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Key Issues Nuclear Weapons The Basics Weapons Basics

Weapons Basics

Introduction

Basic Terms

How the Bomb Works

Immediate Aftermath of a Nuclear Explosion

Effects of Radiation on Humans

Nuclear Materials

Introduction:
This page is an introduction to some of the scientific concepts and history behind nuclear weapons. The first section is a list of some basic terms, followed by an explanation of how a nuclear weapon works and a description of the immediate damage caused by a nuclear explosion. The next section discusses the harmful effects of radiation on humans. The timeline chronicles the discovery of the scientific concepts behind the atomic bomb.

Basic Terms:
Atom: The smallest particle of matter that can have the properties of a chemical element. Atoms

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See Also
The Nuclear Files list of nuclear terms
The Nuclear Files list of acronyms

More on the Web
Lawrence Livermore Laboratory's ABC's of Nuclear Science
Atomic Archive Glossary
Radiation risk

are composed of protons (positively charged particles), electrons (negatively charged particles), and neutrons (uncharged particles). Protons and neutrons are heavy particles that are found in an atom's nucleus (the core). Electrons, which are much smaller and lighter, orbit the nucleus
Source: http://www.academicpress.com

Fission: The splitting of the nucleus of an element into fragments. Heavy elements such as uranium or plutonium release energy when fissioned.

Fusion: The combining of two nuclei to form a heavier one. Fusion of the isotopes of light elements such as hydrogen or lithium gives a large release of energy.

Radiation: Radiation is any energy that is emitted from some source and travels through space. This includes things such as light, sound, and heat. The radiation typically referred to when discussing nuclear weapons or nuclear energy is ionizing radiation, which comes from unstable atoms. To become stable, unstable atoms emit radiation in the form of particles, such as alpha and beta radiation, or in the form of electromagnetic waves, such as gamma radiation and X-rays. Source: http://www.orau.gov/reacts/define.htm

Alpha Radiation: Radiation consisting of helium nuclei (atomic wt. 4, atomic number 2) that are discharged by radioactive disintegration of some heavy elements, including uranium-238, radium-226, and plutonium-239.

Beta Radiation: Radiation consisting of electrons or positrons emitted from atoms at speeds approaching the speed of light.

Gamma Radiation: Electromagnetic waves released during radioactive decay that can ionize atoms and split chemical bonds.

Rad: A unit of absorbed dose of radiation defined as deposition of 100 ergs of energy per gram of tissue. It amounts to approximately one ionization per cubic micron.

Chain Reaction: The process of nuclear fission in which the neutrons released trigger other nuclear fission reactions at the same or greater rate. In a nuclear weapon, an extremely rapid multiplying chain reaction causes an explosive release of energy. In a nuclear reactor the pace of the chain reaction is controlled to produce heat (in a power reactor) or large quantities of neutrons (in a research or production reactor)

Critical Mass: The amount of a fissile substance that will allow a self-sustaining chain reaction. The amount depends both on the properties of the fissile element and on the shape of the mass.

Atom Bomb: A nuclear bomb whose energy comes from the fission of uranium or plutonium

Hydrogen Bomb: A nuclear weapon that derives its energy from the fusion of hydrogen. Also known as a thermonuclear weapon.

Source: Unless otherwise indicated, definitions are prepared by Alyn Ware, Coordinator of the Parliamentary Network for Nuclear Disarmament, a project of the Middle Powers Initiative. http://www.pnnd.org

How the Bomb Works: Nuclear weapons, like conventional bombs, are designed to cause damage through an explosion, i.e. the release of a large amount of energy in a short period of time. In conventional bombs the explosion is created by a chemical reaction, which involves the rearrangement of atoms to form new molecules. The amount of energy released is proportional to the binding energies of the molecules. In nuclear weapons the explosion is created by changing the atoms themselves - they are either split or fused to create new atoms.

The binding energies within atoms are many magnitudes of order greater than the binding energies of molecules. The amount of energy available within an atom is given by Einstein's famous formula E=MC2, where E = energy, M = the mass and C = the speed of light. Thus the energy available equals the mass multiplied by 9,000,000,000,000,000,000. As a result, a nuclear bomb using a kilogram of plutonium could have the same explosive force of approximately 15 million kilograms of TNT.

There are two main types of nuclear weapons:
atom bombs which use fission as the main reaction, i.e. the atoms are split;
hydrogen bombs which use fusion as the main reaction, i.e. the atoms are fused together.

Fission bombs
Materials: The core of a fission bomb is either plutonium or highly enriched uranium. These are the only materials that can achieve a self-sustaining chain reaction.

Plutonium occurs naturally only in minute quantities. Most plutonium is produced in reactors through the fission of uranium. It must then be extracted in a reprocessing facility if it is to be useable.

Naturally occurring uranium is mostly Ur238, which is not suitable for nuclear weapons. Ur235, which is better at sustaining a chain reaction, comprises about 0.7% of natural uranium. An enrichment facility is used to increase the proportion of Ur235 to about 90%, although lower grades can be used. Uranium which is composed of more than 20% Ur235 is known as highly enriched uranium and can be used in a nuclear weapon. Low enriched uranium can be used in nuclear power reactors.

Mechanics: Plutonium and uranium atoms are both heavy, meaning they have a large number of protons and neutrons in the nucleus. Fission of a heavy nucleus can be spontaneous or induced by the absorption of a neutron. During fission, when the heavy nucleus splits into two smaller nuclei, extra neutrons are released. If these neutrons are absorbed by other nuclei, they in turn could split, also releasing neutrons. Generally, the neutrons released by an atom splitting spontaneously "miss" other atoms and so do not stimulate further fission. However, if the atoms are brought together under high pressure, the "hit rate" of neutrons is increased and a chain reaction can occur. In nuclear power plants this chain reaction is controlled by absorbing extra neutrons. In nuclear weapons this chain reaction becomes critical, i.e. uncontrolled.

Achieving criticality in an atom bomb: In order to achieve criticality and thus create an explosion from the fission of atoms, an uncontrolled chain reaction must be generated by compressing the fissile material so that the atoms are close enough for the released neutrons to continue to hit. Such compression can be obtained through a gun method or an implosion method.

Gun method: One mass of uranium is fired down a barrel into another mass of uranium. This is the simplest design and was used for the Hiroshima bomb. However, it is not as efficient as the implosion method.

Implosion method: A sphere of fissile material - plutonium or highly-enriched uranium - is surrounded by conventional high explosives, which are detonated simultaneously. Timing of the detonation is crucial for the material to be compressed sufficiently and uniformly.

Fusion bombs
In fusion bombs, deuterium and tritium - two isotopes of hydrogen - are fused together to create heavier atoms. This is the same reaction as occurs in the centre of the sun. Fusion can only happen at very high temperatures and pressures. In a nuclear weapon these are created through using a fission explosion (i.e. an atom bomb) to trigger the fusion reaction. There is no theoretical limit to the explosive force of a fusion weapon. Typically, fusion weapons are 10 - 100 times as explosive as the fission bombs which nearly destroyed Hiroshima and Nagasaki.

Mechanics: In fusion bombs, deuterium and tritium are fused together to create heavier atoms. This is the same reaction as occurs in the centre of the sun. Fusion can only happen at very high temperatures and pressures. In a nuclear weapon, these temperature and pressure levels are created by using a fission explosion (i.e. an atom bomb) to trigger the fusion reaction. There is no theoretical limit to the explosive force of a fusion weapon. Typically, fusion weapons are 10 - 100 times as explosive as the fission bombs that nearly destroyed Hiroshima and Nagasaki.

Weapons materials: The core of a fission bomb is either plutonium or highly enriched uranium. These are the only materials that can achieve a self-sustaining chain reaction. Plutonium occurs naturally only in minute quantities. Most plutonium is produced in reactors through the fission of Uranium. If it is to be useable it must then be extracted in a reprocessing facility. Naturally occurring uranium is mostly Ur238, which is not suitable for nuclear weapons. Ur235, which is better at sustaining a chain reaction, comprises about 0.7%. An enrichment facility is used to increase the proportion of Ur235 to about 90%, although lower grades could be used. Uranium which comprises more than 20% Ur235 is known as highly enriched uranium and can be used in a nuclear weapon. Low enriched uranium can be used in nuclear power reactors. Deuterium (H2) and tritium (H3) are isotopes of hydrogen which are used in fusion weapons. Tritium is also used in a fission bomb as a source of additional neutrons to assist the fission process.

Source: Prepared by Alyn Ware, Coordinator of the Parliamentary Network for Nuclear Disarmament, a project of the Middle Powers Initiative. http://www.pnnd.org

Further reading:
Crude Nuclear Weapons: Proliferation and the Terrorist Threat , IPPNW, Cambridge 1996
A Call to a New Exodus: An Anti-Nuclear Primer for Pacific People , Suliana Siwatibau and David Williams, Pacific Conference of Churches, Fiji, 1982
Bombing Bombay? Effects of Nuclear Weapons and a Case Study of a Hypothetical Explosion , M.V. Ramana, IPPNW, Cambridge USA, 1999
Security and Survival: The Case for a Nuclear Weapons Convention , Merav Datan and Alyn Ware, IPPNW, Cambridge, 1999

Immediate Aftermath of a Nuclear Explosion
A nuclear explosion produces several distinct forms of energy that have damaging effects: blast, thermal radiation, electromagnetic pulse, direct nuclear radiation, and fallout. The extent of damage will depend on various factors, including the size of the nuclear weapon, the height at which it is detonated, and the geography of the target.

Blast:
The rapid release of energy in an explosion creates a shock wave of overpressure. Very close to the centre of a nuclear explosion, overpressure is equivalent to several thousand pounds per square inch (psi). This is hundreds of times greater than the pressure in a pressure cooker.

The overpressure crushes objects. Human lungs are crushed at about 30 psi overpressure. Brick houses are destroyed at about 10-15 psi overpressure. The blast also generates high velocity winds which can turn humans or objects into missiles. At 15 - 20 psi the winds can fling a person at several hundred kilometres per hour. The pressure of the shock wave can also cause deafness.

Thermal radiation:
Thermal radiation includes light and heat.

A nuclear explosion releases a huge amount of energy as light (utlraviolet, visible, and infrared), which can be seen from hundreds of miles away. The light is so intense that it can make sand explode, blind people many miles away, burn shadows into concrete, and ignite flammable materials at large distances. The thermal radiation also causes burns on human skin. The radius of the flash burns depends on the power of the weapon and the clearness of the atmosphere. An explosion above clouds can diminish the burns suffered from heat flash.

The heat from the explosion is so intense that nearly all materials at the center of the explosion (epicenter) are immediately vaporized. The thermal radiation also creates a fireball which rapidly expands outward, consuming oxygen and, combined with the blast effect, creating near total destruction for some distance from the epicenter.

Electromagnetic pulse:
A nuclear explosion also sends out an electromagnetic pulse, similar to the thermal pulse. Although the electromagnetic pulse does not directly harm humans, it can increse the devastation at the site of a nuclear explosion because it disables all electrical devices in its path, such as medical equipment and the microchips found in newer cars.

Direct nuclear radiation:
A nuclear explosion releases several forms of radiation. Both gamma rays and neutrons easily penetrate solid objects and can be deadly. Beta and alpha particles are generally less dangerous, having much shorter ranges - several meters and several centimeters, respectively. Alpha particles cannot penetrate human skin. If ingested, however, alpha particles will cause the most damage to the human body.

Fallout:
Fallout consists of large numbers of particles, from the earth, buildings and other ground objects, which are propelled upward in the blast and irradiated, mixing with the radioactive products of the explosion. Some of this material will fall back to earth within a few minutes, and radioactive fallout may continue its descent for about 24 hours. The rising and descending debris forms the mushroom cloud that follows a nuclear explosion.

No early fallout is associated with high-altitude explosions, although an explosion well above the ground causes radioactive residues to rise to a great height in the mushroom cloud and descend gradually over a large area.

The distribution of fallout depends on the topography of the land and weather conditions, especilly the direction and speed of winds. Radioactive fallout may travel and settle in areas hundreds of miles from the explosion site.

Radioactive fallout may be the most dangerous effect of a nuclear explosion because the area of exposure to fallout is much wider than that of direct nuclear radiation.

Because there is no known way of neutralising a radioactive substance, apart from sending it through a nuclear reactor, radioactive products are dangerous until they have decayed to such an extent that they no longer emit significant amounts of radiation. This time is usually considered to be 10 times the half-life.

Extent of damage:
The extent of damage depends on the size of the nuclear weapon, the terrain and the height at which it is detonated. Nuclear weapons detonated at ground level generate more fallout as a result of the large amount of ground material which is irradiated by the explosion and thrown in the air, but the effects of thermal radiation and radioactive waves is less than in an air blast.

The nuclear weapon detonated in Hiroshima was about 12kt, i.e. the equivalent of 12,000 tons of TNT. The combined effects of blast, and radiation killed about 300,000 people. Current nuclear weapons range in size from 1 kt to over 1000 kt. Most are about 100kt, i.e. about 10 times the force of the Hiroshima bomb.

Source: Excerpts prepared by Alyn Ware, Coordinator of the Parliamentary Network for Nuclear Disarmament, a project of the Middle Powers Initiative.
http://www.pnnd.org

Additional Sources:
http://www.pbs.org/wgbh/amex/bomb/sfeature/effects.html
http://www.clemson.edu/ep/n_expl.htm


Effects of Radiation on Humans

The effects of radiation on the human body vary, depending on the dosage of radiation, and whether exposure is slow and protracted or large and instantaneous.

Radiation affects cells in the human body that actively divide (e.g. hair, intestine, bone marrow, reproductive organs). The most frequent kind of radiation exposure is exposure of small areas of the body. Damage in localized tissue and to blood vessels in the exposed areas can lead to disturbed organ functioning. Higher doses cause gangrene and/or death of localized tissue.

A large, rapid dose of radiation causes cell death, and effects are immediately apparent - within hours, days, or weeks. With protracted exposure, cells can do some repair over the exposure period. Protracted exposure is generally better tolerated, even when the total dose is high. (It is impossible to measure how much radiation a person has been exposed to over an extended period of time). Radiation doses low enough to avoid cell damage can still induce cellular changes that may be clinically detected sometime in the future, and could potentially be passed on through defective genes.

With radiation exposure due to internally deposited radiation, effects are delayed, and degeneration or destruction of the irradiated tissue may not be as severe. The initiation of cancer is possible, depending on the affected organ and the nature of the radioactive element (its half-life, radiation characteristics, and biochemical behavior).

High whole-body doses of radiation produce a characteristic pattern of injury. Doses are measured in rads.

Extremely high doses: 4000-5000 rads
Radiation exposure in this range severely damages the vascular system. It also causes accumulation of fluid in the brain (cerebral edema), leading to central nervous system syndrome. Symptoms include nausea, vomiting, explosive diarrhea, convulsions, and progressive impairment of cognitive and motor skills. A person exposed to this amount of radiation will enter a coma and die within 48 hours.

High doses: 1000-4000 rads
In this range of radiation exposure, vascular damage is less severe, but there is also a loss of fluids and electrolytes in intercellular spaces and the gastrointestinal tract. Death occurs within ten days, due to fluid and electrolyte imbalance, severe bone-marrow damage, and terminal infection.

Moderate doses: 400-1000 rads
Exposure in this range causes a gastrointestinal form of radiation sickness, with symptoms of nausea, vomiting, and diarrhea. Radiation in this range also destroys bone marrow and disrupts its production of blood cells, leading to infection as the white blood cells count decreases. There would also be a drop in the number of platelets (cell fragments that help blood to clot), which would allow massive hemorrhaging. Death is probable and will occur in approximately four to five weeks.

Low Doses: 100-400 rads
Low doses of radiation cause problems similar to those of moderate exposure. Nausea, vomiting, and diarrhea symptoms cease after a few days. Treatment for radiation exposure in this range can be effective, but death is still a possibility.

Acute Radiation Syndrome (see Extremely High Doses and High Doses above)
Acute radiation syndrome is sickness caused by irradiation of most or all of the body, whether in one large dose or through exposure over time (although it is impossible to measure the amount of radiation a person has accumulated over an extended period of time). Symptoms will be more immediately apparent in the case of a large dose in a short period. Encompassing the most severe effects of radiation exposure, acute radiation syndrome requires immediate medical attention. Without medical treatment, survival is highly unlikely.

Initially patients experience fatigue, loss of appetite, nausea, vomiting, and diarrhea for a day or two. If the dose of radiation is very high, there may also be symptoms such as fever and respiratory problems. Symptoms then disappear for several days to several weeks, after which the illness becomes severe.

Radiation inhibits reproduction of blood cells, leading to bleeding and anemia as the number of red blood cells decreases, and inability for wounds to heal as blood clotting factors are lost. A decreased white blood cell count hinders the body's immune system and leads to more infetions.

There may also be a loss of fluids, electrolytes, and intestinal lining. In more serious cases, accumulation of fluid in the brain can lead to central nervous system syndrome, with symptoms of nausea, vomiting, and diarrhea.

Other symptoms may include temporary sterility in males, clouding in the lens of the eye, and loss of hair. Hair loss occurs because damage to hair-root cells causes hairs to become thinner and break off.

Late Effects
Delayed effects of radiation exposure, largely secondary to blood vessel damage, are the impaired functioning of and degenerative changes in many organs, particularly bone marrow, kidneys, lungs, and the lens of the eye.

The most serious late effect of radiation exposure is a significantly increased incidence of leukemia and thyroid, lung, and breast cancers (compared to the average figure among people exposed to doses of less than 100 rads).

There is also an increased incidence of leukemia, lung cancer, radiation-induced anemia, and bone cancer among people exposed to lower doses of radiation. The type of cancer depends on how the radiation exposure occurs. For example, there was a high incidence of lung cancer among uranium mine workers, who inhaled radioactive dust. Watch painters at the turn of the century licked their radioactive paintbrushes, leading to a high incidence of bone cancer and radiation-induced anemia. There is also a very high incidence of leukemia among Hiroshima survivors who were exposed to 100+ rads.

Radiation exposure can also cause cataracts and hair loss, and increase the risk of infertility and birth defects.

Additional Sources:
http://www.rerf.or.jp/eigo/radefx/early/acute.htm
http://www.orau.gov/reacts/syndrome.htm
http://www.howstuffworks.com/nuclear-bomb.htm/printable