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There are several ways in which the energy released by nuclear explosions
cause mass destruction, including the physical destruction of buildings and infrastructure and immense numbers of human casualties. The destructive effects of nuclear weapons include blast, heat, radiation, fallout, and electromagnetic pulse (EMP). The scale and/or nature of these effects are unique to nuclear weapons. Conventional explosives can cause damage through blast and heat, but at levels thousands or millions of times less severe than those caused by nuclear weapons. Conventional explosives also do not release radiation or cause electromagnetic pulse.
A wood frame house exposed to a nuclear blast at the Nevada Test Site. The
thermal radiation exposure was about one-quarter of that experienced at
ground zero in Hiroshima. |
The major proportion (about 50 percent) of energy from a nuclear weapon exploded on or near the earth's surface or within the atmosphere is released in the form of blast and shock waves. The pressure from a nuclear blast can cause structures to crumble and create hurricane-force winds of hundreds of miles per hour. The blast from a nuclear bomb equivalent in yield to the weapon dropped on Hiroshima (about 13 kt) would flatten all wooden or unreinforced masonry structures within one mile (1.6 km) from the point of explosion ("ground zero"). Shock waves from a nuclear explosion occurring on or under the ground can damage or destroy buildings in a similar manner as an earthquake.
Although a nuclear weapon has never been used in an urban environment such as New York or other cities with numerous skyscrapers and densely packed modern buildings, some experts believe that a Hiroshima-sized weapon detonated at ground or near ground level in this type of environment might show modified blast effects. That is, while a tremendous amount of damage would be expected, the blast damage in this scenario could be significantly less than what would occur from an equivalent yield weapon detonated high above a city. (The Hiroshima bomb detonation height was set for about 2,000 feet to maximize the blast damage.)
About 35 percent of the energy of a nuclear explosion is released as heat. Immediately after a thermonuclear explosion, the temperature at the point of explosion ("ground zero") may exceed 100 million degrees Centigrade (°C). This is about 10 times the temperature of the surface of the sun. At these temperatures, matter cannot exist in its normal solid, liquid, or gaseous state. Instead, atoms are stripped of all their electrons and converted to ionized plasma.
Even from considerable distances, the heat from a nuclear explosion can vaporize objects and living things, ignite combustible material, and cause painful or fatal burns on people and animals. Depending on environmental factors, such as building materials and weather conditions, at the explosion site, the heat can also create a "firestorm" of flames and air heated to over 1000°C, hot enough to melt glass and many metals. In a Hiroshima-sized explosion, this firestorm could incinerate everything within about 1.2 miles (1.9 km) from ground zero.
About 5 percent of the energy from a nuclear explosion is released as radiation. The radiation released immediately after the explosion in the form of neutrons, gamma rays, x-rays, and alpha and beta particles is called
prompt radiation. The prompt radiation from a Hiroshima-sized explosion would expose people within about 1.3 miles (2 km) of ground zero to a 500 rem dose of radiation, creating a 50 percent chance of death from radiation sickness, radiation burns, and other health effects within a few days or weeks. The harmful effects of radiation are described in the
tutorial on terrorism with radiological weapons. The actual radiation exposure a person receives depends on the amount of time exposed, the distance from the radiation source, and the amount of shielding. For instance, within a city such as New York the numerous, dense buildings might significantly reduce the amount of prompt radiation exposure.
About 10 percent of total energy appears in "fallout"—small particles of radioactive "dust" that settle back to earth over a period of minutes to weeks. Radioactive materials pushed high into the atmosphere by the force of a nuclear explosion can travel hundreds of miles before returning (falling) to earth, causing radioactive contamination across thousands of square miles. The intensity and duration of contamination from fallout vary with the size of the nuclear weapon and how close it exploded to the ground. Weapons detonated at or close to ground level generate the most fallout. For two days after a Hiroshima-sized explosion at ground level, anyone within about 1.7 miles (2.75 km) of ground zero could be exposed to a 500 rem radiation dose from fallout (resulting in a 50 percent chance of death). Without extensive decontamination, radiation within the area would not decay to safe levels for 3 to 5 years. Although a 12 kt surface burst would not launch significant amounts of fallout into the stratosphere, low-altitude winds could carry fallout particles many miles beyond ground zero.
The radiation from a nuclear explosion also creates a powerful electromagnetic pulse, or EMP. Radiation from a nuclear explosion ionizes air molecules, giving them an electric charge. These ionized air molecules interact with the earth's magnetic field to cause a surge of electromagnetic energy. The effect is very similar to the electrical surge caused by a lightning bolt, but about one hundred times faster and thousands of times stronger.
EMP from some very large nuclear detonations can cause surges of 25,000 to 50,000 volts per meter. This surge is strong enough to destroy unprotected electronic equipment, shut down power grids and communications networks, and completely erase a computer's memory. If a nuclear weapon explodes on or close to the ground, EMP is generated over a relatively small area. However, according to some calculations, the EMP could be conducted by subterranean electrical transmission lines, resulting in loss of electrical power beyond the immediate zone of the most intense destruction. Moreover, these calculations also indicate that depending on the weapon’s yield, EMP effects could radiate significantly beyond the zone of heaviest blast damage, potentially leading to loss of electrical communications equipment for emergency first responders stationed within miles of ground zero, unless this equipment was properly electrically shielded ahead of time. A nuclear explosion at high altitudes, unlikely to be achievable by terrorists, can generate EMP over a radius of hundreds of miles.
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