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A-bomb, short for atomic bomb, is a type of nuclear weapon. Atomic bomb is sometimes used either to mean all nuclear weapons (fusion or fission). However, this article deals only with fission weapons which are commonly called A-bombs. See also nuclear weapon design.
A-bombs can be fueled by several different fissionable elements, and fissionable isotopes, including U-235, U-233, Pu-239, Pu-240, Pu-242, Americium, Californium, and others.
Key elements
Plutonium 239, and Uranium 235 have so far been the fissionable elements used in atomic bombs. The mean neutron free path (MNP) is the average distance that a free neutron travels after its emission, before it will either be absorbed by a fissionable or non-fissionable atom. The length of the MNP is generally 126.8 millimeters. When a free neutron is absorbed by a fissionable atom it will cause the atom to divide into two or more pieces known as fission fragments. Less than 0.10% of the mass of the fissioned atom is thus converted into atomic energy. Nuclear fission converts less then 1/1000 th of the mass of the nuclear fuel into energy.
Use and Effects
The A-bomb has been one of mankind's most destructive inventions. Their effects and after-effects are terrible. The US bomb at Hiroshima killed more than 140,000 people either immediately or in later months from horrific burns or radiation. See Atomic bombings of Hiroshima and Nagasaki. H-bombs however, are far more destructive then A-bombs. There is in fact no limit: to the distructive power of H-bombs.
Physics
The equivalence of energy and matter can be expressed as
- E = mc.
The amount of energy contained in 1 kg of mass is approximately 9×10 joules. The nuclear fission of a single kilogram of fuel will therefore release approximately 9×10 joules of energy. This is equivalent to the energy that would be released by the detonation of from 17,000–23,000 tons of TNT.
The equation for U-235 fission may be summarized as follows: U-235 + 0 N1 = fission fragments + 2.5 0N1 + 180 MeV. The equation for Pu-239 fission is generally as follows: Pu-239 + 0 N1 = fission fragments + 3 0N1 + 200 MeV. When a heavy atom undergoes nuclear fission it breaks into two or more fission fragments. Each of these fission fragments is an atom of a more lightweight element on the periodic table of the elements.
Weapons grade U-235 is obtained by refining natural uranium ore, enriching the U-235 content from 0.7% to 80% U-235 or more. Plutonium 239 is bred in atomic fission reactors from natural uranium 238 atoms that capture neutrons and then change into Pu239 atoms via the following nuclear reactions:
- U-238 + n = U-239;
- U-239 + β = Np-239;
- Np-239 + β = Pu-239.
The reactions that produce Pu-239 from U-238 that is irradiated by neutrons are beta particle decay processes. Beta particles can be either electrons, or positrons. Beta decay is how a neutron decays to form a proton. The reaction is n + e + neutrino = proton. This reaction can also be n + e + neutrino = proton.
A few sheets of paper, or metal foil is sufficient to completely block alpha particles, and beta particles. When a neutron in the nucleus of a radioactive isotope such as U-239, or Pu-239 undergoes beta decay, the neutron changes into a proton inside the nucleus of an atom. When this occurs the atom of one element is thus changed into the atom of another element. How many protons are present in the nucleus of an atom , determines which element the atom is on the periodic table of the elements. The number of protons that are present inside the nucleus of an atom is known as the atomic number of the element on the periodic table. How many neutrons are present in the nucleus of an atom determines what isotope of a given element on the periodic table the atom is.
U-235 atoms naturally undergo spontaneous nuclear fission at a certain rate over time. For this reason U-235 will naturally generate a self-sustaining nuclear fission reaction when a critical mass of it is assembled by any method. Pu-239 undergoes an alpha particle decay process.
The element helium has two isotopes. These are He and He. Alpha particles are always high energy positively charged ions of He, or He. Some radioactive elements such as radium, plutonium, polonium, and some others undergo radioactive decay by emitting (alpha particles) which are high energy helium ions from their atomic nucleus.
The difference between He, and He, is that He has one more neutron in its atomic nucleus then does He. Alpha particles can be He, or He ions. The half life of U-235 is about 250,000 years. The half life of U-238 is 4,500,000,000 years.
The half life of Pu239 is 24,000 years. The radioactive half life is the amount of time that is required for 50% of the atoms in a given quantity of a radioactive element, or radioactive isotope to decay into stable non-radioactive elements, or isotopes.
A self-sustaining nuclear fission chain reaction requires that each of the 2.5 to 3 neutrons released per fission event must cause another fissionable atom to actually undergo nuclear fission. So when k = 1/2.5 , or k = 1/ 3 this condition is known as a critical mass. When k < 1/2.5 or k > 1/3 this defines a sub-critical mass.
K defines the statistical probability that a lone neutron will cause an atom to undergo nuclear fission by the time it is absorbed. In order to be absorbed, a free neutron must travel on average a distance of 12.68 centimeters from the point of its origin. The assembly of a supercritical mass of fissionable material requires that k = 1.68 /2.5 or K = 2/3. 126.8 millimeters * 0.67 = 84.956 mm which is a radius of 3.345 inches. A solid ball of U-235 metal that has a radius of 3.45 inches will thus be a critical mass.
The purity of the fissionable material directly affects the value of K. The density, and shape of the fissionable material also directly effects the value of K. Whether or not a mass of fissionable material is surrounded by a neutron reflector also directly effects the value of k.
Therefore a supercritical bare metal ball of 100% pure U-235 will have a diameter of 6.69 inches which is 17 centimeters. The density of uranium is 18.9 g/cm³, and the density of plutonium is 19.1 g/cm³. The volume of a solid sphere is equal to 4/3 * 3.14259 * R. These numbers allow us to calculate the exact dimensions, shape, and mass of a ball of critical, or supercritical fissionable material.One inch is eqaul to 2.54
centimeters.
A neutron must travel through a solid sphere of fissionable material that has a radius = .4 * 12.68 Centimeters in order to have a 40% probability of fissioning an atom and thus make a single atomic critical mass. This is a distance of 50.72 mm which is a radius of 1.9968 inches.
The smallest ball of U-235 or Pu-239 that will barely constitute one single critical mass must therefore have a diameter of 101.44 mm. This calculation applies only to bare metal spheres that do not have a neutron reflector wrapped around them.
- In order to trigger an efficient atomic explosion it is necessary to assemble a solid sphere made of two or more critical masses of metallic fissionable material.
The critical mass for a bare metal ball of a pure U-235 ball with no neutron reflector wrapped around it is approximately 50 kg. The critical mass for a bare metal ball of plutonium 239 with no neutron reflector wrapped around it is 16 kilograms. The gun method of detonating an atomic bomb creates a super critical mass by propelling enough fissionable material together into a common metal ball to create a supercritical size.
Elements of fission type nuclear weapon design
In the gun method the pit is assembled by bringing two or more sub-critical masses of fissionable material together using a chemical explosive such as cordite. This may be done in the following manner.
Place two 30 kg hemispheres of 90% pure U-235 metal at opposite ends, of a 2 - 3 meter long, two inch thick steel gun (artillary) barrel. Plug one end of the gun barrel with a steel block. Place the U-235 target hemisphere in front of this steel plug. Then insert a screw on metal cartidge containing a 30 kg hemisphere of U-235: at the opposite end of the gun barrel from the U-235 target hemisphere . Inside the screw on cartidge the U-235 hemisphere (bullet ) is backed by a charge of cordite . Cordite is white smokeless gunpowder.
In the manhattan project during world war 2 : consideration was given to making a gun type A-bomb that was fueled by Plutoniun 239. This A-bomb design was called "Thin Man" . The Plutonium 239 made during the second world war had a large number of Plutonium 240 atoms in it. Pu-239 atoms naturally undergo spontaneous nuclear fission. This greatly increased the number of free neutrons, naturally present inside of the plutonium 239 fuel.
The decison not to build the "Thin Man" A-bomb design was made: because those in the manhattatan project feared this would cause the pre-detonation of the plutonium 239 fuel . A predetonation of the Pu - 239 , would result in an atomic explosion, but it would be a low yield fission fizzle. During the second world war, there was very little weapon grade plutonium 239 fuel available for use.
The 7.33 meters or more long gun barrel required by the "Thin Man" A-bomb design , was too long to carry on a B-29 bomber . The " Thin Man" design was also too heavy to be delivered on a B-29 bomber during the war.After the war ended , post war atomic research ,showed that a successful plutonium 239 fueled Gun type A-bomb could be achieved. This could achieved ,by using a gun barrel that was at least 7.333 meters in length .Thin Man was never actually developed after the second world war. This was due to the considerable successes achieved by the use of the implosion Pu-239 pit assembly system.
The B-29 was the best available american nuclear weapon delivery system until the giant back mounted 8 propeller engine B-36 Bomber, came into use in 1948-1949. Later in 1953-1954 the first B-47 Jet bombers were entered into service with S. A. C. Sac was the U. S. Strategic air command) units of the United States Air Force. Beginning in 1955-1956 the first B-52 Jet bombers entered into service. Some of them are still in service in the year 2005.. These are the B-52 G models. The old B-52s are now being gradually replaced by the newer stealth type, B-1, and B-2 bombers.
In Gun type A-bombs , the cordite charge should be just the right mass to propel the U-235 hemisphere through the gun barrel at a muzzle velocity of 300 to 400 m/s when it explodes. In order to ensure that the explosion of the cordite will not rupture the gun barrel, the hollow cylindrical steel gun barrel should have at least 100 times the mass of the U-235 Hemisphere (bullet). This will assemble the equivalent of at least two critical masses in the pit. This will result in an 8-10 kiloton atomic explosion.
The mass, and size of a basic, primitive device like the Little Boy A-bomb of 1945 is usually very large. For this reason primitive versions of this kind of device are probably useless to anyone other than mid-20th century moderately developed industrial nation states.
It is however, possible to produce advanced gun type atomic weapons that have a much lower mass, and a much smaller size. This is possible if the nuclear weapon technology is sufficiently sophisticated, and modern. Some advanced modern gun-type atomic weapons may have only 5% or 10% of the mass of the original Little Boy A-bomb of 1945. Such advanced designs have often been used to create atomic artillery shells, atomic land mines, atomic bunker busters, atomic depth charges, and military special operations atomic weapons.
The atomic fission yield can be increased by: 1. adding more fissionable fuel to the pit; 2. using fusion neutron boosting on the pit; 3. using a more efficient neutron reflector wrapped around the pit such as tungsten carbide, or berylium metal. It is also possible to add a secondary fusion charge, and use the gun type atomic bomb as the primary to trigger a thermonuclear explosion via the teller-ulam radiation implosion technique.
It is important for the mass of the steel gun barrel to be at least 3000 kg if a 30 kg projectile is fired in the barrel. By the time atomic radiation shielding, the fuze, and any secondary physics package is added the device mass may reach 4000 to 5000 kg, like the Little Boy A-bomb of 1945.
The critical mass can be reduced by placing a hollow spherical tamper made of U-238, Berylium 9, or Tungsten carbide around a solid, or hollow sphere of the fissionable material. The critical mass for a ball of U-235 surrounded by a steel neutron reflector is 27 kg. In addition to reducing the critical mass, the tamper can also increase the yield of the A-bomb when it explodes by slowing down the disassembly of the core. The longer it takes to disassemble a core that is surrounded by a heavy massive tamper, the larger the fraction of the mass of the atomic fuel that will be fissioned during the explosion.
If we surround the U-235 ball with tungsten carbide the critical mass is 23 kg. If we surround the U-235 ball with a berylium neutron reflector the critical mass is reduced to 20 kg. If we surround a ball of plutonium 239 with a neutron reflector made of U-238, the critical mass is reduced to 10 kg of Pu239. If we surround the plutonium core with a berylium neutron reflector the critical mass is reduced to 6.82 kg of Pu239. If the core of fissionable fuel is, for example, only 90% pure then the above critical masses must be actually multiplied by 1.1.
Also, if we increase the density of the fissionable material by a factor of two this will assemble the equivalent of 4 critical masses of fissionable material. This is how the implosion method of detonating an A-bomb converts its atomic core or (pit) into a super-critical mass. In the implosion method of detonating an Atomic bomb, a hollow sphere of U-235 or Pu-239 is placed inside of a hollow spherical tamper made of U-238, berylium 9, or tungsten carbide, which is then placed inside a hollow spherical metal pusher. A neutron source can also be placed inside the hollow sphere of fissionable material if a solid core christy device design is used. The original Fat Man A-bomb of 1945 used a solid core christy device design.
The concentric hollow spheres discussed above make up the pit. Then the pit is placed inside of the center of a hollow sphere of high explosive. This explosive hollow sphere is made of a castable explosive such as TNT, or a plastic bonded high explosive such as C-4 plastic explosive. To improve implosion efficiency the explosive hollow sphere may be made of a combination of both the slowest, and fastest detonating chemical high explosives.
A hollow sphere of castable high explosive can be made of one hollow steel sphere , that is filled with Liquid TNT or liquid preparation B. After half of the liquid castable explosive has been poured and has nearly hardened, the pit may be inserted through a polar service cap on top of the hollow sphere of steel.
Then the rest of the explosive is inserted, the cap is sealed, and the explosive then hardens into a hollow sphere around the pit. This will create a spherical high explosive shaped charge in which the hollow steel sphere serves as a spherical lens that will reflect 100 % of the shock waves created by the detonation of the high explosive onto the pit at the center of the hollow sphere.
The behavior of all the kinds of waves that exist in the universe, are governed by the same universal physical laws. The laws of physics that govern the behavior of all waves, allow us to know exactly how to shape, focus, and control the shock waves that are created by the detonation of any configuration of high explosives.
All high explosive lenses use curved parabolic, conical or spherical wave reflectors to focus all of their shock waves to their focal point. High explosive lenses are often called high explosive shaped charges. There are spherical, conical, tetragonal, hexagonal, octagonal, cutting, and scissors type high explosive lenses et el. To learn more about this, look up and read about shaped high explosive charges. Also look up, and read about high explosives.
There are up to 100 symmetrical high explosive detonators placed on the outer surface of the hollow steel sphere. They are wired into series circuits. These detonators are exploded simultaneously by a pulse of high electric current. This results in the simultaneous detonation of 100% of the hollow sphere of high explosive. The hollow sphere of steel then serves as a spherical mirror that reflects the shock waves to it's focus which has the atomic pit at its center.
In this way the hollow sphere of steel actually serves as a spherical high explosive lens. The diameter of the hollow spherical assembly of high explosives may be from 18 to 60 inches (0.5 to 1.5 m). In order to achieve at least a two-fold increase in the density of the atomic pit, the mass of the high explosive sphere should be at least six times the mass of the amount of U-235, or Pu239 that is actually used in the pit. The mass of the high explosive sphere should not exceed 60 times the mass of the fissionable fuel used in the pit.
A two fold increase in the density of the atomic pit will tend to result in a 10-20 kiloton atomic explosion. A 3-fold compression may produce a 40-45 kiloton atomic yield, a four fold compression may produce a 60-80 kiloton atomic yield, and a five-fold compression of the pit which is very hard to get, may produce an 80-100 kiloton atomic yield. Getting a 5-fold compression of the pit requires a very strong, massive, and very efficient, lens implosion system.
The degree of compression, and thus density, increase achieved by the lens implosion system has a direct effect on the atomic yield, of an A-bomb. Pure fission yields from implosion type A-bombs, without using fusion neutron boosting, rarely exceed 100,000 tons of TNT equivalent. The atomic yield for a weak lens implosion system can be as low as 100 tons of TNT equivalent, or even less. The atomic yield for a strong, and good lens implosion system can be as high as 100,000 or even 150,000 tons of TNT equivalent.
The basic, primitive, and rudimentary versions of these implosion type atomic weapon designs tend to result in large, massive weapons like the original 10,200 pound (4600 kg) Fat Man A-bomb of 1945. For this reason primitive implosion type nuclear devices are in fact useful only to moderately developed mid-20th century style industrial nation states.
There are however, modern, and advanced implosion type atomic weapon designs that are only 5% or 10% of the size, and mass, of devices like the Fat Man A-bomb of 1945. The arsenals of most modern nuclear weapon states are dominated by such low mass, and compact modern nuclear weapon designs. Since the late 1950s almost all nuclear weapon tests, have been done for the purpose of producing, smaller, more efficient, and more lightweight nuclear weapons.
The United States ceased live fire under ground nuclear weapons tests in the year 1994. Now, highly sophisticated computer simulations, and computerized mathematical models of weapon physics are used to evaluate nuclear weapon designs. These computer models are called nuclear codes. These models, and codes have been compiled from 50 years of data, and calculations that were obtained from live nuclear weapon tests. The United States has conducted more than 1400 live nuclear weapon tests underground, underwater, in the atmosphere, and in space since the first test (Trinity )on July 16,1945 near Alamogardo, New Mexico.
Alternatively, the hollow sphere of high explosive used to trigger implosion type A-bombs can be constructed by assembling three layers, of overlapping high explosive charges, around the pit. Each of these specially shaped high explosive charges will be a curved parabolic segment that will serve as a high explosive lens, by reflecting its shock waves to its central focus. Each shock wave from each explosive segment will have a common point of convergence with the other high explosive shock waves.
In order to prevent premature disassembly of the pit by the squeezing, and jetting of the pit through the points at which the shock waves converge, it is necessary for the shock waves of the second high explosive layer to overlap and reinforce the shock waves of the first layer at the 1st layer shock wave convergence points.
Then the shock waves of the third and last layer of explosive charges will overlap and reinforce the shock waves of the second layer at its shock wave convergence points. This allows all the individual shock waves to be merged into one perfect ingoing spherical implosion wave. This implosion wave compresses the pit and thus increases its density by forcefully pushing its atoms closer to each other for a microsecond or so. This renders the pit supercritical, and results in an atomic explosion that could be be equal to the explosive force of 20,000 tons of TNT or more.
Alternatively, a hollow sphere of fissionable material that contains a spherical vacuum cavity inside of it may be used as the pit in an implosion type A-bomb. A gram or so of tritium gas can be inserted inside the vacuum cavity to achieve fusion neutron boosting if this is desired when the A-bomb is detonated. Many modern atomic weapons use hollow core pits, and fusion neutron boosted hollow core pits.
The Orion Project, and the Plow Share Project
From 1958 to 1965 The U. S government ran a project to design a nuclear bomb powered nuclear pulse rocket called Project Orion. Never built, this vessel would use repeated nuclear explosions to propel itself, and was considered surprisingly practical, and thought to be a feasible design for interstallar travel.
From 1958 to 1973 the U. S government also ran a related project called the Plow Share project. The purpose of the Plow Share project was to use peaceful nuclear explosions for moving and lifting enormous amounts of earth and rock during construction projects such as building reservoirs.
At the height of the cold war there were more then 100,000 nuclear weapons stockpiled
on the planet earth. These weapons were the result of the U. S / Soviet cold war.
Plutonium 239 has a half life of 24,000 years, and Uranium 235 has a half life of up to 250,000 years. Many of these weapons have been decommissioned and removed from the nuclear stock piles of the world since the Union of Soviet Socialist republics desolved in December 1991.
References
The information in this article is drawn from many sources including, but not limited to, the following sources.
- Smith atomic weapons report to U. S. Congress, and United Nations 1946-1947.
- Los Alamos Primer 1943.
- Encyclopedia Brittanica article on nuclear weapons.
- Posted histories of U.S nuclear weapons development on internet 1945 - 1993
- The summary of the published history, and records of the Manhattan Project as published by Los Alamos laboratory, 1967.
- Nuclear weapons archive on internet.