The introduction of this weapon has effectively changed the landscape of warfare, as a single atomic bomb can kill millions of people.
See also nuclear war and ABC.
Structure and Design
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). 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.
Plutonium breaks down by spontanous fission, releasing several neutrons (depending on the isotope used). The neutrons, in turn, cause 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 to a size and shape that allows for the runaway reaction, resulting in the actual explosion.
A pit of plutonium in a minimal fission bomb is about 8Kg, roughly the size of a large marble.
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 an atomic bomb. The fission reaction takes place in a strong metal case that can contain the atomic explosion for a fraction of a second, which is long enough for the hydrogen inside the metal case to initiate a fusion reaction. 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.
Later bombs included a 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, 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.
Another variant uses Cobalt in the shell, and the 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 wepon 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.
The design focus of modern weapons is to reduce their cost and increase their reliability. Plutonium currently costs about $800,000 per kilogram because it has to be transmuted from Uranium in large special-purpose reactors, and reprocessing form waste has been politically forbidden. 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. Plutonium has machining and mechanical properties much like stainless steel, but is more toxic and much more dense.
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.
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 controller isotope.
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 to the Japanese city of Nagasaki, after which Japan did surrender.
Only two atomic bombs have ever actually been used in warfare, by the USA against the Japanese cities Hiroshima and Nagasaki. The second explosion led to the immediate surrender of Japan, accelerating the end of World War II.
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 developed 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. 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 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 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 missile]?s 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 is thought to have developed and tested a nuclear weapon, 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.
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Humor and Culture
Nuclear weaponry has become a part of our culture. Films featuring nuclear war include Dr. Strangelove or, How I Learned to Stop Worrying and Love the Bomb, and [The Day After]?.
Remember, almost doesn't count except in horseshoes, hand grenades, and H bombs.
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