and a much faster and more intense deposition of fallout than had been assumed in studies made during the sixties and seventies. The reduction in aggregate strategic arsenal yield that occurred when high yield weapons were retired in favor of more numerous lower yield weapons has actually increased the fallout risk.