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Nuclear weapons are weapons of enormous destructive potential, deriving their energy either from nuclear fission or nuclear fusion. Commonly referred to as an atomic bomb (which usually denotes a fission only device) or a hydrogen bomb (which refers to the hydrogen necessary for a fusion device, initiated by an initial fission reaction). These weapons were initially developed by the United States during the Second World War. Although no nuclear weapon has been used in warfare since World War II, the possibility of mass destruction has had extensive political ramifications on international interactions.


The destructive energy of nuclear weapons is so vast that it is measured in kilotons or megatons, units developed for this purpose.


See also nuclear war, Mutual Assured Destruction, Nuclear Test Ban, Nuclear Non-Proliferation Treaty, Nuclear proliferation, nuclear reactor.


Structure and Design


Simple Nuclear Weapon (also Fission bomb or Atomic bomb)


The first nuclear weapons were pure fission bombs. A fission bomb works by assembling a critical mass of a heavy, unstable isotopes, usually plutonium (Pu-239), though the Hiroshima bomb used uranium-235. Plutonium does not naturally occur. It can be produced in a nuclear reactor by neutron capture in uranium-238, the most plentiful isotope of uranium. The production of fissionable Pu-239 is eased by the chemical distinguishability of Pu-239 from unconverted U-238. In contrast, U-235 must be physically separated from U-238 based on the slight differences in the mass of the two isotopes.


Plutonium breaks down by spontanous fission, releasing several neutrons (depending on the isotope used). The neutrons, in turn, accelerate the fission of other plutonium atoms, resulting in an exponential increase of fission and an exponential release of energy.


For this process to cause an explosion, the cascade must consist of "fast neutrons." Nuclear reactors use reactions based on more controllable, slow neutron reactions.


The volume-to-surface ratio of the plutonium must also be minimal. A spherical shape is optimal. To explode an atomic bomb, conventional explosives compress plutonium, in a process referred to as implosion, to a size and shape that allows for the runaway reaction, resulting in the actual explosion. The Hiroshima bomb did not use implosion, but rather the rapid assembly of two sub-critical masses of U-235 brought about through a gun-like mechanism.


A pit of plutonium in a minimal fission bomb is about 8Kg, roughly the size of a large marble.


Thermonuclear Weapons (also Fusion bomb of Hydrogen bomb)


Even more energy can be released by a fusion reaction of two light atoms, usually hydrogen. Fusion is the energy source powering the sun. A hydrogen bomb works by fusing hydrogen atoms to helium. The extreme pressure necessary for that reaction is generated by radiation presure from a fission bomb. The fusion core of modern fusion weapons is lithium-7 deuteride (deuterium being an isotope of hydrogen).


The heavier the elements used, the less energy is released by fusion. In the other direction, the lighter the elements used, the less energy is released by fission. Both reactions meet at a low point in iron.


The first hydrogen bomb was developed by the U.S., but was the size of a box-car. It used twenty low-yield fission bombs arranged at the corners of an icosahedron surrounding a tank of lquid hydrogen.


In the Soviet Union, Andrei Sakharov developed the modern form of nuclear weapon. This design was later copied by U.S. intelligence services. A single small fission bomb, the trigger, is placed at the point of a cone-shaped arrangement of X-ray mirrors. The mirrors focuse 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 Uranium would be detonated by the intense fast neutrons from the fusion stage, increasing the yield of the bomb many times. The largest bomb ever exploded was of this type, a 60Mt 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 successful Sakharov design is the 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. The planned deployment was that civilians would construct buried radiation shelters, a facility unavailable to an advancing army. When the army would enter their area, the civilians would enter their shelters. The weapons would then be exploded high over the city, perhaps deployed by mortars. The soldiers would die, and the civilians could emerge with trees and grass dead, but the city otherwise undamaged. Experimental prototypes of this weapon have been tested in underground tests by the U.S., but development was halted due to public outcry. An "enhanced radiation warhead" was deployed instead. This is believed to have much higher blast and fallout effects, with higher associated collateral damage. A nuetron bomb would have a fairly small radius of damage compared to other weapons so a more realistic use of a weapon relesing large amounts of neutron flux is as a tactical weapon which could kill soldiers in their fortified tanks or bunkers - it is concievable that the tanks of bunkers would be cool enough to occupy after a period of several days.


Problems in nuclear weapons design


The design focus of modern weapons is to reduce their cost, increase their reliability, and decrease their weight, which increases the range of their delivery systems. Plutonium currently costs about $800,000 per kilogram because it has to be transmuted from Uranium in large special-purpose reactors. Plutonium can also be chemically extracted from the waste of power reactors. Most modern bombs use several techniques to reduce the amount.


First, a subcritical amount, a pit, is compressed to very high densities by explosive lenses that partially coppase the electron shells of the metal. The higher-than-normal densities pack many more nuclei into a smaller-than-normal volume, decreasing the critical mass.


Second, to help the subcritical mass detonate, a highly radioactive neutron emitter, often an isotope of Polonium, is placed in a carefully shaped void in or near the pit.


Third, as a further aid to the detonation, a neutron-emitting electronic tube is sometimes focused on the polonium trigger area. This tube is discharged by a bank of capacitors, and before it melts and implodes, generates a brief, intense beam of neutrons.


Fourth, Tritium is injected. As it is compressed, it fuses and emits additional neutrons.


The electronics sequences these activities in a precise timing 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.


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.


The toxic dust created by machining is contained by nested negative pressure zones. The air sucked from each containment is passed through "HEPA" filters. HEPA stands for high efficiency particulate affinity (or area). HEPA filters are now widely used to remove antigens by filtering air for allrgy sufferers. In most programs, contaminated filters are chemically processed to remove the valuable plutonium, and then trucked away and buried in designated waste dumps.


During storage, the pit's radioactivity damages the rest of the bomb, reducing storage times. Some bombs have shielding of some sort around the pit to increase the bomb's lifetime.


Using a subcritical mass also increases bombs' storage lifetimes. Most nuclear bombs have to be serviced every ten to fifteen years because the highly radioactive pit decays in place. In some cases, the pit has to be replaced because it has decayed. Pits decay fast enough that they feel warm to the touch.


Fusion bombs without transmutation


Some people have said that it might be possible to construct a nuclear weapon without plutonium. The plan would be to use a chemical explosive to compress a magnetic field, and then use the magnetic field to crush a conducting metal container to extremely high temperatures and pressures. The metal container would contain Tritium in some compact chemical form. The Tritium would then fuse. Auxiliary arcs might be used to preheat the Tritium. The result would be a large burst of fast neutrons that could then be used to cascade a raction in the stable isotope of Uranium, U-238. This scenario is believed to be the reason why Tritium is a controlled isotope in most countries.


There are indications that Tritium and Plutonium can be efficiently produced outside reactors by radio-frquency acceleration of particles that are then used to produce neutrons.


Advanced super centrifuges can also be used to refine isotopes of uranium, at far less expense thanthe methods used in the Manhattan project.


These developments may indicate that widespread proliferation of nuclear explosives cannot be easily prevented.


History


The military potential of explosives utilizing nuclear fission of isotopes of uranium and plutonium were understood in the early part of the 20th_century. 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 under the United States military with the goal of producing fission-based explosive devices. As it turned out, the German efforts to produce such weapons were haphazard, disorganized, splintered, and deeply hampered by Nazi policies on race which led to the loss of potentially useful Jewish theoreticians like Albert Einstein, and Hitler was not willing to invest much time or effort into weapons which showed little sign of fruition in a time-table which would have been useful to him.


The secret project to develop atomic weapons in the US during the World War II era was called the Manhattan Project.

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, Hanford, Washington and Oak Ridge, Tennessee 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 without "island hopping". 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 wepons are the only use of atomic weapons in warfare.


The choice of civilian instead of military targets has often been criticized. However, the U.S. already had a policy of massive incendiary attacks against civilial targets in Japan. These dropped 20% explosives, to break up wooden structures and provide fuel, and then dropped 80% (by weight) small incendiary bombs to set the cities on fire. The resulting raids completely destroyed many Japanese cities, including Tokyo, even before atomic weapons were deployed. The allies (Australian and New Zealand forces also participated) performed such attacks because Japanese industry was extremely dispersed among civilian targets, with many tiny family-owned factories operating in the midst of civilian housing.


There is excellent evidence that Japan actively worked toward development of atomic weapons during World War II, and planned to deploy them against the U.S. 7th fleet. The Japanese bomb effort initially started in Tokyo University, and was coordinated with and funded by the powerful Department of the Navy. In early 1945, U.S. bombing raids destroyed the isotope separation plant near Tokyo. To prevent further delays, the program was moved to the Korean highlands (then a secure Japanese possession), to place it out of range of B-29 raids in a historically-secure area with abundant hydroelectric power. This move delayed the program by about three months.


There is some speculation that the Japanese program may have successfully tested a low-yield nuclear weapon in the China sea the day before Nagasaki was bombed. However, the general consensus is that the Japanese program, largely due to industrial resource limitations, never was close to an

actual working bomb.


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. 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.


Heavy water reactors (CANDU reactors) were successfully developed by Canada and are still sold by them.


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.


For a relatively brief time, the United States had a monopoly in the area of nuclear weapons, but Soviet scientists, aided by intelligence agents, were able to produce fission-based explosives.


Early delivery systems for nuclear devices were primarily bombers like the American 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.


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.


South Africa also sucessfully developed and tested a uranium based nuclear weapons, but dismantled its nuclear weapon program in the 1970s. 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.


Effects of a thermonuclear weapon


There are four main effects - blast, wind, thermal and fall-out.

Blast effects are a simple cube root of bomb yield. Solid objects that have no openings are damaged by static overpressure. For more common objects, such as houses, there is crushing damage but also as the shock-wave passes it is followed by a low pressure wave. The pressure difference, and the fact that houses have very little resistance against expansion, means that buildings will usually explode. Houses within 8km of the blast would be destroyed or very badly damaged, with windows being blown out as far as 20km away.


Structures with large surface areas are most vulnerable to wind drag. Depending on distance and weapon yield the effects of wind drag become more significant with larger bombs. Objects such as aircraft, trees and girder towers are more damaged by wind than blast.


Humans are moderately blast resistant. The pressure effect of nuclear weapons lasts several seconds and 3.5kg/sq cm would kill around half those exposed (Conventional blasts have a much shorter pressure effect, and around 15kg/sq cm is need to kill in a fraction of a second). Therefore very few people would be killed by pressure effects from a blast, flying debris would be a much more serious hazard. And this threat would extend out to about 7.5km from ground zero (about 0.21kg/sq cm). Standing in the open a person would be blown over 12km away, or if lying flat moved at 5km away.

Apart from housing, which is very vulnerable, most objects would survive a single small nuclear blast.


Thermal radiation is released from the bomb in two parts - one brief blast at initiation and two seconds later a longer ten second pulse containing around a third of the bomb's energy. In 'ideal' conditions the effective thermal injury distance for a 1MT blast is 12km (causing extensive second-degree burning). Flammable objects may be effected within 14km of a 5MT burst, but the 'firestorm' effect is unlikely.


The effects of fallout are difficult to judge but are likely to be the major hazard from a terrorist or small nuclear attack. Fallout effects vary due to bomb delivery, bomb type, weather conditions, what precautions people take and other factors. Air bursts have much less fallout, because they do not suck the ground into the fire-ball. Air-bursts cause blast damage more effectively than ground bursts and would probably be used in government attacks on cities, battlefields and airfields. Nuclear terrorists, and attacks on hardened missile fields would be ground bursts. The radiation exposure from a 1Mt ground burst would start about two miles wide, at a rate of 30,000 RADs/hr. A dose of 450 Rads accumulated in less than a month is enough to kill half of a population. Over the next three days or so, the fallout expands in a long, narrow wind-borne ellipse. Over this period radiation falls off logarithmically to 100 rads/hr. In this period, the fallout could range as far as several hundred miles away. The number of radiation casualties from a 1MT burst could range from zero to millions depending on the direction of wind, and the population density. See fallout shelter.


(all of the following numbers are for a nominal 1 Megaton weapon)


Air burst

Within a thousandth of a second, the bomb heats to ten million degrees C and emits many soft X-rays.
The X-rays are absorbed in nearby air.
The air is heated to incandescence, and rather resembles a flash-bulb or strobe light. It is brighter than the sun. This is the fire-ball.
The fireball immediately begins to expand.
The surrounding air is heated by compression and an acoustic shock-wave forms around the fireball. In humid air, the shockwave condenses water droplets and looks like a grey shell. The shock-wave expands more quickly than the fireball, heating the air to around 2000 deg. C.
The fireball, expanding behind the shock-wave cools to an apparent temperature of 8000 deg. C and in ten seconds a third of the blast energy is expended as heat, light, ultraviolet and X-rays.
The fireball expands to about 750m in radius, with a blast overpressure at its edge of around 4.2kg/sq cm. The expansion has been so rapid that a vacuum has formed at the centre of the blast. The density of the fireball has fallen below that of the atmosphere and the fireball starts to rise.
The surrounding air forms a ring-shaped annular vortex as the fireball drags it up.
The ring "scrapes off" the edges of the fireball, and leaves a trail as it rises, creating the classic mushroom cloud.
The shock-wave, which causes most of the damage, has continued to expand in two spheres - from the point of detonation and the reflected blast from the ground below.
Ten seconds from starting the emission of thermal radiation is complete, the blast has reached 5000m from ground zero with a wind speed of 290km/h.
By 40 seconds the blast front is 15000m away, overpressure is down to 0.07kg/sq cm and wind speed is 64km/h. The mushroom cloud is 11000m high and the immediate destructive effects are over.

Ground burst

If detonation occurs at less than one tenth of the final fireball diameter above the ground a crater around 400m across and 45m deep is formed. Overpressure effects are reached at about 75% range of air-burst. There is also an earth tremour effect within three crater radii.


Space burst

In a space burst, most damage occurs from the X-rays emitted by the bomb core.


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).


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


Glossary:

  • MIRV -- Multiple Independent Re-entry Vehicles, nuclear devices carried, usually ten or twelve at a time on a single ICBM, allowing a single launched missile to strike a handful of targets, and allowing a few missiles to strike several targets redundantly.
  • SALT II -- A treaty designed to further limit the growth of US and Soviet missile arsenals.
  • START -- STrategic Arms Reductions Treaty -- A treaty proposed by Ronald Reagan to reduce the numbers of missiles and warheads.
  • GLCM -- Ground Launched Cruise Missile.
  • ALCM -- Air Launched Cruise Missile.
  • SLCM -- Submarine Launched Cruise Missile.
  • SLBM -- Submarine Launched Ballistic Missile.
  • Ballistic Missile -- A missile using a ballistic trajectory involving a significant ascent and descent including suborbital and partial orbital trajectories.
  • Cruise Missile -- 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.
  • MAD -- Mutual Assured Destruction. The doctrine of preventing nuclear war by creating a situation in which any use of nuclear weapons would result in the certain destruction of both the attacker and the defender.
  • ABM -- Anti-Ballistic Missile. Missiles designed to intercept and destroy ballistic missiles. Can also refer to the ABM treaty, signed by Richard Nixon and Leonid Brezhnev, which halted the development and use of such systems due to fears that such systems could counter the MAD scenario and, thereby, increase the likelihood that an ABM protected country would use their nuclear weapons aggressively.
  • SDI -- Strategic Defense Initiative, more commonly known as Star Wars. A system proposed by Ronald Reagan to use space-based systems to detect, intercept and destroy ICBMs and MIRVs. Criticized for its costs, doubts that it would be effective, and concerns that it would violate the ABM treaty and offset MAD, it was not supported by the US Congress at that time.


Relevant individuals:



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; as well as less-famous films such as Miracle Mile. 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, U.S. Government Printing Office, 1977 (third edition)Available Online


Smyth, H. DeW., Atomic Energy for Military Purposes, Princeton University Press, 1945.


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


/Talk


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