Basics

Basic Vocabulary

Atom: The smallest particle of matter that can have the properties of a chemical element. Atoms 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 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. Unstable atoms emit particles, such as alpha and beta radiation, or electromagnetic waves, such as gamma radiation and X-rays, to become stable. Source: http://www.orau.gov/reacts/define.htm

Alpha Radiation

The radiation consists of helium nuclei (atomic wt. 4, atomic number 2) discharged by the radioactive disintegration of some heavy elements, including uranium-238, radium-226, and plutonium-239.

Beta Radiation

The radiation consists 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.

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.

Atomic 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: http://www.pnnd.org

How the Bomb Works

Nuclear weapons, like conventional bombs, are designed to cause damage through an explosion, i.e., quickly releasing a large amount of energy. In conventional bombs, the explosion is created by a chemical reaction involving 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 more significant than the binding energies of molecules. The amount of energy available within an atom is given by Einstein’s famous formula, E = mc2. Thus the energy available equals the mass multiplied by 9×10^18. 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 types of nuclear weapons:

  • atom bombs which use fission as the main reaction
  • hydrogen bombs which use fusion as the main reaction

Fission Bombs:

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 to be usable. Naturally occurring Uranium is mostly Ur238, which is unsuitable for nuclear weapons. Ur235, 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, composed of more than 20% Ur235, is highly enriched and can be used in a nuclear weapon. Low-enriched Uranium can be used in nuclear power reactors.

Fusion Bombs:

In fusion bombs, Deuterium (H2) and Tritium (H3) – two hydrogen isotopes – fuse to create heavier atoms. This is the same reaction that occurs in the sun’s center and can only happen at very high temperatures and pressures. A nuclear weapon creates these 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 which nearly destroyed Hiroshima and Nagasaki.

Fission Bombs:

Plutonium and uranium atoms are both heavy, meaning they have many protons and neutrons in the nucleus. The fission of a heavy nucleus can be spontaneous or induced by the absorption of a neutron. During fission, extra neutrons are released when the heavy nucleus splits into two smaller nuclei. If other nuclei absorb these neutrons, they, in turn, could split, releasing neutrons. Generally, the neutrons released by an atom splitting spontaneously “miss” other atoms and do not stimulate further fission. However, if the atoms are brought together under high pressure, neutrons’ “hit rate” increases 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.

Fusion Bombs:

In fusion bombs, Deuterium and Tritium are fused to create heavier atoms. This is the same reaction that occurs in the center of the sun. Fusion can only happen at very high temperatures and pressures. A nuclear weapon creates these temperature and pressure levels 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.

Fission Bombs:

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 most straightforward design and was used for the Hiroshima bomb. However, it is less efficient than the implosion method.

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

Source: How Nuclear Bombs Work

Aftermath of a Nuclear Explosion

A nuclear explosion produces several forms of energy that have damaging effects: blast, thermal radiation, electromagnetic pulse, direct atomic 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.

An explosion’s sudden energy release generates an overpressure shock wave. Overpressure is the equivalent of several thousand pounds per square inch (psi) near the center of a nuclear explosion. This pressure is hundreds of times higher than that of a pressure cooker. The excess pressure crushes the object. At a pressure difference of roughly 30 psi, human lungs are crushed. High-velocity winds produced by the explosion can turn people or items into missiles, throwing a person several hundred kilometers per hour at 15 to 20 psi.

Heat and light are both examples of thermal radiation. A nuclear explosion emits tremendous energy as light (ultraviolet, visible, and infrared), which can be seen from hundreds of kilometers away. The light is so powerful that it can ignite flammable items miles away, cause sand to explode, blind individuals at great distances, and burn shadows into concrete. The weapon’s power and the atmosphere’s clarity affect the radius of flash burns. An explosion above the clouds can lessen the burns caused by a heat flash.

Nearly all materials at the explosion’s epicenter (the center of the explosion) are instantly vaporized due to the explosion’s extreme heat. Along with the blast effect, the heat radiation produces a fireball that quickly spreads outward, consumes oxygen, and causes almost complete devastation a fair distance from the epicenter.

Like a thermal pulse, an electromagnetic pulse is also released during a nuclear explosion. The electromagnetic pulse doesn’t directly kill people. Still, it can worsen the destruction at the site of a nuclear explosion since it destroys all electrical items in its path, including modern cars’ microchips and medical equipment.

Multiple types of radiation are released during a nuclear explosion. Gamma rays and neutrons can be fatal and quickly pass through solid materials. Since beta and alpha particles have far lower ranges—a few meters and a few centimeters, respectively—they are often less harmful. Human skin is impermeable to alpha particles. Alpha particles will harm people the most if they consume them, though.

Fallout comprises numerous particles from the ground, structures, and other ground items that are irradiated, driven into the air by the explosion, and combined with the blast’s radioactive byproducts. Within a few minutes, some of this debris will start to fall back to Earth, and radioactive fallout may continue for about 24 hours. The ascending and descending waste creates a mushroom cloud after a nuclear explosion. High-altitude explosions don’t have any early fallout. Still, they do cause radioactive residues to rise significantly in the mushroom cloud and spread out gradually over a wide area when they occur.

The terrain’s topography and the weather, particularly the direction and speed of the winds, affect how fallout is distributed. Radioactive fallout can reach hundreds of miles from the explosion site and settle there. Because the exposure zone for radioactive fallout is far more extensive than that for direct nuclear radiation, it may be the most deadly result of a nuclear explosion. Radioactive materials are dangerous until they have degraded to the point that they no longer produce substantial levels of radiation since there is no known way to neutralize them other than by passing them through a nuclear reactor. Typically, this period is thought to be ten times the half-life.

Effects of Radiation on Humans

The effects of radiation on the human body vary, depending on the radiation dosage and whether exposure is slow and protracted or significant 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 to small body areas. Damage in localized tissue and blood vessels in the exposed areas can disturb organ functioning. Higher doses cause gangrene or death of localized tissue.

A large, rapid radiation dose causes cell death, and effects are immediately apparent – within hours, days, or weeks. With prolonged 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). Radiation doses low enough to avoid cell damage can still induce cellular changes that may be clinically detected sometime in the future and could 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. Cancer initiation 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.

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.

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.

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.

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.

 

Acute Radiation Syndrome (See Extremely High Doses and High Doses above): Acute radiation syndrome is a condition brought on by exposure to radiation, whether in a single high dose or over time (although it is impossible to determine how much radiation a person has absorbed over a long period). In the case of a high dose over a short period, symptoms will be more noticeable immediately. Acute radiation syndrome, which includes the most severe symptoms of radiation exposure, necessitates prompt medical intervention. Survival without medical care is unlikely.

Initially, patients feel exhaustion, loss of appetite, nausea, vomiting, and diarrhea for a day or two. If the radiation dose is exceptionally high, there may also be symptoms such as fever and respiratory issues. Symptoms disappear for several days or weeks, and the sickness becomes severe.

Radiation suppresses the formation of blood cells, leading to bleeding and anemia as the number of red blood cells declines and the inability of wounds to heal as blood clotting components are depleted. A lower white blood cell count hampers the body’s immune system, leading to more illnesses.

Additionally, electrolytes, fluids, and the intestinal lining may be lost. In more extreme situations, brain fluid buildup can result in central nervous system syndrome, which presents with nausea, vomiting, and diarrhea symptoms.
Hair loss, lens clouding in the eyes, and temporary male sterility are possible further symptoms. Damage to hair-root cells results in thinner, more fragile hairs that eventually fall out and cause hair loss.

 

Late Effects: Many organs, especially the bone marrow, kidneys, lungs, and eye lens, have decreased function and degenerative changes due to radiation exposure that manifest later on. These changes are mainly brought on by blood vessel damage.

A significantly higher incidence of leukemia, thyroid, lung, and breast cancers (relative to the average number among those exposed to doses of less than 100 rads) is the most severe late impact of radiation exposure.

Subsequent to cataracts and hair loss, radiation exposure can increase the risk of infertility and congenital disorders. Those who receive fewer radiation doses are also more likely to develop leukemia, lung cancer, radiation-induced anemia, and bone cancer. The method of radiation exposure affects the type of cancer. For instance, radioactive dust exposure among uranium mine employees was associated with a high risk of lung cancer. Watch painters lick radioactive paintbrushes in the early 20th century, increasing bone cancer risk and radiation-induced anemia. The incidence of leukemia among Hiroshima survivors exposed to 100 rads or more is very high.

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