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Section 5.0 Effects of Nuclear Explosions
Nuclear Weapons Frequently Asked Questions
Version 2.14: 15 May 1997
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5.0 Effects of Nuclear Explosions
Nuclear explosions produce both immediate and delayed destructive effects. Immediate effects (blast, thermal radiation, prompt ionizing radiation) are produced and cause significant destruction within seconds or minutes of a nuclear detonation. The delayed effects (radioactive fallout and other possible environmental effects) inflict damage over an extended period ranging from hours to centuries, and can cause adverse effects in locations very distant from the site of the detonation. These two classes of effects are treated in separate subsections.
The distribution of energy released in the first minute after detonation among the three damage causing effects is:
Low Yield (<100 kt) High Yield (>1 Mt)
Thermal Radiation 35% 45%
Blast Wave 60% 50%
Ionizing Radiation 5% 5%
(80% gamma, 20% neutrons)
The radioactive decay of fallout releases an additional 5-10% over time.
5.1 Overview of Immediate Effects
5.2 Overview of Delayed Effects
5.3 Physics of Nuclear Weapon Effects
5.4 Air Bursts and Surface Bursts
5.5 Electromagnetic Effects
5.6 Mechanisms of Damage and Injury
5.1 Overview of Immediate Effects
The three categories of immediate effects are: blast, thermal radiation (heat), and prompt ionizing or nuclear radiation. Their relative importance varies with the yield of the bomb. At low yields, all three can be significant sources of injury. With an explosive yield of about 2.5 kt, the three effects are roughly equal. All are capable of inflicting fatal injuries at a range of 1 km.
The equations below provide approximate scaling laws for relating the destructive radius of each effect with yield:
r_thermal = Y^0.41 * constant_th
r_blast = Y^0.33 * constant_bl
r_radiation = Y^0.19 * constant_rad
If Y is in multiples (or fractions) of 2.5 kt, then the result is in km (and all the constants equal one). This is based on thermal radiation just sufficient to cause 3rd degree burns (8 calories/cm^2); a 4.6 psi blast overpressure (and optimum burst height); and a 500 rem radiation dose.
The underlying principles behind these scaling laws are easy to explain. The fraction of a bomb's yield emitted as thermal radiation, blast, and ionizing radiation are essentially constant for all yields, but the way the different forms of energy interact with air and targets 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 (this is the familiar inverse square law of electromagnetic radiation). 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 with 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. When it passes through air it simply grows weaker. 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 like thermal radiation. However nuclear radiation is also strongly absorbed by the air it travels through, which causes the intensity to drop off much more rapidly.
These scaling laws show that the effects of thermal radiation grow rapidly with yield (relative to blast), while those of radiation rapidly decline.
In the Hiroshima attack (bomb yield approx. 15 kt) casualties (including fatalities) were seen from all three causes. Burns (including those caused by the ensuing fire storm) were the most prevalent serious injury (two thirds of those who died the first day were burned), and occurred at the greatest range. Blast and burn injuries were both found in 60-70% of all survivors. People close enough to suffer significant radiation illness were well inside the lethal effects radius for blast and flash burns, as a result only 30% of injured survivors showed radiation illness. Many of these people were sheltered from burns and blast and thus escaped their main effects. Even so, most victims with radiation illness also had blast injuries or burns as well.
With 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 damage. A 20 megaton bomb can cause potentially fatal third degree burns at a range of 40 km, where the blast can do little more than break windows and cause superficial cuts.
It should be noted that the atomic bombings of Hiroshima and Nagasaki caused fatality rates were ONE TO TWO ORDERS OF MAGNITUDE higher than the rates in conventional fire raids on other Japanese cities. Eventually on the order of 200,000 fatalities, which is about one-quarter of all Japanese bombing deaths, occurred in these two cities with a combined population of less than 500,000. This is due to the fact that the bombs inflicted damage on people and buildings virtually instantaneously and without warning, and did so with the combined effects of flash, blast, and radiation. Widespread fatal injuries were thus inflicted instantly, and the many more people were incapacitated and thus unable to escape the rapidly developing fires in the suddenly ruined cities. Fire raids in comparison, inflicted few immediate or direct casualties; and a couple of hours elapsed from the raid's beginning to the time when conflagrations became general, during which time the population could flee.
A convenient rule of thumb for estimating the short-term fatalities from all causes due to a 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 outside the contour will die, but the assumption is that these two groups will be roughly equal in size and balance out. This completely ignores any possible fallout effects.
5.2 Overview of Delayed Effects
5.2.1 Radioactive Contamination
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 our 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 thus 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 7*7 hours (49 hours, approx. 2 days), the level drops again by 90%. After 7*2 days (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. The rule of sevens corresponds to an approximate t^-1.2 scaling relationship.
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 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 100 kt or less, the fireball does not rise abve 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 100 kt, 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 that were developed in the US and USSR during the fifties and sixties 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.
5.2.2 Effects on the Atmosphere and Climate
Although not as directly deadly as fallout, other environmental effects can be quite harmful.
126.96.36.199 Harm to the Ozone Layer
The high temperatures of the nuclear fireball, followed by rapid expansion and cooling, cause large amounts of nitrogen oxides to form from the oxygen and nitrogen in the atmosphere (very similar to what happens in combustion engines). Each megaton of yield will produce some 5000 tons of nitrogen oxides. The rising fireball of a high kiloton or megaton range warhead will carry these nitric oxides well up into the stratosphere, where they can reach the ozone layer. A series of large atmospheric explosions could significantly deplete the ozone layer. The high yield tests in the fifties and sixties probably did cause significant depletion, but the ozone measurements made at the time were too limited to pick up the expected changes out of natural variations.
188.8.131.52 Nuclear Winter
The famous TTAPS (Turco, Toon, Ackerman, Pollack, and Sagan) proposal regarding a potential "nuclear winter" is another possible occurrence. This effect is caused by the absorption of sunlight when large amounts of soot are injected into the atmosphere by the widespread burning of cities and petroleum stocks destroyed in a nuclear attack.
Similar events have been observed naturally when large volcanic eruptions have injected large amounts of dust into the atmosphere. The Tambora eruption of 1815 (the largest volcanic eruption in recent history) was followed by "the year without summer" in 1816, the coldest year in the last few centuries.
Soot is far more efficient in absorbing light than volcanic dust, and soot particles are small and hydrophobic and thus tend not to settle or wash out as easily.
Although the initial TTAPS study was met with significant skepticism and criticism, later and more sophisticated work by researchers around the world have confirmed it in all essential details. These studies predict that the amount of soot that would be produced by burning most of the major cities in the US and USSR would severly disrupt climate on a world-wide basis. The major effect would be a rapid and drastic reduction in global temperature, especially over land. All recent studies indicate that if large scale nucelar attack occur against urban or petrochemical targets, average temperature reductions of at least 10 degrees C would occur lasting many months. This level of cooling far exceeds any that has been observed in recorded history, and is comparable to that of a full scale ice age. In areas downwind from attack sites, the cooling can reach 35 degrees C. It is probable that no large scale temperature excursion of this size has occurred in 65 million years.
Smaller attacks would create reduced effects of course. But it has been pointed out that most of the world's food crops are subtropical plants that would have dramatic drops in productivity if an average temperature drop of even one degree were to occur for even a short time during the growing season. Since the world maintains a stored food supply equal to only a few months of consumption, a war during the Northern Hemisphere spring or summer could still cause deadly starvation around the globe from this effect alone even if it only produced a mild "nuclear autumn".
5.3 Physics of Nuclear Weapon Effects
Thermal radiation and blast are inevitable consequences of the near instantaneous release of an immense amount of energy in a very small volume, and are thus characteristic to all nuclear weapons regardless of type or design details. The release of ionizing radiation, both at the instant of explosion and delayed radiation from fallout, is governed by the physics of the nuclear reactions involved and how the weapon is constructed, and is thus very dependent on both weapon type and design.
5.3.1 Fireball Physics
The fireball is the hot ball of gas created when a nuclear explosion heats the bomb itself, and the immediate surrounding environment, to very high temperatures. As this incandescent ball of hot gas expands, it radiates part of its energy away as thermal radiation (including visible and ultraviolet light), part of its energy also goes into creating a shock wave or blast wave in the surrounding environment. The generation of these two destructive effects are thus closely linked by the physics of the fireball. In the discussion below I assume the fireball is forming in open air, unless stated otherwise.
184.108.40.206 The Early Fireball
Immediately after the energy-producing nuclear reactions in the weapon are completed, the energy is concentrated in the nuclear fuels themselves. The energy is stored as (in order of importance): thermal radiation or photons; as kinetic energy of the ionized atoms and the electrons (mostly as electron kinetic energy since free electrons outnumber the atoms); and as excited atoms, which are partially or completely stripped of electrons (partially for heavy elements, completely for light ones).
Thermal (also called blackbody) radiation is emitted by all matter. The intensity and most prevalent wavelength is a function of the temperature, both increasing as temperature increases. The intensity of thermal radiation increases very rapidly - as the fourth power of the temperature. Thus at the 60-100 million degrees C of a nuclear explosion, which is some 10,000 times hotter than the surface of the sun, the brightness (per unit area) is some 10 quadrillion (10^16) times greater! Consequently about 80% of the energy in a nuclear explosion exists as photons. At these temperatures the photons are soft x-rays with energies in the range of 10-200 KeV.
The first energy to escape from the bomb are the gamma rays produced by the nuclear reactions. They have energies in the MeV range, and a significant number of them penetrate through the tampers and bomb casing and escape into the outside world at the speed of light. The gamma rays strike and ionize the surrounding air molecules, causing chemical reactions that form a dense layer of "smog" tens of meters deep around the bomb. This smog is composed primarily of ozone, and nitric and nitrous oxides.
X-rays, particularly the ones at the upper end of the energy range, have substantial penetrating power and can travel significant distances through matter at the speed of light before being absorbed. Atoms become excited when they absorb x-rays, and after a time they re-emit part of the energy as a new lower energy x-ray. By a chain of emissions and absorptions, the x-rays carry energy out of the hot center of the bomb, a process called radiative transport. Since each absorption/re-emission event takes a certain amount of time, and the direction of re-emission is random (as likely back toward the center of the bomb as away from it), the net rate of radiative transport is considerably slower than the speed of light. It is however initially much faster than the expansion of the plasma (ionized gas) making up the fireball or the velocity of the neutrons.
An expanding bubble of very high temperatures is thus formed called the "iso-thermal sphere". It is a sphere were everything has been heated by x-rays to a nearly uniform temperature, initially in the tens of millions of degrees. As soon as the sphere expands beyond the bomb casing it begins radiating light away through the air (unless the bomb is buried or underwater). Due to the still enormous temperatures, it is incredibly brilliant (surface brightness trillions of times more intense than the sun). Most of the energy being radiated is in the x-ray and far ultraviolet range to which air is not transparent. Even at the wavelengths of the near ultraviolet and visible light, the "smog" layer absorbs much of the energy. Then too, at this stage the fireball is only a few meters across. Thus the apparent surface brightness at a distance, and the output power (total brightness) is not nearly as intense as the fourth-power law would indicate.
220.127.116.11 Blast Wave Development and Thermal Radiation Emission
As the fireball expands, it cools and the wavelength of the photons transporting energy drops. Longer wavelength photons do not penetrate as far before being absorbed, so the speed of energy transport also drops. When the isothermal sphere cools to about 300,000 degrees C (and the surface brightness has dropped to being a mere 10 million times brighter than the sun), the rate of radiative growth is about equal to the speed of sound in the fireball plasma. At this point a shock wave forms at the surface of the fireball as the kinetic energy of the fast moving ions starts transferring energy to the surrounding air. This phenomenon, known as "hydrodynamic separation", occurs for a 20 kt explosion about 100 microseconds after the explosion, when the fireball is some 13 meters across. A shock wave internal to the fireball caused by the rapidly expanding bomb debris may overtake and reinforce the fireball surface shock wave a few hundred microseconds later.
The shock wave initially moves at some 30 km/sec, a hundred times the speed of sound in normal air. This compresses and heats the air enormously, up to 30,000 degrees C (some five times the sun's surface temperature). At this temperature the air becomes ionized and incandescent. Ionized gas is opaque to visible radiation, so the glowing shell created by the shock front hides the much hotter isothermal sphere inside. The shock front is many times brighter than the sun, but since it is much dimmer than the isothermal sphere it acts as an optical shutter, causing the fireball's thermal power to drop rapidly.
The fireball is at its most brilliant just as hydrodynamic separation occurs, the great intensity compensating for the small size of the fireball. The rapid drop in temperature causes the thermal power to drop ten-fold, reaching a minimum in about 10 milliseconds for a 20 kt bomb (100 milliseconds for 1 Mt bomb). This "first pulse" contains only about 1 percent of the bomb's total emitted thermal radiation. At this minimum, the fireball of a 20 kt bomb is 180 meters across.
As the shock wave expands and cools to around 3000 degrees, it stops glowing and gradually also becomes transparent. This is called "breakaway" and occurs at about 15 milliseconds for a 20 kt bomb, when the shock front has expanded to 220 meters and is travelling at 4 km/second. The isothermal sphere, at a still very luminous 8000 degrees, now becomes visible and both the apparent surface temperature and brightness of the fireball climb to form the "second pulse". The isothermal sphere has grown considerably in size and now consists almost entirely of light at wavelengths to which air is transparent, so it regains much of the total luminosity of the first peak despite its lower temperature. This second peak occurs at 150 milliseconds for a 20 kt bomb, at 900 milliseconds for a 1 Mt bomb. After breakaway, the shock (blast) wave and the fireball do not interact further.
A firm cutoff for this second pulse is impossible to provide because the emission rate gradually declines over an extended period. Some rough guidelines are that by 300 milliseconds for a 20 kt bomb (1.8 seconds for a 1 Mt) 50% of the total thermal radiation has been emitted, and the rate has dropped to 40% of the second peak. These figures become 75% total emitted and 10% peak rate by 750 milliseconds (20 kt) and 4.5 second (1 Mt). The emission time scales roughly as the 0.45 power of yield (Y^0.45).
Although this pulse never gets as bright as the first, it emits about 99% of the thermal radiation because it is so much longer.
5.3.2 Ionizing Radiation Physics
There are four types of ionizing radiation produced by nuclear explosions that can cause significant injury: neutrons, gamma rays, beta particles, and alpha particles. Gamma rays are energetic (short wavelength) photons (as are X-rays), beta particles are energetic (fast moving) electrons, and alpha particles are energetic helium nuclei. Neutrons are damaging whether they are energetic or not, although the faster they are, the worse their effects.
They all share the same basic mechanism for causing injury though: the creation of chemically reactive compounds called "free radicals" that disrupt the normal chemistry of living cells. These radicals are produced when the energetic radiation strikes a molecule in the living issue, and breaks it into ionized (electrically charged) fragments. Fast neutrons can do this also, but all neutrons can also transmute ordinary atoms into radioactive isotopes, creating even more ionizing radiation in the body.
The different types of radiation present different risks however. Neutrons and gamma rays are very penetrating types of radiation. They are the hardest to stop with shielding. They can travel through hundreds of meters of air and the walls of ordinary houses. They can thus deliver deadly radiation doses even if an organism is not in immediate contact with the source. Beta particles are less penetrating, they can travel through several meters of air, but not walls, and can cause serious injury to organisms that are near to the source. Alpha particles have a range of only a few centimeters in air, and cannot even penetrate skin. Alphas can only cause injury if the emitting isotope is ingested.
The shielding effect of various materials to radiation is usually expressed in half-value thickness, or tenth-value thickness: in other words, the thickness of material required to reduce the intensity of radiation by one-half or one-tenth. Successive layers of shielding each reduce the intensity by the same proportion, so three tenth-value thickness reduce the intensity to one-thousandth (a tenth-value thickness is about 3.3 half-value thicknesses). Some example tenth-value thicknesses for gamma rays are: steel 8.4-11 cm, concrete 28-41 cm, earth 41-61 cm, water 61-100 cm, and wood 100-160 cm. The thickness ranges indicate the varying shielding effect for different gamma ray energies.
Even light clothing provides substantial shielding to beta rays.
18.104.22.168 Sources of Radiation
22.214.171.124.1 Prompt Radiation
Radiation is produced directly by the nuclear reactions that generate the explosion, and by the decay of radioactive products left over (either fission debris, or induced radioactivity from captured neutrons).
The explosion itself emits a very brief burst (about 100 nanoseconds) of gamma rays and neutrons, before the bomb has blown itself apart. The intensity of these emissions depends very heavily on the type of weapon and the specific design. In most designs the initial gamma ray burst is almost entirely absorbed by the bomb (tamper, casing, explosives, etc.) so it contributes little to the radiation hazard. The neutrons, being more penetrating, may escape. Both fission and fusion reactions produce neutrons. Fusion produces many more of them per kiloton of yield, and they are generally more energetic than fission neutrons. Some weapons (neutron bombs) are designed specifically to emit as much energy in the form as neutrons as possible. In heavily tamped fission bombs few if any neutrons escape. It is estimated that no significant neutron exposure occurred from Fat Man, and only 2% of the total radiation dose from Little Boy was due to neutrons.
The neutron burst itself can be a significant source of radiation, depending on weapon design. As the neutrons travel through the air they are slowed by collisions with air atoms, and are eventually captured. Even this process of neutron attenuation generates hazardous radiation. Part of the kinetic energy lost by fast neutrons as they slow is converted into gamma rays, some with very high energies (for the 14.1 MeV fusion neutrons). The duration of production for these neutron scattering gammas is about 10 microseconds. The capture of neutrons by nitrogen-14 also produces gammas, a process completed by 100 milliseconds.
Immediately after the explosion, there are substantial amounts of fission products with very short half-lifes (milliseconds to minutes). The decay of these isotopes generate correspondingly intense gamma radiation that is emitted directly from the fireball. This process is essentially complete within 10 seconds.
The relative importance of these gamma ray sources depends on the size of the explosion. Small explosions (20 kt, say) can generate up to 25% of the gamma dose from the direct gammas and neutron reactions. For large explosions (1 Mt) this contribution is essentially zero. In all cases, the bulk of the gammas are produced by the rapid decay of radioactive debris.
126.96.36.199.2 Delayed Radiation
Radioactive decay is the sole source of beta and alpha particles. They are also emitted during the immediate decay mentioned above of course, but their range is too short to make any prompt radiation contribution. Betas and alphas become important when fallout begins settling out. Gammas remain very important at this stage as well.
Fallout is a complex mixture of different radioactive isotopes, the composition of which continually changes as each isotope decays into other isotopes. Many isotopes make significant contributions to the overall radiation level. Radiation from short lived isotopes dominates initially, and the general trend is for the intensity to continually decline as they disappear. Over time the longer lived isotopes become increasingly important, and a small number of isotopes emerge as particular long-term hazards.
Radioactive isotopes are usually measured in terms of curies. A curie is the quantity of radioactive material that undergoes 3.7x10^10 decays/sec (equal to 1 g of radium-226). More recently the SI unit bequerel has become common in scientific literature, one bequerel is 1 decay/sec . The fission of 57 grams of material produces 3x10^23 atoms of fission products (two for each atom of fissionable material). One minute after the explosion this mass is undergoing decays at a rate of 10^21 disintegrations/sec (3x10^10 curies). It is estimated that if these products were spread over 1 km^2, then at a height of 1 m above the ground one hour after the explosion the radiation intensity would be 7500 rads/hr.
Isotopes of special importance include iodine-131, strontium-90 and 89, and cesium-137. This is due to both their relative abundance in fallout, and to their special biological affinity. Isotopes that are readily absorbed by the body, and concentrated and stored in particular tissues can cause harm out of proportion to their abundance.
Iodine-131 is a beta and gamma emitter with a half-life of 8.07 days (specific activity 124,000 curies/g) Its decay energy is 970 KeV; usually divided between 606 KeV beta, 364 KeV gamma. Due to its short half-life it is most dangerous in the weeks immediately after the explosion, but hazardous amounts can persist for a few months. It constitutes some 2% of fission-produced isotopes - 1.6x10^5 curies/kt. Iodine is readily absorbed by the body and concentrated in one small gland, the thyroid.
Strontium-90 is a beta emitter (546 KeV, no gammas) with a half-life of 28.1 years (specific activity 141 curies/g), Sr-89 is a beta emitter (1.463 MeV, gammas very rarely) with a half-life of 52 days (specific activity 28,200 Ci/g). Each of these isotopes constitutes about 3% of total fission isotopes: 190 curies of Sr-90 and 3.8x10^4 curies of Sr-89 per kiloton. Due to their chemical resemblance to calcium these isotopes are absorbed fairly well, and stored in bones. Sr-89 is an important hazard for a year or two after an explosion, but Sr-90 remains a hazard for centuries. Actually most of the injury from Sr-90 is due to its daughter isotope yttrium-90. Y-90 has a half-life of only 64.2 hours, so it decays as fast as it is formed, and emits 2.27 MeV beta particles.
Cesium-137 is a beta and gamma emitter with a half-life of 30.0 years (specific activity 87 Ci/g). Its decay energy is 1.176 MeV; usually divided by 514 KeV beta, 662 KeV gamma. It comprises some 3-3.5% of total fission products - 200 curies/kt. It is the primary long-term gamma emitter hazard from fallout, and remains a hazard for centuries.
Although not important for acute radiation effects, the isotopes carbon-14 and tritium are also of interest because of possible genetic injury. These are not direct fission products. They are produced by the interaction of fission and fusion neutrons with the atmosphere and, in the case of tritium, as a direct product of fusion reactions. Most of the tritium generated by fusion is consumed in the explosion but significant amounts survive. Tritium is also formed by the capture of fast neutrons by nitrogen atoms in the air: N-14 + n -> T + C-12. Carbon-14 in also formed by neutron-nitrogen reactions: N-14 + n -> C-14 + p. Tritium is a very weak beta emitter (18.6 KeV, no gamma) with a half-life of 12.3 years (9700 Ci/g).
Carbon-14 is also a weak beta emitter (156 KeV, no gamma), with a half-life of 5730 years (4.46 Ci/g). Atmospheric testing during the fifties and early sixties produced about 3.4 g of C-14 per kiloton (15.2 curies) for a total release of 1.75 tonnes (7.75x10^6 curies). For comparison, only about 1.2 tonnes of C-14 naturally exists, divided between the atmosphere (1 tonne) and living matter (0.2 tonne). Another 50-80 tonnes is dissolved in the oceans. Due to carbon exchange between the atmosphere and oceans, the half-life of C-14 residing in the atmosphere is only about 6 years. By now the atmospheric concentration has returned to within 1% or so of normal. High levels of C-14 remain in organic material formed during the sixties (in wood, say, or DNA).
5.4 Air Bursts and Surface Bursts
It might seem logical that the most destructive way of using a nuclear weapon would be to explode it right in the middle of its target - i.e. ground level. But for most uses this is not true. Generally nuclear weapons are designed to explode above the ground - as air bursts (the point directly below the burst point is called the hypocenter). Surface (and sub-surface) bursts can be used for special purposes.
5.4.1 Air Bursts
When an explosion occurs it sends out a shock wave like an expanding soap bubble. If the explosion occurs above the ground the bubble expands and when it reaches the ground it is reflected - i.e. the shock front bounces off the ground to form a second shock wave travelling behind the first. This second shock wave travels faster than the first, or direct, shock wave since it is travelling through air already moving at high speed due to the passage of the direct wave. The reflected shock wave tends to overtake the direct shock wave and when it does they combine to form a single reinforced wave.
This is called the Mach Effect, and produces a skirt around the base of the shock wave bubble where the two shock waves have combined. This skirt sweeps outward as an expanding circle along the ground with an amplified effect compared to the single shock wave produced by a ground burst.
The higher the burst altitude, the weaker the shock wave is when it first reaches the ground. On the other hand, the shock wave will also affect a larger area. Air bursts therefore reduce the peak intensity of the shock wave, but increase the area over which the blast is felt. For a given explosion yield, and a given blast pressure, there is a unique burst altitude at which the area subjected to that pressure is maximized. This is called the optimum burst height for that yield and pressure.
All targets have some level of vulnerability to blast effects. When some threshold of blast pressure is reached the target is completely destroyed. Subjecting the target to pressures higher than that accomplishes nothing. By selecting an appropriate burst height, an air burst can destroy a much larger area for most targets than can surface bursts.
The Mach Effect enhances shock waves with pressures below 50 psi. At or above this pressure the effect provides very little enhancement, so air bursts have little advantage if very high blast pressures are desired.
An additional effect of air bursts is that thermal radiation is also distributed in a more damaging fashion. Since the fireball is formed above the earth, the radiation arrives at a steeper angle and is less likely to be blocked by intervening obstacles and low altitude haze.
5.4.2 Surface Bursts
Surface bursts are useful if local fallout is desired, or if the blast is intended to destroy a buried or very hard structure like a missile silo or a dam. Shock waves are transmitted through the soil more effectively if the bomb is exploded in immediate contact with it, so ground bursts would be used for destroying buried command centers and the like. Some targets, like earth-fill dams, require actual cratering to be destroyed and would be ground burst targets.
5.4.3 Sub-Surface Bursts
Exploding a bomb below ground level can be even more effective for producing craters and destroying buried structures. It can also eliminate thermal radiation and reduce the range of blast effects substantially. The problem, of course is getting the bomb underground. Earth-penetrating bombs have been developed that can punch over one hundred feet into the earth.
5.5 Electromagnetic Effects
The high temperatures and energetic radiation produced by nuclear explosions also produce large amounts of ionized (electrically charged) matter which is present immediately after the explosion. Under the right conditions, intense currents and electromagnetic fields can be produced, generically called EMP (Electromagnetic Pulse), that are felt at long distances. Living organisms are impervious to these effects, but electrical and electronic equipment can be temporarily or permanently disabled by them. Ionized gases can also block short wavelength radio and radar signals (fireball blackout) for extended periods.
The occurrence of EMP is strongly dependent on the altitude of burst. It can be significant for surface or low altitude bursts (below 4,000 m); it is very significant for high altitude bursts (above 30,000 m); but it is not significant for altitudes between these extremes. This is because EMP is generated by the asymmetric absorption of instantaneous gamma rays produced by the explosion. At intermediate altitudes the air absorbs these rays fairly uniformly and does not generate long range electromagnetic disturbances.
The formation EMP begins with the very intense, but very short burst of gamma rays caused by the nuclear reactions in the bomb. About 0.3% of the bomb's energy is in this pulse, but it lasts for only 10 nanoseconds or so. These gamma rays collide with electrons in air molecules, and eject the electrons at high energies through a process called Compton scattering. These energetic electrons in turn knock other electrons loose, and create a cascade effect that produces some 30,000 electrons for every original gamma ray.
In low altitude explosions the electrons, being very light, move much more quickly than the ionized atoms they are removed from and diffuse away from the region where they are formed. This creates a very strong electric field which peaks in intensity at 10 nanoseconds. The gamma rays emitted downward however are absorbed by the ground which prevents charge separation from occurring. This creates a very strong vertical electric current which generates intense electromagnetic emissions over a wide frequency range (up to 100 MHZ) that emanate mostly horizontally. At the same time, the earth acts as a conductor allowing the electrons to flow back toward the burst point where the positive ions are concentrated. This produces a strong magnetic field along the ground. Although only about 3x10^-10 of the total explosion energy is radiated as EMP in a ground burst (10^6 joules for 1 Mt bomb), it is concentrated in a very short pulse. The charge separation persists for only a few tens of microseconds, making the emission power some 100 gigawatts. The field strengths for ground bursts are high only in the immediate vicinity of the explosion. For smaller bombs they aren't very important because they are strong only where the destruction is intense anyway. With increasing yields, they reach farther from the zone of intense destruction. With a 1 Mt bomb, they remain significant out to the 2 psi overpressure zone (5 miles).
High altitude explosions produce EMPs that are dramatically more destructive. About 3x10^-5 of the bomb's total energy goes into EMP in this case, 10^11 joules for a 1 Mt bomb. EMP is formed in high altitude explosions when the downwardly directed gamma rays encounter denser layers of air below. A pancake shaped ionization region is formed below the bomb. The zone can extend all the way to the horizon, to 2500 km for an explosion at an altitude of 500 km. The ionization zone is up to 80 km thick at the center. The Earth's magnetic field causes the electrons in this layer to spiral as they travel, creating a powerful downward directed electromagnetic pulse lasting a few microseconds. A strong vertical electrical field (20-50 KV/m) is also generated between the Earth's surface and the ionized layer, this field lasts for several minutes until the electrons are recaptured by the air. Although the peak EMP field strengths from high altitude bursts are only 1-10% as intense as the peak ground burst fields, they are nearly constant over the entire Earth's surface under the ionized region.
The effects of these field on electronics is difficult to predict, but can be profound. Enormous induced electric currents are generated in wires, antennas, and metal objects (like missiles, airplanes, and building frames). Commercial electrical grids are immense EMP antennas and would be subjected to voltage surges far exceeding those created by lightning, and over vastly greater areas. Modern VLSI chips are extremely sensitive to voltage surges, and would be burned out by even small leakage currents. Military equipment is generally designed to be resistant to EMP, but realistic tests are very difficult to perform and EMP protection rests on attention to detail. Minor changes in design, incorrect maintenance procedures, poorly fitting parts, loose debris, moisture, and ordinary dirt can all cause elaborate EMP protections to be totally circumvented. It can be expected that a single high yield, high altitude explosion over an industrialized area would cause massive disruption for an indeterminable period, and would cause huge economic damages (all those damaged chips add up).
A separate effect is the ability of the ionized fireball to block radio and radar signals. Like EMP, this effect becomes important with high altitude bursts. Fireball blackout can cause radar to be blocked for tens of seconds to minutes over an area tens of kilometers across. High frequency radio can be disrupted over hundreds to thousands of kilometers for minutes to hours depending on exact conditions.
5.6 Mechanisms of Damage and Injury
The different mechanisms are discussed individually, but it should be no surprise that in combination they often accentuate the harm caused by each other. I will discuss such combined effects wherever appropriate.
5.6.1 Thermal Damage and Incendiary Effects
Thermal damage from nuclear explosions arises from the intense thermal (heat) radiation produced by the fireball. The thermal radiation (visible and infrared light) falls on exposed surfaces and is wholly or partly absorbed. The radiation lasts from about a tenth of a second, to several seconds depending on bomb yield (it is longer for larger bombs). During that time its intensity can exceed 1000 watts/cm^2 (the maximum intensity of direct sunlight is 0.14 watts/cm^2). For a rough comparison, the effect produced is similar to direct exposure to the flame of an acetylene torch.
The heat is absorbed by the opaque surface layer of the material on which it falls, which is usually a fraction of a millimeter thick. Naturally dark materials absorb more heat than light colored or reflective ones. The heat is absorbed much faster than it can be carried down into the material through conduction, or removed by reradiation or convection, so very high temperatures are produced in this layer almost instantly. Surface temperatures can exceed 1000 degrees C close to the fireball. Such temperatures can cause dramatic changes to the material affected, but they do not penetrate in very far.
More total energy is required to inflict a given level of damage for a larger bomb than a smaller one since the heat is emitted over a longer period of time, but this is more than compensated for by the increased thermal output. The thermal damage for a larger bomb also penetrates further due to the longer exposure.
Thermal radiation damage depends very strongly on weather conditions. Cloud cover, smoke, or other obscuring material in the air can considerably reduce effective damage ranges over clear air conditions.
For all practical purposes, the emission of thermal radiation by a bomb is complete by the time the shock wave arrives. Regardless of yield, this generalization is only violated in the area of total destruction around a nuclear explosion where 100% mortality would result from any one of the three damage effects.
Incendiary effects refer to anything that contributes to the occurrence of fires after the explosion, which is a combination of the effects of thermal radiation and blast.
188.8.131.52 Thermal Injury
The result of very intense heating of skin is to cause burn injuries. The burns caused by the sudden intense thermal radiation from the fireball are called "flash burns". The more thermal radiation absorbed, the more serious the burn. The table below indicates the amount of thermal radiation required to cause different levels of injury, and the maximum ranges at which they occur, for different yields of bombs. The unit of heat used are gram-calories, equal to 4.2 joules (4.2 watts for 1 sec). Skin color significantly affects susceptibility, light skin being less prone to burns. The table assumes medium skin color.
SEVERITY 20 Kilotons 1 Megaton 20 Megatons