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Depending on the design of the weapon and the environment in which it is detonated the energy distributed to these categories can be increased or decreased. The blast effect is created by the coupling of immense amounts of energy, spanning the electromagnetic spectrum, with the surroundings. Locations such as submarine, surface, air burst, or exo-atmospheric determine how much energy is produced as blast and how much as radiation. In general, denser media around the bomb, like water, absorb more energy, and create more powerful shockwaves while at the same time limiting the area of its effect.
When an air burst occurs, lethal blast and thermal effects proportionally scale much more rapidly than lethal radiation effects, as higher and higher yield nuclear weapons are used.
The physical-damage mechanisms of a nuclear weapon (blast and thermal radiation) are identical to those of conventional explosives, but the energy produced by a nuclear explosive is millions of times more powerful per gram and the temperatures reached are briefly in the tens of millions of degrees.
Energy from a nuclear explosive is initially released in several forms of penetrating radiation. When there is a surrounding material such as air, rock, or water, this radiation interacts with and rapidly heats it to an equilibrium temperature (i.e. so that the matter is at the same temperature as the atomic bomb's matter). This causes vaporization of surrounding material resulting in its rapid expansion. Kinetic energy created by this expansion contributes to the formation of a shockwave. When a nuclear detonation occurs in air near sea level, much of the released energy interacts with the atmosphere and creates a shockwave which expands spherically from the hypocenter. Intense thermal radiation at the hypocenter forms a nuclear fireball and if the burst is low enough, it is often associated with a mushroom cloud. In a burst at high altitude, where the air density is low, more energy is released as ionizing gamma radiation and x-rays than an atmosphere-displacing shockwave.
In 1942 there was some initial speculation among the scientists developing the first nuclear weapons that there might be a possibility of igniting the Earth's atmosphere with a large enough nuclear explosion. This would concern a nuclear reaction of two nitrogen atoms forming a carbon and an oxygen atom, with release of energy. This energy would heat up the remaining nitrogen enough to keep the reaction going until all nitrogen atoms were consumed. Hans Bethe was assigned the task of studying whether there was a possibility in the very early days, and concluded there was no possibility due to inverse Compton effect cooling of the fireball. Richard Hamming, a mathematician, was asked to make a similar calculation just before Trinity, with the same result. Nevertheless, the notion has persisted as a rumor for many years, and was the source of black humor at the Trinity test.
The high temperatures and radiation cause gas to move outward radially in a thin, dense shell called "the hydrodynamic front". The front acts like a piston that pushes against and compresses the surrounding medium to make a spherically expanding shock wave. At first, this shock wave is inside the surface of the developing fireball, which is created in a volume of air heated by the explosion's "soft" X-rays. Within a fraction of a second the dense shock front obscures the fireball, and continues to move past it, now expanding outwards, free from the fireball, causing the characteristic double pulse of light seen from a nuclear detonation, with the dip causing the double pulse due to the shock wave–fireball interaction. It is this unique feature of nuclear explosions that is exploited when verifying that an atmospheric nuclear explosion has occurred and not simply a large conventional explosion, with radiometer instruments known as Bhangmeters capable of determining the nature of explosions.
For air bursts at or near sea-level, 50–60% of the explosion's energy goes into the blast wave, depending on the size and the yield of the bomb. As a general rule, the blast fraction is higher for low yield weapons. Furthermore, it decreases at high altitudes because there is less air mass to absorb radiation energy and convert it into blast. This effect is most important for altitudes above 30 km, corresponding to <1 per cent of sea-level air density.
Much of the destruction caused by a nuclear explosion is due to blast effects. Most buildings, except reinforced or blast-resistant structures, will suffer moderate damage when subjected to overpressures of only 35.5 kilopascals (kPa) (5.15 pounds-force per square inch or 0.35 atm). Data obtained from the Japanese surveys found that 8 psi (55 kPa) was sufficient to destroy all wooden and brick residential structures. This can reasonably be defined as the pressure capable of producing severe damage.
The blast wind at sea level may exceed one thousand km/h, or ~300 m/s, approaching the speed of sound in air. The range for blast effects increases with the explosive yield of the weapon and also depends on the burst altitude. Contrary to what one might expect from geometry the blast range is not maximal for surface or low altitude blasts but increases with altitude up to an "optimum burst altitude" and then decreases rapidly for higher altitudes. This is due to the nonlinear behaviour of shock waves. When the blast wave from an air burst reaches the ground it is reflected. Below a certain reflection angle the reflected wave and the direct wave merge and form a reinforced horizontal wave, this is known as the 'Mach stem' (named after Ernst Mach) and is a form of constructive interference.   This constructive interference is the phenomenon responsible for the bumps or 'knees' in the above overpressure range graph.
For each goal overpressure there is a certain optimum burst height at which the blast range is maximized over ground targets. In a typical air burst, where the blast range is maximized to produce the greatest range of severe damage, i.e. the greatest range that ~10 psi (69 kPa) of pressure is extended over, is a GR/ground range of 0.4 km for 1 kiloton (kt) of TNT yield; 1.9 km for 100 kt; and 8.6 km for 10 megatons (Mt) of TNT. The optimum height of burst to maximize this desired severe ground range destruction for a 1 kt bomb is 0.22 km; for 100 kt, 1 km; and for 10 Mt, 4.7 km.
Two distinct, simultaneous phenomena are associated with the blast wave in air:
Most of the material damage caused by a nuclear air burst is caused by a combination of the high static overpressures and the blast winds. The long compression of the blast wave weakens structures, which are then torn apart by the blast winds. The compression, vacuum and drag phases together may last several seconds or longer, and exert forces many times greater than the strongest hurricane.
Acting on the human body, the shock waves cause pressure waves through the tissues. These waves mostly damage junctions between tissues of different densities (bone and muscle) or the interface between tissue and air. Lungs and the abdominal cavity, which contain air, are particularly injured. The damage causes severe hemorrhaging or air embolisms, either of which can be rapidly fatal. The overpressure estimated to damage lungs is about 70 kPa. Some eardrums would probably rupture around 22 kPa (0.2 atm) and half would rupture between 90 and 130 kPa (0.9 to 1.2 atm).
Blast winds: The drag energies of the blast winds are proportional to the cubes of their velocities multiplied by the durations. These winds may reach several hundred kilometers per hour.
Nuclear weapons emit large amounts of thermal radiation as visible, infrared, and ultraviolet light, to which the atmosphere is largely transparent. This is known as "Flash". The chief hazards are burns and eye injuries. On clear days, these injuries can occur well beyond blast ranges, depending on weapon yield. Fires may also be started by the initial thermal radiation, but the following high winds due to the blast wave may put out almost all such fires, unless the yield is very high, where the range of thermal effects vastly out ranges blast effects, like that observed in the multi-megaton range. This is because the intensity of the blast effects drops off with the third power of distance from the explosion, while the intensity of radiation effects drops off with the second power of distance. This results in the range of thermal effects increasing markedly more than blast range as higher and higher device yields are detonated. Thermal radiation accounts for between 35-45% of the energy released in the explosion, depending on the yield of the device. In urban areas, the extinguishing of fires ignited by thermal radiation may matter little, as in a surprise attack fires may also be started by blast-effect-induced electrical shorts, gas pilot lights, overturned stoves, and other ignition sources, as was the case in the breakfast-time bombing of Hiroshima. Whether or not these secondary fires will in turn themselves be snuffed out as modern noncombustible brick and concrete buildings collapse in on themselves from the same blast wave is uncertain, not least of which, because of the masking effect of modern city landscapes on thermal and blast transmission are continually examined. When combustible frame buildings were blown down in Hiroshima and Nagasaki, they did not burn as rapidly as they would have done had they remained standing. The noncombustible debris produced by the blast frequently covered and prevented the burning of combustible material. Fire experts suggest that unlike Hiroshima, due to the nature of modern U.S. city design and construction, a firestorm in modern times is unlikely after a nuclear detonation. This does not exclude fires from being started, but means that these fires will not form into a firestorm, due largely to the differences between modern building materials and that used in World War II era Hiroshima.
There are two types of eye injuries from the thermal radiation of a weapon:
Flash blindness is caused by the initial brilliant flash of light produced by the nuclear detonation. More light energy is received on the retina than can be tolerated, but less than is required for irreversible injury. The retina is particularly susceptible to visible and short wavelength infrared light, since this part of the electromagnetic spectrum is focused by the lens on the retina. The result is bleaching of the visual pigments and temporary blindness for up to 40 minutes.
A retinal burn resulting in permanent damage from scarring is also caused by the concentration of direct thermal energy on the retina by the lens. It will occur only when the fireball is actually in the individual's field of vision and would be a relatively uncommon injury. Retinal burns may be sustained at considerable distances from the explosion. The height of burst, and apparent size of the fireball, a function of yield and range will determine the degree and extent of retinal scarring. A scar in the central visual field would be more debilitating. Generally, a limited visual field defect, which will be barely noticeable, is all that is likely to occur.
When thermal radiation strikes an object, part will be reflected, part transmitted, and the rest absorbed. The fraction that is absorbed depends on the nature and color of the material. A thin material may transmit a lot. A light colored object may reflect much of the incident radiation and thus escape damage, like anti-flash white paint. The absorbed thermal radiation raises the temperature of the surface and results in scorching, charring, and burning of wood, paper, fabrics, etc. If the material is a poor thermal conductor, the heat is confined to the surface of the material.
Actual ignition of materials depends on how long the thermal pulse lasts and the thickness and moisture content of the target. Near ground zero where the energy flux exceeds 125 J/cm2, what can burn, will. Farther away, only the most easily ignited materials will flame. Incendiary effects are compounded by secondary fires started by the blast wave effects such as from upset stoves and furnaces.
In Hiroshima on August 6, 1945, a tremendous firestorm developed within 20 minutes after detonation and destroyed many more buildings and homes, built out of predominantly 'flimsy' wooden materials. A firestorm has gale force winds blowing in towards the center of the fire from all points of the compass. It is not peculiar to nuclear explosions, having been observed frequently in large forest fires and following incendiary raids during World War II. Despite fires destroying a large area of the city of Nagasaki, no true firestorm occurred in the city, even though a higher yielding weapon was used. Many factors explain this seeming contradiction, including a different time of bombing than Hiroshima, terrain, and crucially, a lower fuel loading/fuel density in the city than that of Hiroshima.
Nagasaki probably did not furnish sufficient fuel for the development of a fire storm as compared to the many buildings on the flat terrain at Hiroshima.
As thermal radiation travels, more or less, in a straight line from the fireball (unless scattered) any opaque object will produce a protective shadow that provides protection from the flash burn. Furthermore, depending on the properties of the underlying surface material, the exposed area outside the protective shadow will be either burnt to a darker color, such as charring wood, or a brighter color, such as asphalt. If such a weather phenomenon as fog or haze is present at the point of the nuclear explosion, it scatters the flash, now reaching burn sensitive substances from all directions, under these conditions, opaque objects are therefore less effective than they would otherwise be without scattering, as they demonstrate maximum shadowing effect in an environment of perfect visibility and therefore zero scattering. Similar to a foggy or overcast day, although there are few, if any, shadows produced by the sun on such a day, the solar energy that reaches the ground from the sun's infrared rays is nevertheless considerably diminished, due to it being absorbed by the water of the clouds and the energy also being scattered back into space. Analogously, so too is the intensity at range of burning flash energy attenuated, in units of J/cm2, along the slant/horizontal range of a nuclear explosion, during fog or haze conditions. So despite any object that casts a shadow being rendered ineffective as a shield from the flash by fog or haze, due to scattering, the fog fills the same protective role, but generally only at the ranges that survival in the open is just a matter of being protected from the explosion's flash energy.
The thermal pulse also is responsible for warming the atmospheric nitrogen close to the bomb, and causing the creation of atmospheric NOx smog components. This, as part of the mushroom cloud, is shot into the stratosphere where it is responsible for dissociating ozone there, in exactly the same way as combustion NOx compounds do. The amount created depends on the yield of the explosion and the blast's environment. Studies done on the total effect of nuclear blasts on the ozone layer have been at least tentatively exonerating after initial discouraging findings.
Gamma rays from a nuclear explosion produce high energy electrons through Compton scattering. For high altitude nuclear explosions, these electrons are captured in the Earth's magnetic field at altitudes between twenty and forty kilometers where they interact with the Earth's magnetic field to produce a coherent nuclear electromagnetic pulse (NEMP) which lasts about one millisecond. Secondary effects may last for more than a second.
The pulse is powerful enough to cause moderately long metal objects (such as cables) to act as antennas and generate high voltages due to interactions with the electromagnetic pulse. These voltages can destroy unshielded electronics. There are no known biological effects of EMP. The ionized air also disrupts radio traffic that would normally bounce off the ionosphere.
Electronics can be shielded by wrapping them completely in conductive material such as metal foil; the effectiveness of the shielding may be less than perfect. Proper shielding is a complex subject due to the large number of variables involved. Semiconductors, especially integrated circuits, are extremely susceptible to the effects of EMP due to the close proximity of the PN junctions, but this is not the case with thermionic tubes (or valves) which are relatively immune to EMP. A Faraday cage does not offer protection from the effects of EMP unless the mesh is designed to have holes no bigger than the smallest wavelength emitted from a nuclear explosion.
Large nuclear weapons detonated at high-altitudes also cause geomagnetically induced current in very long electrical conductors. The mechanism by which these geomagnetically induced currents are generated is entirely different from the gamma ray induced pulse produced by Compton electrons.
The heat of the explosion causes air in the vicinity to become ionized, creating the fireball. The free electrons in the fireball affect radio waves, especially at lower frequencies. This causes a large area of the sky to become opaque to radar, especially those operating in the VHF and UHF frequencies, which is common for long-range early warning radars. The effect is less for higher frequencies in the microwave region, as well as lasting a shorter time - the effect falls off both in strength and the effected frequencies as the fireball cools and the electrons begin to re-form onto free nuclei.
A second blackout effect is caused by the emission of beta particles from the fission products. These can travel long distances, following the Earth's magnetic field lines. When they reach the upper atmosphere they cause ionization similar to the fireball, but over a wider area. Calculations demonstrate that one megaton of fission, typical of a two megaton H-bomb, will create enough beta radiation to black out an area 400 kilometres (250 mi) across for five minutes. Careful selection of the burst altitudes and locations can produce an extremely effective radar-blanking effect.
The physical effects giving rise to blackout are those that also cause EMP, which itself can cause power blackouts. The two effects are otherwise unrelated, and the similar naming can be confusing.
About 5% of the energy released in a nuclear air burst is in the form of ionizing radiation: neutrons, gamma rays, alpha particles and electrons moving at speeds up to the speed of light. Gamma rays are high energy electromagnetic radiation; the others are particles that move slower than light. The neutrons result almost exclusively from the fission and fusion reactions, while the initial gamma radiation includes that arising from these reactions as well as that resulting from the decay of short-lived fission products.
The intensity of initial nuclear radiation decreases rapidly with distance from the point of burst because the radiation spreads over a larger area as it travels away from the explosion (the inverse-square law). It is also reduced by atmospheric absorption and scattering.
The character of the radiation received at a given location also varies with distance from the explosion. Near the point of the explosion, the neutron intensity is greater than the gamma intensity, but with increasing distance the neutron-gamma ratio decreases. Ultimately, the neutron component of initial radiation becomes negligible in comparison with the gamma component. The range for significant levels of initial radiation does not increase markedly with weapon yield and, as a result, the initial radiation becomes less of a hazard with increasing yield. With larger weapons, above 50 kt (200 TJ), blast and thermal effects are so much greater in importance that prompt radiation effects can be ignored.
The neutron radiation serves to transmute the surrounding matter, often rendering it radioactive. When added to the dust of radioactive material released by the bomb itself, a large amount of radioactive material is released into the environment. This form of radioactive contamination is known as nuclear fallout and poses the primary risk of exposure to ionizing radiation for a large nuclear weapon.
Details of nuclear weapon design also affect neutron emission: the gun-type assembly Hiroshima bomb leaked far more neutrons than the implosion type 21 kt Nagasaki bomb because the light hydrogen nuclei (protons) predominating in the exploded TNT molecules (surrounding the core of the Nagasaki bomb) slowed down neutrons very efficiently while the heavier iron atoms in the steel nose forging of the Hiroshima bomb scattered neutrons without absorbing much neutron energy.
It was found in early experimentation that normally most of the neutrons released in the cascading chain reaction of the fission bomb are absorbed by the bomb case. Building a bomb case of materials which transmitted rather than absorbed the neutrons could make the bomb more intensely lethal to humans from prompt neutron radiation. This is one of the features used in the development of the neutron bomb.
The pressure wave from an underground explosion will propagate through the ground and cause a minor earthquake. Theory suggests that a nuclear explosion could trigger fault rupture and cause a major quake at distances within a few tens of kilometers from the shot point.
The following table summarizes the most important effects of single nuclear explosions under ideal, clear skies, weather conditions. Tables like these are calculated from nuclear weapons effects scaling laws. Advanced computer modelling of real world conditions and how they impact on the damage to modern urban areas has found that most scaling laws are too simplistic and tend to overestimate nuclear explosion effects. As it is only simplistic and unclassified scaling laws that are commonly encountered, that do not take important things like varying land topography into account to ease calculation time and equation length. The scaling laws that were used to produce the table below, assume among other things, a perfectly level target area, no attenuating effects from urban terrain masking, e.g. skyscraper shadowing, and no enhancement effects from reflections and tunneling by city streets. As a point of comparison in the chart below, the most likely nuclear weapons to be used against countervalue city targets in a global nuclear war are in the sub-megaton range. Weapons of yields from 100 to 475 kilotons have become the most numerous in the US and Russian nuclear arsenals; for example the warheads equipping the Russian Bulava submarine launched ballistic missile (SLBM) have a yield of 150 kilotons. US examples are the W76 and W88 warheads, with the lower yield W76 being over twice as numerous as the W88 in the US nuclear arsenal.
|Effects||Explosive yield / height of burst|
|1 kt / 200 m||20 kt / 540 m||1 Mt / 2.0 km||20 Mt / 5.4 km|
|Blast—effective ground range GR / km|
|Urban areas completely levelled (20 psi or 140 kPa)||0.2||0.6||2.4||6.4|
|Destruction of most civilian buildings (5 psi or 34 kPa)||0.6||1.7||6.2||17|
|Moderate damage to civilian buildings (1 psi or 6.9 kPa)||1.7||4.7||17||47|
|Railway cars thrown from tracks and crushed
(62 kPa; values for other than 20 kt are extrapolated using the cube-root scaling)
|Thermal radiation—effective ground range GR / km|
|Third degree burns||0.6||2.5||12||38|
|Second degree burns||0.8||3.2||15||44|
|First degree burns||1.1||4.2||19||53|
|Effects of instant nuclear radiation—effective slant range1 SR / km|
|Lethal2 total dose (neutrons and gamma rays)||0.8||1.4||2.3||4.7|
|Total dose for acute radiation syndrome2||1.2||1.8||2.9||5.4|
1 For the direct radiation effects the slant range instead of the ground range is shown here, because some effects are not given even at ground zero for some burst heights. If the effect occurs at ground zero the ground range can be derived from slant range and burst altitude (Pythagorean theorem).
Further complicating matters, under global nuclear war scenarios, with conditions similar to that during the Cold War, major strategically important cities, like Moscow, and Washington are likely to be hit not once, but numerous times from sub megaton multiple independently targetable re-entry vehicles, in a cluster bomb or "cookie cutter" configuration. It has been reported that during the height of the Cold War in the 1970s Moscow was targeted by up to 60 warheads. The reasons that the cluster bomb concept is preferable in the targeting of cities is twofold, the first is down to the fact that large singular warheads are much easier to neutralize as both tracking and successful interception by anti-ballistic missile systems than it is when several smaller incoming warheads are approaching. This strength in numbers advantage to lower yield warheads is further compounded by such warheads tending to move at higher incoming speeds, due to their smaller, more slender physics package size, assuming both nuclear weapon designs are the same (a design exception being the advanced W88). The second reason for this cluster bomb, or ‘layering’ (using repeated hits by accurate low yield weapons), is that this tactic along with limiting the risk of failure, also reduces individual bomb yields, and therefore reduces the possibility of any serious collateral damage to non-targeted nearby civilian areas, including that of neighboring countries. This concept was pioneered by Philip J. Dolan and others.
Gamma rays from the nuclear processes preceding the true explosion may be partially responsible for the following fireball, as they may superheat nearby air and/or other material. The vast majority of the energy that goes on to form the fireball is in the soft X-ray region of the electromagnetic spectrum, with these X-rays being produced by the inelastic collisions of the high speed fission and fusion products. It is these reaction products and not the gamma rays which contain most of the energy of the nuclear reactions in the form of kinetic energy. This kinetic energy of the fission and fusion fragments is converted into internal and then radiation energy by approximately following the process of blackbody radiation emitting in the soft X-ray region. As a result of numerous inelastic collisions, part of the kinetic energy of the fission fragments is converted into internal and radiation energy. Some of the electrons are removed entirely from the atoms, thus causing ionization, others are raised to higher energy (or excited) states while still remaining attached to the nuclei. Within an extremely short time, perhaps a hundredth of a microsecond or so, the weapon residues consist essentially of completely and partially stripped (ionized) atoms, many of the latter being in excited states, together with the corresponding free electrons. The system then immediately emits electromagnetic (thermal) radiation, the nature of which is determined by the temperature. Since this is of the order of 107 degrees, most of the energy emitted within a microsecond or so is in the soft X-ray region. To understand this one must remember that temperature depends on the average internal energy/heat of the particles in a certain volume, and internal energy or heat is due to kinetic energy.
For an explosion in the atmosphere, the fireball quickly expands to maximum size, and then begins to cool as it rises like a balloon through buoyancy in the surrounding air. As it does so it takes on the flow pattern of a vortex ring with incandescent material in the vortex core as seen in certain photographs. This effect is known as a mushroom cloud.
At the explosion of nuclear bombs lightning discharges sometimes occur.
Smoke trails are often seen in photographs of nuclear explosions. These are not from the explosion; they are left by sounding rockets launched just prior to detonation. These trails allow observation of the blast's normally invisible shock wave in the moments following the explosion.
The heat and airborne debris created by a nuclear explosion can cause rain; debris is thought to do this by acting as cloud condensation nuclei. During the city firestorm which followed the Hiroshima explosion, drops of water were recorded to have been about the size of marbles. This was termed black rain, and has served as the source of a book and film by the same name. Black rain is not unusual following large fires, and is commonly produced by pyrocumulus clouds during large forest fires. The rain directly over Hiroshima on that day is said to have begun around 9 a.m. with it covering a wide area from the hypocenter to the north-west, raining heavily for one hour or more in some areas. The rain directly over the city may have carried neutron activated building material combustion products, but it did not carry any appreciable nuclear weapon debris or fallout, although this is generally to the contrary to what other less technical sources state. The "oily" black soot particles, are a characteristic of incomplete combustion in the city firestorm.
The element einsteinium was discovered when analyzing nuclear fallout.
A side-effect of the Pascal-B nuclear test during Operation Plumbbob may have resulted in the first man-made object launched into space. The so-called "thunder well" effect from the underground explosion may have launched a metal cover plate into space at six times Earth's escape velocity, although the evidence remains subject to debate.
This is highly dependent on factors such as if one is indoors or out, the size of the explosion, the proximity to the explosion, and to a lesser degree the direction of the wind carrying fallout. Death is highly likely and radiation poisoning is almost certain if one is caught in the open with no terrain or building masking effects within a radius of 0–3 km from a 1 megaton airburst, and the 50% chance of death from the blast extends out to ~8 km from the same 1 megaton atmospheric explosion.
To highlight the variability in the real world, and the effect that being indoors can make, despite the lethal radiation and blast zone extending well past her position at Hiroshima, Akiko Takakura survived the effects of a 16 kt atomic bomb at a distance of 300 meters from the hypocenter, with only minor injuries, due mainly to her position in the lobby of the Bank of Japan, a reinforced concrete building, at the time. In contrast, the unknown person sitting outside, fully exposed, on the steps of the Sumitomo Bank, next door to the Bank of Japan, received lethal third degree burns and was then likely killed by the blast, in that order, within two seconds.
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