Nuclear weapons are weapons of enormous destructive potential, deriving their energy from nuclear fission and nuclear fusion reactions. Atomic weapons, atomic bombs, thermonuclear weapons and hydrogen bombs are all alternative names for nuclear weapons. These weapons were initially developed by the United States during the Second World War in the Manhattan Project. A considerable amount of international negotiating has focused on the threat of nuclear warfare and the proliferation of nuclear weapons to new nations or groups.

Structure and Design

Fission bomb (or Atomic bomb)

The simplest nuclear weapons are pure fission bombs, these were the first types of nuclear weapons built during the Manhattan Project and they are a building block for all advanced nuclear weapons designs.

In a fission reaction, a heavy atom in a barely stable nuclear arrangement splits into two smaller atoms when it is perturbed by a neutron, as it splits it also releases energy and additional neutrons. These neutrons split other fissile atoms, releasing more energy and neutrons. If enough atoms are split and neutrons released the reaction sustains itself as a chain reaction and will grow exponentially larger with time. A mass of fissile material is called critical when it is capable of a sustained chain reaction, which depends upon the size, shape and purity of the material as well as what surrounds the material. A numerical measure of whether a mass is critical or not is available as the neutron multiplication factor, k, where

k = f - l

Where f is the average number of neutrons released per fission event and l is the average number of neutrons lost by either leaving the system or being captured in a non-fission event.When k=1 the mass is critical, k<1 is subcritical and k>1 is supercritical. A fission bomb works by rapidly changing a subcritical mass of fissile material into a supercritical assembly, causing a chain reaction which rapidly releases large amounts of energy. In practice the mass is not made slightly critical, but goes from slightly subcritical (k=.9) to highly supercritical (k= 2 or 3), so that each neutron creates several new neutrons and the chain reaction advances more quickly. The main challenge in producing an efficient explosion using nuclear fission is to keep the bomb together long enough for a substantial fraction of the available nuclear energy to be released.

Until detonation is desired, the weapon must consist of a number of separate pieces each of which is below the critical size either because they are too small or unfavorably shaped. To produce detonation, the fissile material must be brought together rapidly. In the course of this assembly process the chain reaction is likely to start causing the material to heat up and expand, preventing the material from reaching its most compact (and most efficient) form. It may turn out that the explosion is so inefficient as to be practically useless. The majority of the technical difficulties of designing and manufacturing a fission weapon are based on the need to both reduce the time of assembly of a supercritcal mass to a minimum and reduce the number of stray (pre-detonation) neutrons to a minimum.

The isotopes desirable for a nuclear weapon are those which have a high probability of fission reaction, yield a high number of excess neutrons, have a low probability of absorbing nuetrons without a fission reaction, and do not release a large number of spontaneous neutrons. The primary isotopes which fit these criteria are U-235, Pu-239 and U-233, although U-233 has never been used to make a weapon.

Enriched Materials

Naturally occuring uranium consists mostly of U-238, with a small part U-235. The U-238 isotope has a high probability of absorbing a neutron without a fission, and also a higher rate of spontaneous fission. For weapons uranium is enriched through isotope separation. Uranium which is more than 80% U-235 is called highly enriched uranium (HEU), and weapons grade uranium is at least 93.5% U-235. U-235 has a spontaneous fission rate of 0.16 fissions/sec-kg. which is low enough to make super critical assembly relatively easy. The critical mass for an unreflected sphere of U-235 is about 50 kg, which is a sphere with a radius of 8 cm. This size can be reduced to about 15 kg with the use of a neutron reflector surrounding the sphere.

Plutonium does not occur in nature and is manufactured by exposing U-238 to a neutron source (i.e. a nuclear reactor). When U-238 absorbs a neutron the resulting U-239 isotope then beta decays twice into Pu-239. The plutonium can then be chemically separated from the uranium and be isolated for weapons use. Pu-239 has a higher probability for fission than U-235, and a larger number of neutrons produced per fission event, resulting in a smaller critical mass. Pure Pu-239 also has a reasonably low rate of neutron emission due to spontaneous fission (10 fission/sec-kg), making it feasible to assemble a super critical mass before predetonation. The Pu-239 will be invariably contaminated by Pu-240, however, due to the fact that the freshly made Pu-239 captures a neutron to make Pu-240. Pu-240 has a high rate of spontaneous fission events (415,000 fission/sec-kg), making it extrememly dificult to assemble a super critical mass before the neutrons emitted from spontaneous fission start a premature chain reaction and cause the weapon to fizzle. Weapons grade plutonium must contain no more than 7% Pu-240 - and is obtained by only exposing U-238 samples to neutron sources for short periods of time to reduce the amount of Pu-240 made. The critical mass for an unreflected sphere of plutonium is 16 kg, but through the use of a neutron reflecting tamper the pit of plutonium in a fission bomb is reduced to 10 kg, roughly the size of a large marble.

Combination Methods

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The simplest technical mechanism for assembling a supercritical mass is to shoot one piece of fissile material as a projectile against a second part as a target, usually called the gun method. This is how the Little Boy weapon which was detonated over Hiroshima worked. This method of combination can only be used for U-235 because of the relatively long amount of time it takes to combine the materials, making predetonation likely for Pu-239 which has a higher spontaneous neutron release due to Pu-240 contamination.

The more difficult, but superior, method of combination is referred to as the implosion method and uses conventional explosives surrounding the material to rapidly compress the mass to a supercritical state. For Pu-239 assemblies a contamination of only 1% Pu-240 produces so many neutrons that implosion systems are required to produce efficient bombs. This is the reason that the more technically difficult implosion method was used on the plutonium Fat Man weapon which was detonated over Nagasaki.

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Weapons assembled with this method also tend to be more efficient than the weapons employing the gun method of combination. The reason that the implosion method is more efficient is because it not only combines the masses, but also increases the density of the mass. The neutron multiplication factor, k, of a fissionable assembly is proportional to the density squared, meaning that k goes up by a factor of four if the density is doubled. Most modern weapons use a hollow plutonium core with an implosion mechanism for detonation.

This precision compression of the pit creates a need for very precise design and machining of the pit and explosive lenses. The milling machines used are so precise that they could cut the polished surfaces of eyeglass lenses. Machining plutonium is difficult not only because of its toxicity but also because plutonium has many different metallic phases and changing phases distorts the metal.

Tamper / Neutron Reflector

In a uranium graphite chain reacting pile the critical size may be considerably reduced by surrounding the pile with a layer of graphite, since such an envelope reflects many neutrons back into the pile. A similar envelope can be used to reduce the critical size of a weapon, but here the envelope has an additional role: its very inertia delays the expansion of the reacting material. For this reason such an envelope is often called a tamper. As has already been remarked, the weapon tends to fly to bits as the reaction proceeds and this tends to stop the reaction, so the use of a tamper makes for a longer lasting, more energetic, and more efficient explosion. The most effective tamper is the one having the highest density; high tensile strength turns out to be unimportant because no material will hold together under the extreme pressures of a nuclear weapon. It is a fortunate coincidence that materials of high density are also excellent as reflectors of neutrons.

While the effect of a tamper is to increase the efficiency - both by reflecting neutrons and by delaying the expansion of the bomb, the effect on the efficiency is not as great as on the critical mass. The reason for this is that the process of reflection is relatively time consuming and may not occur extensively before the chain reaction is terminated.

Neutron trigger / Initiator

need info here on the use of neutron sources which are timed to release a large amount of neutrons right as the weapon reaches its most reactive state in order to beef up efficiency neutron sources more important for implosion design because of short time in critical state.

To help the supercritical mass detonate, a neutron source is placed in a carefully shaped void in or near the pit. Early neutron sources consisted of a highly radioactive isotope of Polonium (Po-210), which is a strong alpha emitter combined with beryllium which will absorb alphas and emit neutrons. When the two elements were mixed by the implosion of the weapon, they emitted enough neutrons to start a chain reaction while the mass was at an optimal criticality.

Another method of providing source neutrons, is through a pulsed neutron emitter which is a small ion accelerator with a metal hydride target. When the ion source is turned on to create a plasma of deuterium or tritium, a large voltage is applied across the tube which accelerates the ions into tritium rich metal (usually scandium). The ions are accelerated so that tehre is a high probablity of nuclear fusion occuring. The deuterium-tritium fusion reactions emit 14 MeV neutrons as a by product which will jump start the fission chain reaction.

These sequences are electronically timed precisely to cause a proper cascade of neutrons in the pit. If the cascade grows improperly, the bomb usually fizzles. That is, it explodes with only a small fraction of the designed yield.

Practical Limitations of the Fission Bomb

A pure fission bomb is practically limited to a yield of a few hundred kilotons by the large amounts of fissile material needed to make a large weapon. It is technically difficult to keep a large amount of fissile material in a subcritical assembly while waiting for detonation, and it is also difficult to physically transform the subcritical assembly into a supercritical one quick enough that the device explodes rather than prematurely detonating such that a majority of the fuel is unused (innefficient predetonation). The most efficient pure fission bomb would still only consume 20% of its fissile material before being blown apart, and can often be much les efficient (Fat Man only had an efficiency of 1.4%). Large yield, pure fission weapons are also unnatractive due to the weight, size and cost of using large amounts of highly enriched material.

Thermonuclear Weapons (also Fusion bomb or Hydrogen bomb)

The amount of energy released by a weapon can be greatly increased by the addition of nuclear fusion reactions. Fusion releases even more energy per reaction than fission, and can also be used as a source for additional neutrons. The light weight of the elements used as fusion fuel, combined with the larger energy release, means that fusion is a very efficient fuel by weight, making it possible to build extremely high yield weapons which are still portable enough to easily deliver. Fusion is the combination of two light atoms, usually isotopes of hydrogen, to form a more stable heavy atom and release excess energy. The fusion reaction requires the atoms involved to have a high thermal energy, which is why the reaction is called thermonuclear. The extreme temperatures and densities necessary for a fusion reaction are easily generated by a fission explosion.

The simplest way to utilize fusion is to put a mixture of deuterium and tritium inside the hollow core of an implosion style plutonium pit. When the imploding fission chain reaction brings the fusion fuel to a sufficient pressure, the fusion reaction occurs fairly quickly and releases a large number of energetic neutrons into the surrounding fissile material, which allows the fissile material to burn more efficiently. The efficiency (And therefore yield) of a pure fission bomb can be doubled through the use of a fusion boosted core, with very little increase in the size and weight of the device. The amount of energy released through fusion is very small compared to the energy from fission, so the fusion chiefly increses the fission efficiency by providing a burst of additional neutrons. The fusion core of modern fusion weapons is lithium-7 deuteride.

Staged thermonuclear weapons

The basic principles behind modern thermonuclear weapons were discovered independently by scientists in different countries. Edward Teller and Stanislaw Ulam at Los Alamos worked out the idea of staged detonation coupled with radiation implosion in what is known in the United States as the Teller-Ulam design. Soviet physicist Andrei Sakharov independently arrived at the same answer (which he called his Third Idea) a short time later. A single small fission bomb, the trigger, is placed at the point of a cone-shaped arrangement of X-ray mirrors. The mirrors focus the X-rays from the fission explosive on a column of lithium deuteride. The radiation pressure of the X-rays heats and pressurizes the deuterium enough to fuse into helium, and emit copious neutrons. The neutrons transmute the lithium to tritium, which then also fuses and emits large amount of gamma rays. A heavy, U-238 cone between the fission bomb and the column prevented the premature collapse of the column by direct X-ray pressure.

Advanced Thermonuclear Weapons Designs

The largest modern fission-fusion-fission weapons include a fissionable outer shell of U-238, the more inert waste isotope of Uranium, or constructed the X-ray mirrors of polished U-238. This otherwise inert U-238 would be detonated by the intense fast neutrons from the fusion stage, increasing the yield of the bomb many times. For maximum yield, however, moderately enriched uranium is preferable as a jacket material. The largest bomb ever exploded was of this type, a 60 Megaton bomb exploded by the Soviet Union in Siberia.

The Cobalt bomb uses Cobalt in the shell, and the fusion neutrons convert the Cobalt into Cobalt-60, a powerful long-term emitter of Gamma rays. The primary purpose of this weapon is to create extremely radioactive fallout to permanently deny a region to an advancing army, a sort of wind-deployed mine-field. It was actually tested by the British in Central Australia, in areas that remain uninhabitable to this day.

A final variant of the thermonuclear weapons is the enhanced radiation weapon, or neutron bomb. This uses Chromium or Nickel for the X-ray mirrors and shell. The neutrons are permitted to escape. The neutron bomb was conceived as a mercy weapon to kill attacking armies while sparing civilians and property. A neutron bomb would have a fairly small radius of damage compared to other weapons so a more realistic use of a weapon releasing large amounts of neutron flux is as a tactical weapon which could kill soldiers in their fortified tanks or bunkers - it is conceivable that the tanks or bunkers would be cool enough to occupy after a period of several days.

Effects of a thermonuclear weapon

An explosion, in general, results from the very rapid release of a large amount of energy within a limited space. This is true for a conventional explosive, such as TNT, as well as for a nuclear explosion, although the energy is produced in quite different ways. The sudden liberation of energy causes a considerable increase of temperature and pressure, so that all the materials present are converted into hot, compressed gases. Since these gases are at very high temperatures and pressures, they expand rapidly and thus initiate a pressure wave, called a shock wave (or sometimes blast wave), in the surrounding medium of air, water, or earth.

Nuclear weapons, like conventional explosives, deliver most of their destructive impact in the form of a blast or shock. One important difference is that nuclear explosions have a large proportion of their energy released in the form of light and heat, collectively referred to as thermal radiation. The basic reason for this difference is that, weight for weight, the energy produced by a nuclear explosive is millions of times as great as that produced by a chemical explosive. Consequently, the temperatures reached in nuclear explosions are much higher, namely, tens of millions of degrees in a nuclear explosion compared with a few thousands in a conventional explosion. As a result of this great difference in temperature, the distribution of the explosion energy is quite different in the two cases. The nuclear explosion is accompanied by highly penetrating electromagnetic radiation. Finally the substances remaining after a nuclear explosion are radioactive, emitting radiation over an extended period of time, called residual radioactivity or fallout.

Nuclear explosions release approximately 85 percent of their explosive energy as air blast, thermal radiation, and heat. The remaining 15 percent of the energy is released as various nuclear radiations. Of this, 5 percent constitutes the initial nuclear radiation, defined as that produced within a minute or so of the explosion. 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) after the explosion. In a thermonuclear device, in which only about half of the total energy a rises from fission, the residual nuclear radiation carries only 5 percent of the energy released in the explosion. It should be noted that there are no nuclear radiations from a conventional explosion since the nuclei are unaffected in the chemical reactions which take place.

By convention the yield of a nuclear weapon will represent that portion of the energy delivered within a minute or so, and will exclude the contribution of the residual nuclear radiation. For a pure fission device 10 percent of the total fission energy is released in the form of residual nuclear radiation, where a thermonuclear weapon typically releases 5 percent of its total energy as residual radiation. Hence, in a pure fission weapon the explosion energy is about 90 percent of the total fission energy, and in a thermonuclear device it is, on the average, about 95 percent of the total energy of the fission and fusion reactions.

When a nuclear weapon explodes, temperature equilibrium is rapidly established in the residual material. Within about one microsecond after the explosion, some 70 to 80 percent of the explosion energy is emitted as primary thermal radiation, mostly soft X-rays. Almost all of the rest of the energy is in the form of kinetic energy of the weapon debris. The interaction of the primary thermal radiation and the debris particles with the surroundings will vary on the type of material and its density (ie altitude) and will determine the ultimate partition of energy between the thermal radiation received at a distance and shock.

When a nuclear detonation occurs in the air, where the atmospheric pressure (and density) is near to sealevel conditions, the soft X rays in the primary thermal radiation are completely absorbed within a distance of a few feet. Some of the radiations are degraded to lower energies, e.g., into the ultraviolet region, but most of the energy of the primary thermal radiation serves to heat the air immediately surrounding the nuclear burst. It is in this manner that the fireball is formed. Part of the energy is then reradiated at a lower temperature from the fireball and the remainder is converted into shock (or blast) energy. This explains why only about 35 to 45 percent of the fission energy from an air burst is received as thermal radiation energy at a distance, although the primary thermal radiation may constitute as much as 70 to 80 percent of the total. Furthermore, because the secondary thermal radiation is emitted at a lower temperature, it lies mainly in the region of the spectrum with longer wavelengths (lower photon energies), i.e., ultraviolet, visible, and infrared.

In the event of a burst at high altitudes, where the air density is low, the soft X rays travel long distances before they are degraded and absorbed. At this stage, the available energy is spread throughout such a large volume (and mass) that most of the atoms and molecules in the air cannot get very hot. Although the total energy emitted as thermal radiation in a high-altitude explosion is greater than for an air burst closer to sea level, about half is reradiated so slowly by the heated air that it has no great significance as a cause of damage. The remainder, however, is radiated very much more rapidly, i.e., in a shorter time interval, than is the case at lower altitudes. A shock wave is generated from a high-altitude burst, but at distances of normal practical interest it produces a smaller pressure increase than from an air burst of the same yield. Severe disruption in communications can occur following high altitude bursts. They also lead to generation of an intense electromagnetic pulse (EMP) which can significantly degrade performance of or destroy sophisticated electronic equipment. There are no known biological effects of EMP; however, indirect effects may result from failure of critical medical equipment.

Blast Damage

As a result of the very high temperatures and pressures at the point of detonation, the hot gaseous residues move outward radially from the center of the explosion with very high velocities. Most of this material is contained within a relatively thin, dense shell known as the hydrodynamic front. Acting much like a piston that pushes against and compresses the surrounding medium, the front transfers energy to the atmosphere by impulse and generates a steep-fronted, spherically expanding blast or shock wave. At first, this shock wave lags behind the surface of the developing fireball. However, within a fraction of a second after detonation, the rate of expansion of the fireball decreases to such an extent that the shock catches up with and then begins to move ahead of the fireball. For a fraction of a second, the dense shock front will obscure the fireball, accounting for the characteristic double peak of light seen with a nuclear detonation.

A large part of the destruction caused by a nuclear explosion is due to blast effects. Objects within the path of the blast wave are subjected to severe, sharp increases in atmospheric pressure and to extraordinarily severe transient winds. Most buildings, with the exception of reinforced or blast- resistant structures, will suffer moderate to severe damage when subjected to overpressures of only 35.5 kiloPascals (kPa) (0.35 Atm). The velocity of the accompanying blast wind may exceed several hundred km/hr. The range for blast effects increases significantly with the explosive yield of the weapon. In a typical air burst, these values of overpressure and wind velocity noted above will prevail at a range of 0.7 km for 1 kiloton (Kt) yield; 3.2 km for 100 Kt; and 15.0 km for 10 Mt.

Two distinct though simultaneous phenomena are associated with the blast wave in air:

Static overpressure, i.e., the sharp increases in pressure due to compression of the atmosphere. These pressures are those which are exerted by the dense wall of air that comprises the wave front. The magnitude of the overpressure at any given point is directly proportional to the density of the air in the wave.

Dynamic pressures, i.e., drag forces exerted by the strong transient blast winds associated with the movement of air required to form the blast wave. These forces are termed dynamic because they tend to push, tumble, and tear apart objects and cause their violent displacement.

Most of the material damage caused by a nuclear air burst is caused by a

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combination of the high static overpressures and the dynamic or blast wind pressures. The relatively long duration of the compression phase of the blast wave is also significant in that structures weakened by the initial impact of the wave front are literally torn apart by the forces and pressures which follow. The compression and drag force phases together may last several seconds or longer, during which forces many times greater than those in the strongest hurricane are present. These persist even through the negative phase of a blast wave when a partial vacuum is present because of the violent displacement of air.

When the blast wave acts directly upon a resilient target such as the human body, rapid compression and decompression result in transmission of pressure waves through the tissues. These waves can be quite severe and will result in damage primarily at junctions between tissues of different densities (bone and muscle) or at the interface between tissue and air spaces. Lung tissue and the gastrointestinal system, both of which contain air, are particularly susceptible to injury. The resulting tissue disruptions can lead to severe hemorrhage or to air embolism, either of which can be rapidly fatal. The threshold level of overpressure which is estimated to cause lung damage is about 68.9 kPa for a simple unreinforced, unreflected blast wave. Perforation of the ear drums would be a common but a minor blast injury. The threshold value for eardrum rupture is probably around 22 kPa (0.2 atm) and that overpressure associated with a 50% probability of eardrum rupture ranges from 90 to 130 kPa (0.9 to 1.2 atm).

Blast Winds: The drag forces of the blast winds are proportional to the velocities and duration times of those winds, which in turn vary with distance from the point of detonation, yield of the weapon, and altitude of the burst. These winds are relatively short in duration but are extremely severe. They can be much greater in velocity than the strongest hurricane winds and may reach several hundred kilometers per hour. Considerable injury can result, due either to missiles or to the physical displacement of human bodies against objects and structures in the environment.

Thermal radiation

Large amounts of electromagnetic radiation in the visible, infrared, and ultraviolet regions of the electromagnetic spectrum are emitted from the surface of the fireball within the first minute or less after detonation. This thermal radiation travels outward from the fireball at the speed of light. The chief hazard of thermal radiation is the production of burns and eye injuries in exposed personnel. Such thermal injuries may occur even at distances where blast and initial nuclear radiation effects are minimal. Absorption of thermal radiation will also cause the ignition of combustible materials and may lead to fires which then spread rapidly among the debris left by the blast. The range of thermal effects increases markedly with weapon yield.

Most of the energy released in the fission or fusion processes is initially in the form of the kinetic energy of the products of the reactions (e.g., fission fragments, etc.). Within millionths of a second after detonation, numerous inelastic collisions of these vaporized atoms give rise to a plasma of intensely hot weapon residues. Since the temperature of this system is of several tens of million degrees centigrade, it emits enormous quantities of energy in the form of electromagnetic radiation. This radiation is subsequently absorbed by the surrounding atmosphere, which is heated to extremely high temperatures, causing it to emit additional radiation of slightly lower energy. This complex process of radiative transfer of energy is the basic mechanism by which the fireball is formed and expands.

Because this thermal radiation travels at the speed of light, and its mean free path (distance between point of emission and point of absorption) is relatively long, the initial expansion of the fireball is extremely rapid, much more so than the outward motion of gaseous material from the center of the burst responsible for production of the blast wave. Consequently, the blast wave front at first lags behind the radiative front (surface of the fireball).

However, as the fireball expands and its energy is deposited in an ever-increasing volume its temperature decreases and the transfer of energy by thermal radiation becomes less rapid. At this point, the blast wave front begins to catch up with the surface of the fireball and then moves ahead of it, a process called hydrodynamic separation. Due to the tremendous compression of the atmosphere by the blast wave, the air in front of the fireball is heated to incandescence. Thus, after hydrodynamic separation, the fireball actually consists of two concentric regions: the hot inner core known as the isothermal sphere; and an outer layer of luminous shock-heated air.

The outer layer initially absorbs much of the radiation from the isothermal sphere and hence the apparent surface temperature of the fireball and the amount of radiation emitted from it decreases after separation. But, as the shock front advances still farther, the temperature of the shocked air diminishes and it becomes increasingly transparent. This results in an unmasking of the still incandescent isothermal region and an increase in the apparent surface temperature of the fireball. This phenomena is referred to as breakaway.

Since thermal radiation travels in straight lines from the fireball (unless scattered) any opaque object interposed between the fireball and the target will act as a shield and provide significant protection from thermal radiation. If a significant amount of scattering is present, as is the case when visibility is poor, thermal radiation will be received from all directions and shielding will be less effective.

When thermal radiation strikes an object, part will be reflected, part will be transmitted, and the rest will be absorbed. The fraction of the incident radiation that is absorbed depends on the nature and color of the material. A thin material may transmit a large part of the radiant energy striking it. A light colored object may reflect much of the incident radiation and thus escape damage. Thermal damage and injury is due to the absorption of large amounts of thermal energy within relatively short periods of time. The absorbed thermal radiation raises the temperature of the absorbing surface and results in scorching, charring, and possible ignition of combustible organic materials, such as wood, paper, fabrics, etc. If the target material is a poor thermal conductor, the absorbed energy is largely confined to a superficial layer of the material.

Actual ignition of materials exposed to thermal radiation is highly dependent on the width of the thermal pulse (which is dependent on weapon yield) and the nature of the material, particularly its thickness and moisture content. At locations close to ground zero where the radiant thermal exposure exceeds 125 Joules/sq cm, almost all ignitable materials will flame, although burning may not be sustained. On the other hand, at greater distances only the most easily ignited materials will flame, although charring of exposed surfaces may occur. The probability of significant fires following a nuclear explosion depends on the density of ignition points, the availability and condition of combustible material (whether hot, dry, wet), wind, humidity, and the character of the surrounding area. Incendiary effects are compounded by secondary fires started by the blast wave effects such as from upset stoves and furnaces, broken gas lines, etc. In Hiroshima, a tremendous fire storm developed within 20 minutes after detonation. A fire storm burns in upon itself with great ferocity and is characterized by gale force winds blowing in towards the center of the fire from all points of the compass. It is not, however, a phenomenon peculiar to nuclear explosions, having been observed frequently in large forest fires and following incendiary raids during World War II.

An Electromagnetic Pulse (EMP) is caused when a bomb detonates just above the atmosphere. The pulse of X-raysknocks electrons from the air molecules in the top of the atmosphere. The happens over a wide area for each bomb. The moving electric charge causes a single wide-frequency radio pulse. The pulse is powerful enough so that most long metal objects would act as antennas, and generate high voltages when the pulse passes. These voltages and the associated high currents could destroy unshielded electronics and even many wires. One can shield ordinary radios and car ignition parts by wrapping them completely in aluminum foil, or any other form of Faraday Cage. (Of course radios cannot operate, because broadcast radio waves can't reach them).

The thermal radiation emitted by a nuclear detonation causes burns in two ways, by direct absorption of the thermal energy through exposed surfaces (flash burns) or by the indirect action of fires caused in the environment (flame burns). The relative importance of these two processes will depend upon the nature of the environment. If a nuclear weapon detonation occurs in easily flammable surroundings, indirect flame burns could possibly outnumber all other types of injury.

There are also 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 particularity susceptible to visible and short wavelength infrared light, since this part of the electromagnetic spectrum is focused by the lens with concentration of energy at the retinal surface. 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, however, may be sustained at considerable distances from the explosion. The apparent size of the fireball, a function of yield and range will determine the degree and extent of retinal scarring. The location of the scar will determine the degree of interference with vision, with a scar in the central visual field being potentially much more debilitating. Generally, a limited visual field defect, which will be barely noticeable, is all that is likely to occur.

Radiation

About 5% of the energy released in a nuclear air burst is transmitted in the form of initial neutron and gamma radiation. The neutrons result almost exclusively from the energy producing 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 due to the spread of radiation over a larger area as it travels away from the explosion, and to absorption, scattering, and capture by the atmosphere. 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, blast and thermal effects are so much greater in importance that prompt radiation effects can be ignored.

The residual radiation hazard from a nuclear explosion is in the form of radioactive fallout and neutron-induced activity. Residual ionizing radiation arises from:

Fission Products. These are intermediate weight isotopes which are formed when a heavy uranium or plutonium nucleus is split in a fission reaction. There are over 300 different fission products that may result from a fission reaction. Many of these are radioactive with widely differing half-lives. Some are very short, i.e., fractions of a second, while a few are long enough that the materials can be a hazard for months or years. Their principal mode of decay is by the emission of beta and gamma radiation. Approximately 60 grams of fission products are formed per kiloton of yield. The estimated activity of this quantity of fission products 1 minute after detonation is equal to that of 1.1 x 1021 Bq (30 million kilograms of radium) in equilibrium with its decay products.

Unfissioned Nuclear Material. Nuclear weapons are relatively inefficient in their use of fissionable material, and much of the uranium and plutonium is dispersed by the explosion without undergoing fission. Such unfissioned nuclear material decays by the emission of alpha particles and is of relatively minor importance.

Neutron-Induced Activity. If atomic nuclei capture neutrons when exposed to a flux of neutron radiation, they will, as a rule, become radioactive (neutron-induced activity) and then decay by emission of beta and gamma radiation over an extended period of time. Neutrons emitted as part of the initial nuclear radiation will cause activation of the weapon residues. In addition, atoms of environmental material, such as soil, air, and water, may be activated, depending on their composition and distance from the burst. For example, a small area around ground zero may become hazardous as a result of exposure of the minerals in the soil to initial neutron radiation. This is due principally to neutron capture by sodium (Na), manganese, aluminum, and silicon in the soil. This is a negligible hazard because of the limited area involved.

Fall Out

Worldwide Fallout: After an air burst the fission products, unfissioned nuclear material, and weapon residues which have been vaporized by the heat of the fireball will condense into a fine suspension of very small particles 0.01 to 20 micrometers in diameter. These particles may be quickly drawn up into the stratosphere, particularly so if the explosive yield exceeds 10 Kt. They will then be dispersed by atmospheric winds and will gradually settle to the earth's surface after weeks, months, and even years as worldwide fallout. The radiobiological hazard of worldwide fallout is essentially a long-term one due to the potential accumulation of long-lived radioisotopes, such as strontium-90 and cesium-137, in the body as a result of ingestion of foods which had incorporated these radioactive materials. This hazard is much less serious than those which are associated with local fallout and, therefore, is not discussed at length in this publication. Local fallout is of much greater immediate operational concern.

Local Fallout: In a land or water surface burst, large amounts of earth or water will be vaporized by the heat of the fireball and drawn up into the radioactive cloud. This material will become radioactive when it condenses with fission products and other radiocontaminants or has become neutron-activated. There will be large amounts of particles of less than 0.1 micrometer to several millimeters in diameter generated in a surface burst in addition to the very fine particles which contribute to worldwide fallout. The larger particles will not rise into the stratosphere and consequently will settle to earth within about 24 hours as local fallout. Severe local fallout contamination can extend far beyond the blast and thermal effects, particularly in the case of high yield surface detonations. Whenever individuals remain in a radiologically contaminated area, such contamination will lead to an immediate external radiation exposure as well as a possible later internal hazard due to inhalation and ingestion of radiocontaminants. In severe cases of fallout contamination, lethal doses of external radiation may be incurred if protective or evasive measures are not undertaken. In cases of water surface (and shallow underwater) bursts, the particles tend to be rather lighter and smaller and so produce less local fallout but will extend over a greater area. The particles contain mostly sea salts with some water; these can have a cloud seeding affect causing local rainout and areas of high local fallout. For subsurface bursts, there is an additional phenomenon present called "base surge." The base surge is a cloud that rolls outward from the bottom of the column produced by a subsurface explosion. For underwater bursts the visible surge is, in effect, a cloud of liquid (water) droplets with the property of flowing almost as if it were a homogeneous fluid. After the water evaporates, an invisible base surge of small radioactive particles may persist. For subsurface land bursts, the surge is made up of small solid particles, but it still behaves like a fluid. A soil earth medium favors base surge formation in an underground burst.

Meteorological Effects: Meteorological conditions will greatly influence fallout, particularly local fallout. Atmospheric winds are able to distribute fallout over large areas. For example, as a result of a surface burst of a 15 Mt thermonuclear device at Bikini Atoll on March 1, 1954, a roughly cigar-shaped area of the Pacific extending over 500 km downwind and varying in width to a maximum of 100 km was severely contaminated. Snow and rain, especially if they come from considerable heights, will accelerate local fallout. Under special meteorological conditions, such as a local rain shower that originates above the radioactive cloud, limited areas of heavy contamination may be formed.

Blast and thermal injuries in many cases will far outnumber radiation injuries. However, radiation effects are considerably more complex and varied than are blast or thermal effects and are subject to considerable misunderstanding. A wide range of biological changes may follow the irradiation of an animal, ranging from rapid death following high doses of penetrating whole-body radiation to an essentially normal life for a variable period of time until the development of delayed radiation effects, in a portion of the exposed population, following low dose exposures.

Median Lethal Dose (LD₅₀): When comparing the effects of various types or circumstances, that dose which is lethal to 50% of a given population is a very useful parameter. The term is usually defined for a specific time, being limited, generally, to studies of acute lethality. The common time periods used are 30 days or less for most small laboratory animals and to 60 days for large animals and humans. It should be understood that the LD₅₀ assumes that the individuals did not receive other injuries or medical treatment.

For yields of 5-10 Kt (or less), initial nuclear radiation is the dominant casualty producer on the battlefield. Military personnel receiving an acute incapacitation dose (30 Gy) will become performance degraded almost immediately and combat ineffective within several hours. However, they will not die until 5-6 days after exposure if they do not receive any other injuries which make them more susceptible to the radiation dose. Soldiers receiving less than a total of 150 cGy will remain combat effective. Between those two extremes, military personnel receiving doses greater than 150 cGy will become degraded; some will eventually die. A dose of 530-830 cGy is considered lethal but not immediately incapacitating. Personnel exposed to this amount of radiation will become performance degraded within 2-3 hours, depending on how physically demanding the tasks they must perform are, and will remain in this degraded state at least 2 days. However, at that point they will experience a recovery period and be effective at performing nondemanding tasks for about 6 days, after which they will relapse into a degraded state of performance and remain so for about 4 weeks. At this time they will begin exhibiting radiation symptoms of sufficient severity to render them totally ineffective. Death follows at approximately 6 weeks after exposure.

Late or delayed effects of radiation occur following a wide range of doses and dose rates. Delayed effects may appear months to years after irradiation and include a wide variety of effects involving almost all tissues or organs. Some of the possible delayed consequences of radiation injury are life shortening, carcinogenesis, cataract formation, chronic radiodermatitis, decreased fertility, and genetic mutations.

For videos and more on the effects of a thermonuclear device check [1]

Weapons delivery

The term strategic nuclear weapons is often used to denote large weapons which would be used to destroy large targets, such as cities. Tactical nuclear weapons' are smaller weapons used to destroy specific targets (military, communications, infrastructure).

Basic methods of delivery are:

Bombers such as the B-52 and V bomber
Ballistic missiles - a missile using a ballistic trajectory involving a significant ascent and descent including suborbital and partial orbital trajectories. Most commonly ICBM. Modern weapons also deliver Multiple Independent Re-entry Vehicles (MIRV) each of which carries a warhead and allows a single launched missile to strike a handful of targets.
Cruise missiles - A missile using a low altitude trajectory intended to avoid detection by radar systems. Cruise missiles have shorter range and lower payloads than ballistic missiles, usually, and are not know to carry MIRVs
artillery shells - for tactical use
hand held (suitcase)

History

This military potential in the minds of leaders during the Second World War, and the Allied nations were particularly concerned about Germany producing such weapons, and so began the Manhattan Project which brought the top minds in nuclear physics together, led by Robert Oppenheimer under the United States military with the goal of producing fission based explosive devices.

A massive industrial and scientific undertaking, it involved many of the world's great physicists in the scientific and development aspects, whose work was centered around the laboratories at Los Alamos, New Mexico. As part of the project, the world's first sustained and controlled nuclear chain reaction was achieved at the University of Chicago under the supervision of Enrico Fermi. Apart from Chicago and Los Alamos, the Hanford Site in the state of Washington and Oak Ridge were the sites of large scale production and purification of fissionable material.

The Manhattan Project was unable to produce fission-based weapons prior to the fall of the Third Reich and the end of the European War, but were able to produce a test device and two deliverable devices, one using uranium 235 known as Little Boy, and another using plutonium and known as Fat Man. Facing the prospects of a long, bloody, grueling "island hopping" campaign to conquer Japan, US President Harry Truman chose to use the newly developed nuclear weapons to produce enough terror in the minds of the Japanese leadership that they would surrender. Little Boy was delivered to the Japanese city of Hiroshima by the bomber Enola Gay, and Fat Man was delivered by Bocks Car to the Japanese city of Nagasaki, after which Japan did surrender. These two bombings are still the only instances when atomic weapons were used in warfare.

The Third Reich attempted to develop nuclear weapons, but erroneously concluded that they were impossible, because slow neutron fission cannot cascade fast enough to cause an explosion. The physicists, including Werner Heisenberg and Otto Hahn, working for the Reich may have deliberately refused to notice fast neutron fission but this is unlikely. The Reich attempted to develop power reactors, but its supplies of graphite were poisoned by Boron, a neutron absorber injected by the boron electrodes then used to provide commercial graphite. As a result, the Nazi reactor program attempted to develop heavy water reactors, and was hampered by supply and purity problems. Leo Szilard, a Hungarian physicist trained as an industrial chemist, successfully prevented this problem with American reactors, which used graphite.

The first thermonuclear device was developed by the United States and tested as Operation Ivy on November 1 1952 on Elugelab Island in the Enewetak (or Eniwetok) Atoll of the Marshall Islands, code-named Mike. It yielded 10.4 MT, over 450 times the power of the bomb that fell on Nagasaki. Mike used liquid deuterium as the fusion fuel and had a 92 point ignition system. It was 20ft high, 6ft 8 in wide, and weighting 140,000 lb (164,000lb including attached refrigeration and measuring equipment). The detonation obliterated Elugelab, leaving an underwater crater 6240ft wide and 164ft deep where an island had once been. The largest pure fission bomb (King at 500KT) was tested in the Enewetak atoll two weeks later on November 15, 1952. The USSR exploded its first thermonuclear device on August 12 1953. Great Britain (May 15 1957. Operation Grapple, test off Malden Island), France (February 13 1960), and China (October 16 1964) have also exploded thermonuclear weapons.

After World War II, the balance of power between eastern and western forces, resulting in the fear of global destruction, prevented the further military use of atomic bombs. This fear was even a central part of Cold War strategy, referred to as the doctrine of Mutally Assured Destruction (or, appropriately, "MAD" for short). So important was this balance to international political stability that a treaty, the Antiballistic Missile Treaty (or ABM treaty) was signed by the US and the USSR in 1972 to curtail the development of defenses against nuclear weapons and the ballistic missiles which carry them.

Early delivery systems for nuclear devices were primarily bombers like the American B-29 Superfortress and B-36 Peacemaker, and later the B-52 Stratofortress. Ballistic missile systems, based on designs used by Germany under Wernher Von Braun and known as the V-2, were developed by both American and Soviet teams of captured scientists and engineers from this program. These systems, after testing, were used to launch satellites, such as Sputnik, and to propel the Space Race, but they were primarily developed to create the capability of Intercontinental Ballistic Missiles (ICBMs) with which nuclear powers could deliver that destructive force anywhere on the globe. These systems continued to be developed throughout the Cold War, although plans and treaties, beginning with the Strategic Arms Limitation Treaty (SALT I), restricted deployment of these systems until, after the fall of the Soviet Union, system development essentially halted, and many weapons were disabled and destroyed (see nuclear disarmament).

The end of the Cold War failed to bring an end to the threat of the use of nuclear weapons, although global fears of nuclear war reduced substantially. France made a point of conducting above-ground tests of nuclear weapons in the 1990s, and both India and Pakistan successfully tested nuclear devices in that decade, raising concerns that they would use nuclear weapons on each other, although India's first test was in the 1970s with Smiling Buddha. The fall of the Soviet Union spread the posession and control of the Soviet nuclear arsenal over several former Soviet republics, and their economic need and the lack of opportunity for Soviet nuclear physicists created opportunities for Third World countries to hire developers and buy materials and supplies, which may have accelerated the nuclear programs of nations such as India, Pakistan and Iraq. The proliferation of nuclear weapons to these nations is prohibited by the international Nuclear Non-Proliferation Treaty.

South Africa also had an active program to develop uranium based nuclear weapons, but dismantled its nuclear weapon program in the 1990s. It is unclear whether it actually tested such a weapon. In the late 1970's, American spy satellites detected what appeared to be a flash of gamma rays in the south Atlantic. As most of the records of the South African program were destroyed when it was dismantled, it remains unclear whether what was observed was a South African nuclear test. Israel is also thought to possess an arsenal of potentially up to several hundred nuclear warheads, but this has never been openly confirmed. The United Kingdom has its own nuclear weapons but has not run an independent development program since the failure of Blue Streak missile in the 1960s, buying American delivery systems and fitting British warheads instead (Polaris Sales Agreement). China also posesses a small arsenal of nuclear warheads.

Humor and Culture

Nuclear weaponry has become a part of our culture.

Films featuring nuclear war or the threat of it include Dr. Strangelove or, How I Learned to Stop Worrying and Love the Bomb, On The Beach, The Day After, The War Game (1966), Threads (1985), War Games (1983 - "Let's play Global Thermonuclear War"); as well as less-famous films such as Miracle Mile and Broken Arrow ("I don't know which worries me more: the fact that our government lost a nuclear missile, or the fact that it happens so often they have code name for it"). Godzilla is considered by some to be an analogy to the nuclear weapons dropped on Japan.

A memorable episode of The Bionic Woman featured the threat of a cobalt bomb.

A main character in Repo Man was a designer of the neutron bomb: "Ever heard of the neutron bomb? Kills people, leaves buildings standing - so small, it fits in a suitcase, noone knows it's there and then blammo - eyes melt, skin explodes, everybody dies. It's so immoral working on the thing could drive you mad."

"Close doesn't count except in horseshoes, hand grenades, and H bombs."

References

Glasstone, Samuel and Dolan, Philip J., The Effects of Nuclear Weapons (third edition), U.S. Government Printing Office, 1977. (Available Online)
NATO Handbook on the Medical Aspects of NBC Defecsive Operations (Part I - Nuclear), Departments of the Army, Navy, and Air Force, Washington, D.C., 1996. (Available Online)
Smyth, H. DeW., Atomic Energy for Military Purposes, Princeton University Press, 1945. (Available Online)
Rhodes, Richard. Dark Sun: The Making of the Hydrogen Bomb. Simon & Schuster, New York, 1995
Rhodes, Richard. The Making of the Atomic Bomb. Simon & Schuster, New York, 1986
Carey Sublette's High Energy Weapons Archive is a reliable source of information and has links to other sources: http://fas.org/nuke/hew/

For discussions of how nuclear weapons might be used and the political and military strategies of their use see nuclear warfare and Mutual Assured Destruction.

For discussion on how nuclear weapons spread see nuclear proliferation, Nuclear Test Ban, Nuclear Non-Proliferation Treaty