Nuclear weapon


A nuclear weapon is a weapon deriving its energy from nuclear reactions.
These weapons have enormous destructive potential.

Types of 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.

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".

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 bomb 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 (as in the salted bombs
discussed above).

For more technical details see: Nuclear weapon design

Effects of a nuclear explosion

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
iteself 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.

[Atomic blast.jpg]
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. Incendiary effects are compounded by
secondary fires started by the blast wave effects such as from upset stoves
and furnaces.

In Hiroshima, a tremendous fire storm developed within 20 minutes after
detonation. A fire storm 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.

Electromagnetic pulse

At altitudes above the majority of the air, the x-rays ionize the upper air,
moving large numbers of electrons. 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. There are no known
biological effects of EMP except from failure of critical medical and
transportation equipment. The ionized air also disrupts radio traffic that
would normally bounce from the ionosphere.

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 when shielded, because broadcast radio waves can't reach them.

Radiation

About 5% of the energy released in a nuclear air burst 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.

Nuclear fallout

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.

In an explosion near the surface 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, mixed with
fission products and other radiocontaminants that have become
neutron-activated. The larger particles will settle back to the earth's
surface near ground zero (depending on wind and weather conditions of
course) within 24 hours, while fine particles will rise to the stratosphere
and be distributed globally over the course of weeks or months.

Severe local fallout contamination can extend far beyond the blast and
thermal effects, particularly in the case of high yield surface detonations.
In detonations near a water surface, the particles tend to be lighter and
smaller and 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.

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
incorporating these radioactive materials. The hazard of worldwide fallout
is much less serious than the hazards which are associated with local
fallout.

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 animals, ranging from rapid death following high doses of
penetrating whole-body radiation to essentially normal lives for a variable
period of time until the development of delayed radiation effects, in a
portion of the exposed population, following low dose exposures.

For more technical details see: nuclear explosion

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 such as
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 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

Nuclear weapons in culture

Nuclear weaponry has become a part of our culture, the decades post-WW II
being can be termed the atomic age. The stunning power and the astonishing
visual effects are a strong influence on art, from Andy Warhol's silkscreen
Atomic Bomb (1965) and James Rosenquist's F-111 (1964-65) to Gregory Green's
constructions and the efforts of artist James Acord to use uranium in his
sculptures.

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), WarGames (1983); as well as
less-famous films such as Miracle Mile and Broken Arrow (1996). Also the
series of movies Planet of the Apes finish with the launching of cobalt
bombs. 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.

Nuclear weapons are a staple element in science fiction novels. The
so-called dirty bomb was predicted in a 1943 article by Robert A. Heinlein
titled "Solution Unsatisfactory" which caused him to be investigated by the
FBI, concerned that there had been a breach of security on the Manhattan
Project.