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Nuclear Weapons

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 the explosion disperses much of the uranium and plutonium without undergoing fission. Such unfissioned nuclear material decays slowly 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 various elements, such as sodium, manganese, aluminum and silicon in the soil. This is a negligible hazard because of the limited area involved.

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.

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.

Designs of Nuclear Bombs

To build an atomic bomb, you need:

  • A source of fissionable or fusionable fuel

  • A triggering device

  • A way to allow the majority of fuel to fission or fuse before the explosion occurs (otherwise the bomb will fizzle out)

Fission Bombs (Atomic Bombs)

A fission bomb uses an element like uranium-235 to create a nuclear explosion. Uranium-235 has an extra property that makes it useful for both nuclear-power production and nuclear-bomb production -- U-235 is one of the few materials that can undergo induced fission. If a free neutron runs into a U-235 nucleus, the nucleus will absorb the neutron without hesitation, become unstable and split immediately.

If a uranium-235 nucleus with a neutron approaching from the top. As soon as the nucleus captures the neutron, it splits into two lighter atoms and throws off two or three new neutrons (the number of ejected neutrons depends on how the U-235 atom happens to split). The two new atoms then emit gamma radiation as they settle into their new states. There are three things about this induced fission process that make it interesting:

  • The probability of a U-235 atom capturing a neutron as it passes by is fairly high. In a bomb that is working properly, more than one neutron ejected from each fission causes another fission to occur. This condition is known as super criticality.

  • The process of capturing the neutron and splitting happens very quickly, on the order of picoseconds (1*10E-12 seconds).

  • An incredible amount of energy is released, in the form of heat and gamma radiation, when an atom splits. The energy released by a single fission is due to the fact that the fission products and the neutrons, together, weigh less than the original U-235 atom.

The difference in weight is converted to energy at a rate governed by the equation e = mc2. A pound of highly enriched uranium as used in a nuclear bomb is equal to something on the order of a million gallons of gasoline. When you consider that a pound of uranium is smaller than a baseball and a million gallons of gasoline would fill a cube that is 50 feet per side (50 feet is as tall as a five-story building), you can get an idea of the amount of energy available in just a little bit of U-235.

In order for these properties of U-235 to work, a sample of uranium must be enriched. Weapons-grade uranium is composed of at least 90-percent U-235.

Fusion Bombs

Fission bombs worked, but they weren't very efficient. Fusion bombs, also called thermonuclear bombs, have higher kiloton yields and greater efficiencies than fission bombs. To design a fusion bomb, some problems have to be solved:

  • Deuterium and tritium, the fuel for fusion, are both gases, which are hard to store.

  • Tritium is in short supply and has a short half-life, so the fuel in the bomb would have to be continuously replenished.

  • Deuterium or tritium has to be highly compressed at high temperature to initiate the fusion reaction.

First, to store deuterium, the gas could be chemically combined with lithium to make a solid lithium-deuterate compound. To overcome the tritium problem, the bomb designers recognized that the neutrons from a fission reaction could produce tritium from lithium (lithium-6 plus a neutron yields tritium and helium-4; lithium-7 plus a neutron yields tritium, helium-4 and a neutron). That meant that tritium would not have to be stored in the bomb. Finally, it was recognized that the majority of radiation given off in a fission reaction was X-rays, and that these X-rays could provide the high temperatures and pressures necessary to initiate fusion. Therefore, by encasing a fission bomb within a fusion bomb, several problems could be solved.

Teller-Ulam Design of a Fusion Bomb

To understand this bomb design, imagine that within a bomb casing you have an implosion fission bomb and a cylinder casing of uranium-238 (tamper). Within the tamper are the lithium deuteride (fuel) and a hollow rod of plutonium-239 in the center of the cylinder. Separating the cylinder from the implosion bomb is a shield of uranium-238 and plastic foam that fills the remaining spaces in the bomb casing. Detonation of the bomb caused the following sequence of events:

  1. The fission bomb imploded, giving off X-rays.

  2. These X-rays heated the interior of the bomb and the tamper; the shield prevented premature detonation of the fuel.

  3. The heat caused the tamper to expand and burn away, exerting pressure inward against the lithium deuterate.

  4. The lithium deuterate was squeezed by about 30-fold.

  5. The compression shock waves initiated fission in the plutonium rod.

  6. The fissioning rod gave off radiation, heat and neutrons.

  7. The neutrons went into the lithium deuterate, combined with the lithium and made tritium.

  8. The combination of high temperature and pressure were sufficient for tritium-deuterium and deuterium-deuterium fusion reactions to occur, producing more heat, radiation and neutrons.

  9. The neutrons from the fusion reactions induced fission in the uranium-238 pieces from the tamper and shield.

  10. Fission of the tamper and shield pieces produced even more radiation and heat.

  11. The bomb exploded.

All of these events happened in about 600 billionths of a second (550 billionths of a second for the fission bomb implosion, 50 billionths of a second for the fusion events). The result was an immense explosion that was more than 700 times greater than the Little Boy explosion: It had a 10,000-kiloton yield.

Critical Mass

In a fission bomb, the fuel must be kept in separate sub critical masses, which will not support fission, to prevent premature detonation. Critical mass is the minimum mass of fissionable material required to sustain a nuclear fission reaction. This separation brings about several problems in the design of a fission bomb that must be solved:

  • The two or more sub critical masses must be brought together to form a supercritical mass, which will provide more than enough neutrons to sustain a fission reaction, at the time of detonation.

  • Free neutrons must be introduced into the supercritical mass to start the fission.

  • As much of the material as possible must be fissioned before the bomb explodes to prevent fizzle.

To bring the sub critical masses together into a supercritical mass, two techniques are used:

Gun-Triggered Fission Bomb

The simplest way to bring the sub critical masses together is to make a gun that fires one mass into the other. A sphere of U-235 is made around the neutron generator and a small bullet of U-235 is removed. The bullet is placed at the one end of a long tube with explosives behind it, while the sphere is placed at the other end. A barometric-pressure sensor determines the appropriate altitude for detonation and triggers the following sequence of events:

  • The explosives fire and propel the bullet down the barrel.

  • The bullet strikes the sphere and generator, initiating the fission reaction.

  • The fission reaction begins.

  • The bomb explodes.

Little Boy was this type of bomb and had a 14.5-kiloton yield (equal to 14,500 tons of TNT) with an efficiency of about 1.5 percent. That is, 1.5 percent of the material was fissioned before the explosion carried the material away.

Implosion-Triggered Fission Bomb

Early in the Manhattan Project, the secret U.S. program to develop the atomic bomb, scientists working on the project recognized that compressing the sub critical masses together into a sphere by implosion might be a good way to make a supercritical mass. There were several problems with this idea, particularly how to control and direct the shock wave uniformly across the sphere. But the Manhattan Project team solved the problems. The implosion device consisted of a sphere of uranium-235 (tamper) and a plutonium-239 core surrounded by high explosives. When the bomb was detonated, this is what happened:

  • The explosives fired, creating a shock wave.

  • The shock wave compressed the core.

  • The fission reaction began.

  • The bomb exploded.

Fat Man was this type of bomb and had a 23-kiloton yield with an efficiency of 17 percent. These bombs exploded in fractions of a second. The fission usually occurred in 560 billionths of a second.

In a later modification of the implosion-triggered design, here is what happens:

  • The explosives fire, creating a shock wave.

  • The shock wave propels the plutonium pieces together into a sphere.

  • The plutonium pieces strike a pellet of beryllium/polonium at the center.

  • The fission reaction begins.

  • The bomb explodes.

Neutrons are introduced by making a neutron generator. This generator is a small pellet of polonium and beryllium, separated by foil within the fissionable fuel core. In this generator

  • The foil is broken when the sub critical masses come together and polonium spontaneously emits alpha particles.

  • These alpha particles then collide with beryllium-9 to produce beryllium-8 and free neutrons.

  • The neutrons then initiate fission.

Finally, the fission reaction is confined within a dense material called a tamper, which is usually made of uranium-238. The tamper gets heated and expanded by the fission core. This expansion of the tamper exerts pressure back on the fission core and slows the core's expansion. The tamper also reflects neutrons back into the fission core, increasing the efficiency of the fission reaction.

Nuclear Weapon Accident

An unexpected event involving nuclear weapons or nuclear weapons components that results in any of the following:

  • Accidental or unauthorized launching, firing, or use, by U.S. forces or supported allied forces, of a nuclear-capable weapon system which could create the risk of an outbreak of war;

  • Nuclear detonation, non-nuclear detonation or burning of a nuclear weapon or radioactive weapon component, including a fully assembled nuclear weapon, an unassembled nuclear weapon, or radioactive nuclear weapon components;

  • Radioactive contamination;

  • Seizure, theft, or loss of a nuclear weapon component, including jettisoning;

  • Public hazard, actual or implied.

Nuclear Weapons Incident

  • An unexpected event involving a nuclear weapon, facility, or component, resulting in any of the following, but not constituting a nuclear weapons accident:

  • An increase in possibility of explosion or radioactive contamination;

  • Errors committed in the assembly, testing, loading or transportation of equipment, and or the malfunctioning of equipment and material which could lead to an unintentional operation of all or part of the weapon arming and/or firing sequence, or which could lead to a substantial change in yield, or increased dud probability;

  • Any act of God, unfavorable environment, or conditions resulting in damage to the weapon, facility or component.

Consequences of Nuclear Explosions

The detonation of a nuclear bomb over a target such as a populated city causes immense damage. The degree of damage depends upon the distance from the center of the bomb blast, which is called the hypocenter or ground zero. The closer one is to the hypocenter, the more severe the damage. Several things cause the damage:

• A wave of intense heat from the explosion

• Pressure from the shock wave created by the blast

• Radiation

• Radioactive fallout (clouds of fine radioactive particles of dust and bomb debris that fall back to the ground)

At the hypocenter, everything is immediately vaporized by the high temperature (up to 500 million degrees Fahrenheit or 300 million degrees Celsius). Outward from the hypocenter, burns from the heat, injuries from the flying debris of buildings collapsed by the shock wave, and acute exposure to the high radiation cause most casualties. Beyond the immediate blast area, casualties are caused from the heat, radiation, and fires spawned from the heat wave. In the long-term, radioactive fallout occurs over a wider area because of prevailing winds. The radioactive fallout particles enter the water supply and are inhaled and ingested by people at a distance from the blast.

Health Risks

Scientists have studied survivors of the Hiroshima and Nagasaki bombings to understand the short-term and long-term effects of nuclear explosions on human health. Radiation and radioactive fallout affect those cells in the body that actively divide (hair, intestine, bone marrow, reproductive organs). Some of the resulting health conditions include:

• Nausea, vomiting and diarrhea

• Cataracts

• Hair loss

• Loss of blood cells

These conditions often increase the risk of :

• Leukemia

• Cancer

• Infertility

• Birth defects

Scientists and physicians are still studying the survivors of the bombs dropped on Japan and expect more results to appear over time.

In the 1980s, scientists assessed the possible effects of nuclear warfare (many nuclear bombs exploding in different parts of the world) and proposed the theory that a nuclear winter could occur. In the nuclear-winter scenario, the explosion of many bombs would raise great clouds of dust and radioactive material that would travel high into Earth's atmosphere. These clouds would block out sunlight. The reduced level of sunlight would lower the surface temperature of the planet and reduce photosynthesis by plants and bacteria. The reduction in photosynthesis would disrupt the food chain, causing mass extinction of life (including humans). This scenario is similar to the asteroid hypothesis that has been proposed to explain the extinction of the dinosaurs. Proponents of the nuclear-winter scenario pointed to the clouds of dust and debris that traveled far across the planet after the volcanic eruptions of Mount St. Helens in the United States and Mount Pinatubo in the Philippines.

Nuclear weapons have incredible, long-term destructive power that travels far beyond the original target. This is why the world's governments are trying to control the spread of nuclear-bomb-making technology and materials and reduce the arsenal of nuclear weapons deployed during the Cold War.

Nuclear Aftermath

All of humanity would most like be destroyed, if all the nuclear weapons in the world were used. This is for several reasons. Firstly, incoming warheads would destroy most major cities. However, this would leave some areas untouched. These areas would most likely be reached by radioactive fall-out blown by the wind. These would be the immediate repercussions. Later, the world would go into what is called "Nuclear Winter". Global temperatures would drop significantly, as well as the amount of sunlight received by the earth. This is very similar to what is believed happened to the dinosaurs. It is believed that a large asteroid collided with the earth, and stirred up a lot of dust into the atmosphere. This blotted out the sun, and plants died. With very few plants to eat, the dinosaurs (and many other animals) went extinct. Nuclear winter would be a lot like this. The only difference is that there the dust would be raised up by impacting nuclear warheads and their explosions. Additionally, the dust would be radioactive. The combination of radioactivity, lack of food, and lowering temperatures cause a Nuclear Holocaust, with the chances of humans surviving it very low.

The OTA Study

The Office of Technology Assessment (1979) estimated the effects of a large-scale nuclear attack on U.S. military and economic targets. This scenario assumes a direct attack on 250 U.S. cities, with a total yield of 7,800 megatons. The most immediate effects would be the loss of millions of human lives, accompanied by similar incomprehensible levels of injuries, and the physical destruction of a high percentage of U.S. economic and industrial capacity. The full range of effects resulting from several thousand warheads - most having yields of a megaton or greater - impacting on or near U.S. cities can only be discussed in terms of uncertainty and speculation. It is estimated that 100 million to 165 million people would be killed.

This map shows a possible long-range fallout pattern over the United States.

Although this type of attack is less likely than during the Cold War, the risk of a limited nuclear strike by one of the smaller nuclear powers still remains a possibility.

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