Nuclear Weapons
Nuclear Weapons

A nuclear weapon derives its energy from nuclear reactions of fission or fusion. These weapons have enormous destructive potential and are possessed by only a handful of nations.

Since 1945, when the Manhattan Project team in the US exploded the first nuclear bomb, nuclear weapons have proliferated across the globe. Currently, the US has about 7,000 warheads and the nations of the former Soviet Union have approximately 6,000. There are enough nuclear weapons in the world to destroy all civilization, as we know it. They are perhaps the most powerful forces that man has ever wielded. When first developed, nuclear weapons were completely strategic weapons. That is, they were not designed to destroy enemy weaponry; they were designed to destroy entire cities. However, there are now small, tactical nuclear weapons in addition to the others. Besides how powerful a nuclear weapon is, there are other differences between them. They can be either a fusion or a fission device, and they can be dropped from an airplane, fired from an artillery gun, or attached to various types of rockets. They explode with a mushroom cloud. In the history of warfare, nuclear weapons have been used only twice, both during the closing days of World War II.

On the morning of 6 August 1945,the first event occurred; when the United States dropped a uranium gun-type device code-named "Little Boy" on the Japanese city of Hiroshima. The second event occurred three days later when a plutonium implosion-type device code-named "Fat Man" was dropped on the city of Nagasaki. The use of the weapons, which resulted in the immediate deaths of at least 200,000 individuals (mostly civilians) and about twice that number over time, was and remains controversial — critics charged that they were unnecessary acts of mass killing, while others claimed that they ultimately reduced casualties on both sides by hastening the end of the war.

Since that time, nuclear weapons have been detonated on over two thousand occasions, mostly for testing purposes, chiefly by the United States, Soviet Union, United Kingdom, France, People's Republic of China, India and Pakistan. These countries are the declared nuclear powers (with Russia inheriting the weapons of the Soviet Union after its collapse). Various other countries may hold nuclear weapons, but they have never publicly admitted possession, or their claims to possession have not been verified. For example, Israel has modern airborne delivery systems and appears to have an extensive nuclear program; North Korea has recently stated that it has nuclear capabilities (although it has made several changing statements about the abandonment of its nuclear weapons programs, often dependent on the political climate at the time) and Iran was accused by a number of governments of attempting to develop nuclear capabilities, and now acknowledges that it is trying to obtain nuclear power, supposedly for peaceful purposes.

Apart from their use as weapons, nuclear explosives have been proposed for various non-military uses.

The Energy from a Nuclear Weapon

Difference between a nuclear and a conventional explosion is that nuclear explosions can be many thousands (or millions) of times more powerful than the largest conventional detonations. Both types of weapons rely on the destructive force of the blast or shock wave. However, the temperatures reached in a nuclear explosion are very much higher than in a conventional explosion, and a large proportion of the energy in a nuclear explosion is emitted in the form of light and heat, generally referred to as thermal energy. This energy is capable of causing skin burns and of starting fires at considerable distances. Nuclear explosions are also accompanied by various forms of radiation, lasting a few seconds to remaining dangerous over an extended period of time.

Approximately 85 percent of the energy of a nuclear weapon produces air blast (and shock), thermal energy (heat). The remaining 15 percent of the energy is released as various type of nuclear radiation. Of this, 5 percent constitutes the initial nuclear radiation, defined as that produced within a minute or so of the explosion, are mostly gamma rays and neutrons. The final 10 percent of the total fission energy represents that of the residual (or delayed) nuclear radiation, which is emitted over a period of time. This is largely due to the radioactivity of the fission products present in the weapon residues, or debris, and fallout after the explosion. A more extensive analysis on the damage produced by a nuclear weapon is discussed later in the article.

The "yield" of a nuclear weapon is a measure of the amount of explosive energy it can produce. The yield is given in terms of the quantity of TNT that would generate the same amount of energy when it explodes. Thus, a 1 kiloton nuclear weapon is one which produces the same amount of energy in an explosion as does 1 kiloton (1,000 tons) of TNT. Similarly, a 1 megaton weapon would have the energy equivalent of 1 million tons of TNT. One megaton is equivalent to 4.18 x 1015 joules.

In evaluating the destructive power of a weapons system, it is customary to use the concept of equivalent megatons (EMT). Equivalent mega tonnage is defined as the actual mega tonnage rose to the two-thirds power:

EMT = Y2/3 where Y is in megatons.

This relation arises from the fact that the destructive power of a bomb does not vary linearly with the yield. The volume the weapon's energy spreads into varies as the cube of the distance, but the destroyed area varies at the square of the distance.

Thus 1 bomb with a yield of 1 megaton would destroy 80 square miles. While 8 bombs, each with a yield of 125 kilotons would destroy 160 square miles. This relationship is one reason for the development of delivery systems that could carry multiple warheads (MIRVs).

Types of nuclear weapons

Fission bombs derive their power from nuclear fission, where heavy nuclei (uranium or plutonium) split into lighter elements when bombarded by neutrons (produce more neutrons which bombard other nuclei, triggering a chain reaction). These are historically called atom bombs or A-bombs, though this name is not precise due to the fact that chemical reactions release energy from atomic bonds and fusion is no less atomic than fission. Despite this possible confusion, the term atom bomb has still been generally accepted to refer specifically to nuclear weapons, and most commonly to pure fission devices.

Fusion bombs are based on nuclear fusion where light nuclei such as hydrogen and helium combine together into heavier elements and release large amounts of energy. Weapons, which have a fusion stage, are also referred to as hydrogen bombs or H-bombs because of their primary fuel, or thermonuclear weapons because fusion reactions require extremely high temperatures for a chain reaction to occur.

There are other types of nuclear weapons as well. For example, a boosted fission weapon is a fission bomb, which increases its explosive yield through a small amount of fusion reactions, but it is not a hydrogen bomb. Some weapons are designed for special purposes; a neutron bomb is a nuclear weapon that yields a relatively small explosion but a relatively large amount of radiation. The detonation of a nuclear weapon is accompanied by a blast of neutron radiation. Surrounding a nuclear weapon with suitable materials (such as cobalt or gold) creates a weapon known as a salted bomb. This device can produce exceptionally large quantities of radioactive contamination. Most variety in nuclear weapon design is in different yields of nuclear weapons for different types of purposes.

Nuclear weapons are often described as either fission or fusion devices based on the dominant source of the weapon's energy. The distinction between these two types of weapon is blurred by the fact that they are combined in nearly all complex modern weapons: a smaller fission bomb is first used to reach the necessary conditions of high temperature and pressure to allow fusion to occur. On the other hand, a fission device is more efficient when a fusion core first boosts the weapon's energy. Since the distinguishing feature of both fission and fusion weapons is that they release energy from transformations of the atomic nucleus, the best general term for all types of these explosive devices is "nuclear weapon". A major challenge in all nuclear weapon designs is ensuring that a significant fraction of the fuel is consumed before the weapon destroys itself.

Nuclear strategy

The United States' Peacekeeper missile was a MIRVed delivery system. Each missile could contain up to ten nuclear warheads, each of which could be aimed at a different target. These were developed to make missile defense very difficult for an enemy country nuclear warfare strategy is a way for either fighting or avoiding a nuclear war. The policy of trying to ward off a potential attack by a nuclear weapon from another country by threatening nuclear retaliation is known as the strategy of nuclear deterrence. The goal in deterrence is to always maintain a second strike status (the ability of a country to respond to a nuclear attack with one of its own) and potentially to strive for first strike status (the ability to completely destroy an enemy's nuclear forces before they could retaliate). During the Cold War, policy and military theorists in nuclear-enabled countries worked out models of what sorts of policies could prevent one from ever being attacked by a nuclear weapon.

Different forms of nuclear weapons delivery allow for different types of nuclear strategy, primarily by making it difficult to defend against them and difficult to launch a pre-emptive strike against them. Sometimes this has meant keeping the weapon locations hidden, such as putting them on submarines or train cars whose locations are very hard for an enemy to track, and other times this means burying them in hardened bunkers. Other responses have included attempts to make it seem likely that the country could survive a nuclear attack, by using missile defense (to destroy the missiles before they land) or by means of civil defense (using early warning systems to evacuate citizens to a safe area before an attack). Weapons, which are designed to threaten large populations or to generally deter attacks, are known as "strategic" weapons and weapons, which are designed to actually be used on a battlefield in military situations, are known as "tactical" weapons.

Critics are there for the idea of "nuclear strategy" for waging nuclear war that have suggested that a nuclear war between two nuclear powers would result in mutual annihilation. From this point of view, the significance of nuclear weapons is purely to deter war because any nuclear war would immediately escalate out of mutual distrust and fear, resulting in mutually assured destruction. This threat of destruction has been a strong motivation for anti-nuclear weapons activism.Critics from the peace movement and within the military establishment have questioned the usefulness of such weapons in the current military climate. The use of such weapons would generally be contrary to the rules of international law applicable in armed conflict.

Nuclear Weapon Testing

Nuclear weapons are “sophisticated but not complicated.” That is, the working principles are straightforward, although the equipment needed to make a device function, and function reliably, is quite sophisticated and requires high-quality engineering to design and build. Although it is generally believed that a proliferator need not test a conservatively designed device at full yield to have confidence in it, some experimentation and testing along the way is necessary to demonstrate the behavior of the non-nuclear components including the firing set, detonators, and neutron generators. If there is not to be a full-yield nuclear test, then the non-nuclear experiments must be carried out with greater care and competence.

For believing that a full-yield nuclear test is unnecessary, there is one reason that each of the six states known to have tested nuclear devices has achieved a nuclear detonation on the first try. The first nuclear weapon used in combat used an untested gun-assembled design, but a very simple and inefficient one. The first implosion device was tested on July 16, 1945, near Alamogordo, New Mexico, and an identical “physics package” (the portion of the weapon including fissile and fusion fuels plus high explosives) was swiftly incorporated into the bomb dropped on Nagasaki.

The term “nuclear testing” encompasses all experiments in which special nuclear material is placed in contact with high explosives, which are then detonated, or with a propellant, which is ignited. This limitation deliberately excludes activities, which are more scientific in nature and not intimately connected with the progression from fissile material and/or fusion fuel to a nuclear explosive device. This definition is far broader than that of the Comprehensive Test Ban Treaty (CTBT) of 1996, which prohibits only nuclear weapon test explosions and other nuclear explosions. Many states of concern for nuclear proliferation have subscribed to the CTBT, and may, therefore, find it difficult to conduct full-yield tests either underground or in the atmosphere. At the lowest end of the nuclear yield distribution from hydro nuclear tests, some states might reckon that the knowledge gained from a small explosive release of nuclear energy would be worth the risk of getting caught. Generally, within the U.S. Government, the condition of prompt nuclear criticality distinguishes, under the CTBT, a prohibited test of an explosively assembled device from one, which is allowed.

Weapons Delivery

The technology and systems used to bring a nuclear weapon to its target—is an important aspect of nuclear weapons relating both to nuclear weapon design and nuclear strategy. The term strategic nuclear weapons are 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 such as military, communications, and 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 sub orbital and partial orbital trajectories. Most commonly ICBM and SLBM. 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 known to carry MIRVs

  • Artillery shells - for tactical use

  • Hand held

Advanced Thermonuclear Weapons Designs

The largest modern weapons include a fissionable outer shell of uranium. The intense fast neutrons from the fusion stage of the weapon will cause even natural (that is unenriched) uranium to fission, increasing the yield of the weapon many times. The cobalt bomb uses cobalt in the shell, and the fusion neutrons convert the cobalt into cobalt-60, a powerful long-term (5 years) emitter of gamma rays. In general this type of weapon is a salted bomb and variable fallout effects can be obtained by using different salting isotopes. Gold has been proposed for short-term fallout (days), tantalum and zinc for fallout of intermediate duration (months), and cobalt for long-term contamination (years). The primary purpose of this weapon is to create extremely radioactive fallout making a large region uninhabitable. No cobalt or other salted bomb has been built or tested publicly.

A final variant of the thermonuclear weapons is the enhanced radiation weapon, or neutron bombs, which are small thermonuclear weapons in which the burst of neutrons generated by the fusion reaction is intentionally not absorbed inside the weapon, but allowed to escape. The X-ray mirrors and shell of the weapon are made of chromium or nickel so that the neutrons are permitted to escape. This intense burst of high-energy neutrons is the principle destructive mechanism. Neutrons are more penetrating than other types of radiation so many shielding materials that work well against gamma rays are rendered less effective. The term "enhanced radiation" refers only to the burst of ionizing radiation released at the moment of detonation, not to any enhancement of residual radiation in fallout
Effects of a nuclear explosion.

Types of Damage

The energy released from a nuclear weapon comes in four primary categories:

• Blast 40-60% of total energy

• Thermal radiation - 30-50% of total energy

• Ionizing radiation - 5% of total energy

• Residual radiation (fallout) 5-10% of total energy

The amount of energy released in each form depends on the design of the weapon, and the environment in which it is detonated. The residual radiation of fallout is a delayed release of energy; the other three forms of energy release are immediate.

The dominant effects of a nuclear weapon (the blast and thermal radiation) are the same physical damage mechanisms as conventional explosives. The primary difference is that nuclear weapons are capable of releasing much larger amounts of energy at once. Most of the damage caused by a nuclear weapon is not directly related to the nuclear process of energy release, but would be present for any explosion of the same magnitude.

The damage done by each of the three initial forms of energy release differs with the size of the weapon. Thermal radiation drops off the slowest with distance, so the larger the weapon the more important this effect becomes. Ionizing radiation is strongly absorbed by air, so it is only dangerous by itself for smaller weapons. Blast damage falls off more quickly than thermal radiation but more slowly than ionizing radiation.

When a nuclear weapon explodes, the bomb's material comes to an equilibrium temperature in about a microsecond. At this time about 75% of the energy is emitted as primary thermal radiation, mostly soft X-rays. Almost all of the rest of the energy is kinetic energy in rapidly moving weapon debris. The interaction of the x-rays and debris with the surroundings determines how much energy is produced as blast and how much as light. In general, the denser the medium around the bomb, the more it will absorb, and the more powerful the shockwave will be. When a nuclear detonation occurs in air near sea level, most of the soft X-rays in the primary thermal radiation are absorbed within a few feet. Some energy is reradiated in the ultraviolet, visible light and infrared, but most of the energy heats a spherical volume of air. This forms the fireball.

In a burst at high altitudes, where the air density is low, the soft X rays travel long distances before they are absorbed. The energy is so diluted that the blast wave may be half as strong or less. The rest of the energy is dissipated as a more powerful thermal pulse.

Blast Damage

Much of the destruction caused by a nuclear explosion is due to blast effects. Most buildings, except reinforced or blast-resistant structures, will suffer moderate to severe damage when subjected to moderate overpressures. The blast wind may exceed several hundred km/hr. The range for blast effects increases with the explosive yield of the weapon.

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

• Static overpressure, i.e., the sharp increase in pressure exerted by the shock wave. The overpressure at any given point is directly proportional to the density of the air in the wave.

• Dynamic pressures, i.e., drag exerted by the blast winds required to form the blast wave.

These winds push, tumble and tear objects.

Most of the material damage caused by a nuclear air burst is caused by a combination of the high static overpressures and the blast winds. The long compression of the blast wave weakens structures, which are then torn apart by the blast winds. The compression, vacuum and drag phases together may last several seconds or longer, and exert forces many times greater than the strongest hurricane.

Thermal radiation

Nuclear weapons emit large amounts of electromagnetic radiation as visible, infrared, and ultraviolet light. The chief hazards are burns and eye injuries. On clear days, these injuries can occur well beyond blast ranges. The light is so powerful that it can start fires that spread rapidly in the debris left by a blast. The range of thermal effects increases markedly with weapon yield. Since thermal radiation travels in straight lines from the fireball (unless scattered) any opaque object will produce a protective shadow. If fog or haze scatters the light, it will heat things from all directions and shielding will be less effective.

When thermal radiation strikes an object, part will be reflected, part transmitted, and the rest absorbed. The fraction that is absorbed depends on the nature and color of the material. A thin material may transmit a lot. A light colored object may reflect much of the incident radiation and thus escape damage. The absorbed thermal radiation raises the temperature of the surface and results in scorching, charring, and burning of wood, paper, fabrics, etc. If the material is a poor thermal conductor, the heat is confined to the surface of the material.

Actual ignition of materials depends on the how long the thermal pulse lasts and the thickness and moisture content of the target. Near ground zero where the light is most intense, what can burn, will. Farther away, only the most easily ignited materials will flame. Secondary fires started by the blast wave effects such as from upset stoves and furnaces compound incendiary effects.

In Hiroshima, a tremendous firestorm developed within 20 minutes after detonation. A firestorm has 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.

Radiation

About 5-10% of the energy released in a nuclear airburst is in the form of initial neutron and gamma radiation. The neutrons result almost exclusively from the fission and fusion reactions, while the initial gamma radiation includes that arising from these reactions as well as that resulting from the decay of short-lived fission products.

The intensity of initial nuclear radiation decreases rapidly with distance from the point of burst because the radiation spreads over a larger area as it travels away from the explosion. It is also reduced by atmospheric absorption and scattering.

The character of the radiation received at a given location also varies with distance from the explosion. Near the point of the explosion, the neutron intensity is greater than the gamma intensity, but with increasing distance the neutron-gamma ratio decreases. Ultimately, the neutron component of initial radiation becomes negligible in comparison with the gamma component. The range for significant levels of initial radiation does not increase markedly with weapon yield and, as a result, the initial radiation becomes less of a hazard with increasing yield. With larger weapons, above 50 Kt, blast and thermal effects are so much greater in importance that prompt radiation effects can be ignored.


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