Effects of Nuclear Weapons

Type of physical science: Nuclear physics

Field of study: Nuclear weapons

The explosion of a nuclear weapon produces a blast equivalent to the detonation of thousands of tons of chemical explosives in addition to thermal and nuclear radiation unique to nuclear explosions. Detonation of many nuclear weapons may cause long-lasting environmental effects.

Overview

A nuclear weapon releases its nuclear energy in about a millionth of a second. For small nuclear fission weapons, the energy, the yield of the weapon, is equivalent to the explosion of several thousands of tons of TNT. Large thermonuclear weapons can have yields equivalent to tens of megatons of TNT. When the nuclear reactions stop, the energy is confined to a relatively small volume.

Most of the energy of the explosion (around 85 percent) appears as the kinetic energy of the fragments of the fissioning nuclei and neutrons--that is, heat energy--so that temperatures at the center of the detonation reach tens of millions of degrees Celsius. The extremely hot remnants of the explosion radiate X rays that are absorbed in the material surrounding the explosion and superheat it to form the glowing fireball characteristic of nuclear explosions. The fireball starts several meters across and expands at supersonic speeds, creating a shock wave traveling outward from the explosion.

The highest-energy electromagnetic radiation, γ and very high-energy X rays, cannot be absorbed in air and travel outward at the speed of light. The hot fireball radiates a substantial portion of its energy as electromagnetic radiation, just as a hot stove glows red.

Because electromagnetic radiation travels at the speed of light, the thermal radiation outruns the blast wave from the explosion. Thus, an observer some distance from an explosion taking place in air first experiences a pulse of electromagnetic energy from the initial explosion, followed by the thermal radiation from the hot gases of the fireball. The thermal radiation is capable of causing fires and killing. For example, a 1-megaton detonation in air at a height designed to create maximum blast damage will produce third-degree burns on exposed skin at a distance of 14 kilometers from the burst. Some time later, the blast wave arrives, traveling at the speed of sound. Conventional explosions produce hot central regions and blast waves, but their temperatures rarely exceed 5,000 degrees Celsius, so they produce very little electromagnetic radiation.

The fireball cools as it expands. Most of the remaining energy of the detonation is trapped in the nuclear fragments left over from the nuclear fission reactions that took place in the explosion. Neutrons from the detonation may interact with materials from the bomb casing or the environment producing other nuclei with stored energy. Nuclei lose this energy as radioactive decay in times that vary with the species of radioactive nuclide involved. These radioactive materials are swept along with the rest of the fireball and form a radioactive cloud.

The detailed effects of a nuclear weapon depend upon the environment in which the detonation takes place. For example, nuclear explosions in air have different effects from those of underground nuclear explosions. Underground nuclear explosions are used mainly to test nuclear warhead designs, although there has been some discussion of designing nuclear warheads that would penetrate the ground in order to destroy intercontinental ballistic missiles in hardened silos. In the case of an underground explosion, the weapon detonates and forms a fireball containing vaporized earth and rock. The heat of the nuclear explosion melts and vaporizes solid rock, and the enormous pressure of the expanding fireball (millions of atmospheres) causes the spherical bubble of hot gas to expand until it cools to the point where it no longer melts rock.

The gases then recoil from the cavity walls, sending a spherically symmetric shock wave into the surrounding rock.

If the weapon was deeply buried, its effects are confined to carving out a spherical cavity on whose walls are deposited the radioactive debris from the blast and a shock wave, which may act like a minor earthquake at close distances and can be detected by seismic equipment at distances of thousands of kilometers. Enough radioactivity is produced by a nuclear explosion that nuclear explosives cannot be used for such peaceful applications as fracturing the rock in oil and gas fields to allow the contents to flow to a collection center. The Soviet Union does occasionally use nuclear explosives to generate seismic waves for petroleum exploration and other geological studies. Toward the surface, the rocks are shattered in a narrow chimney of rubble, which may later collapse into the cavity of the explosion, creating a subsidence crater.

Rock around the cavity is shattered. The unclassified RANIER test of a small (1.7-kiloton) nuclear bomb carved out a spherical cavity of radius of 19 meters with crushed rock extending to a radius of 40 meters and fractured rock to 55 meters. The chimney produced by the blast extended upward 122 meters.

If, on the other hand, the shock wave of the explosion is strong enough to lift the surface of the earth, the outer layers of earth and rock may collapse back into the cavity, creating a circular depression on the surface that is called a crater. The nuclear test sites of both the United States and the Soviet Union are pockmarked with craters and subsidence craters produced by underground nuclear tests.

For tests in the atmosphere, the fireball immediately begins both to expand and to rise.

If the explosion is on the surface or the fireball touches the surface, materials from the earth are vaporized and many of them are made radioactive by the neutrons from the nuclear explosion.

These materials are carried upward by the rising fireball. As the fireball rises, it expands and cools to form a dark cloud as the debris it contains begins to condense. Dust and debris from the surface are caught in the updraft of the rising fireball and rise to form a stem leading up to the cloud formed by the cooling fireball. This forms the mushroom cloud that is the symbol of the nuclear age. One and one-tenth minutes after a 1-megaton air burst, the cloud is rising at 354 kilometers per hour at a height of 10 kilometers toward its final height of 19 kilometers. Beneath the cloud, the blast gouges out a crater and dumps debris around its rim. Meanwhile, the shock wave from the burst has spread out spherically until it strikes the earth's surface from which it is reflected. The reflected wave merges with the arriving wave to form the Mach front, which travels outward along the surface. Objects in its path experience crushing overpressures and high winds that first push in the direction of the arriving blast wave and then pull them back toward the site of the detonation. A 1-megaton nuclear weapon detonated at a height above the surface designed to produce a maximum shock wave can produce winds of 160 kilometers per hour with an overpressure of 3 pounds per square inch at a distance of 10 kilometers from the burst.

The nuclear detonation emits a burst of radioactive decay products and neutrons that produce lethal radiation levels in the vicinity of the explosion. This prompt radiation is generally of little concern to those assessing the destructive effects of nuclear weapons, because people in this region would almost certainly be killed by blast or thermal radiation before they had time to succumb to radiation sickness. Enhanced radiation warheads, popularly known as neutron bombs, are specially designed to produce levels of lethal radiation beyond the radius of extreme damage from blast and thermal radiation.

Most of the radioactivity resulting from the explosion is swept into the fireball and rises along with it. As the fireball begins to cool, the vaporized debris condenses to form radioactive particles of various sizes, which settle out of the cloud, the heaviest falling first. If the cloud encounters rain, extra particles are washed down to create fallout. Details of fallout plumes depend on weather and prevailing winds at the time the weapon exploded, but lethal levels of radioactivity can be expected at distances 320 kilometers from multiple explosions (used in an attack on a missiles field, for example) in the direction of the prevailing winds. Portions of the cloud will be carried into the global circulation and girdle the globe with a belt of radioactivity.

The belt gradually spreads to cover an entire hemisphere and within about a year deposits detectable fallout in the spring rains of the other hemisphere.

Applications

The primary application of nuclear weapons is to destroy targets of military or economic value in times of war. If the target in question can be easily destroyed by blast, it is called a soft target. Soft targets include cities or industrial complexes. Hard targets resist destruction by blast and include buried command bunkers and missiles in hardened silos. If a soft target is to be destroyed, the warhead will be fused to explode at a preset altitude so that destructive blast will be felt over the widest possible area.

A 1-megaton weapon exploded in this manner will carve a crater a tenth of a kilometer in radius and collapse houses at a distance of 6.8 kilometers. Beyond the radius of total destruction, the destruction will still be severe, with buildings partially destroyed and streets blocked with rubble. The thermal radiation will start widespread fires, and fire equipment will be unable to reach them through clogged streets. In some cases, the fires may coalesce to form a gigantic central core where enormous heat consumes all oxygen. Winds up to 97 kilometers per hour will rush into the central core, carrying oxygen and new fuel to the growing central firestorm. In other cases, the fires may form the walls of flame that characterize large forest fires.

No one knows how a large modern city will burn.

Further from the blast, people will not be killed immediately. There will be local damage and downed power lines. Drivers who happen to look toward the fireball may be temporarily blinded, creating accidents and massive traffic jams. The survivors will be immediately subject to nuclear fallout and must take shelter to avoid lethal doses of radiation.

The population will have to remain totally sheltered for at least two weeks, after which short trips outside could be made. Water and food supplies will be contaminated, unless they have been protected; wildlife and livestock will die without shelter.

If the nuclear weapon is exploded either high in the atmosphere or near the earth's surface, the blast will produce asymmetrical ionization of the surrounding air, which in turn can generate a powerful burst of radio frequency electromagnetic radiation, the electromagnetic pulse or EMP. EMP can burn out radio and television transmitters, stop computers, or even knock out portions of the electric power grid. Thus, unless equipment is specially protected, the shock of a nuclear detonation will fall on a population that will find itself unable to communicate with its leaders until radio transmitters can be repaired.

Although several nations have exploded single nuclear weapons and studied their effects, no one is certain how the enormous energy released during a nuclear war, when many warheads would be detonated, would affect the earth's environment. Nuclear explosions produce large quantities of nitrogen oxides that attack the ozone in the atmosphere. Certainly, a large nuclear war would damage the ozone layer, although current thinking is that the damage would be relatively minor compared to other global effects of exploding many nuclear weapons. In particular, the dust from explosions and even more important, the smoke from raging fires will be carried into the stratosphere. If these pollutants are trapped there, they can block the sun's light from the earth's surface, creating patchy global cooling. No one is certain how long the smoke particles will remain aloft. They may be quickly carried down by rain, or they may absorb sunlight, creating a hot layer high in the atmosphere to form a permanent temperature inversion.

Current atmospheric models cannot predict how severe or exactly where the effects of this nuclear winter will be felt, or even that it will be more than a nuclear early fall. Nevertheless, the ignorance of the exact nature of nuclear winter demonstrates how poorly the effects of nuclear war on the global ecology are understood.

If nuclear weapons attack hardened targets, they will be detonated on the earth's surface above the target so that the target will become part of the crater or be crushed by the blast wave of the detonation as it travels through earth. A direct hit by a large nuclear weapon is capable of severely damaging all man-made military targets.

Context

News of the discovery of nuclear fission reached the United States in January 1939. In a massive national research effort, the Manhattan Project, scientists employed by the United States government developed nuclear fission weapons, atomic bombs, in order to give the US and its allies a military advantage during World War II (1939–45). Two atomic bombs were detonated over Japan, one at Hiroshima and one at Nagasaki before the Japanese surrendered. The destruction of the atomic bombs far surpassed that of any other weapon and surprised the bomb's creators, who had not anticipated firestorms or radiation sickness. The awesome destructive effect of nuclear weapons shocked the global community to such an extent that no nuclear weapon has been used since Nagasaki.

Although no nation has used them, nuclear weapons have become a symbol of global power. Five nations (United States, Soviet Union, Great Britain, France, and China) developed nuclear weapons shortly after World War II and stockpiled them. India and Pakistan are both considered nuclear states as well, as is North Korea, which developed nuclear weapons despite intense international opposition. Several other nations, including Israel and South Africa, are believed to have at least some nuclear weapons capability, and a number of nations have the capability to develop them.

The United States and the Soviet Union embarked on a race to develop first larger numbers of atomic bombs. Then each side developed thermonuclear weapons or hydrogen bombs. The next phase of the arms race revolved around the development of delivery systems to allow precise placement of nuclear weapons on hardened targets such as the other side's intercontinental ballistic missiles. Sophisticated delivery systems allowed the use of warheads with lower yields so that the arms race focused next on developing smaller warheads tailored to specific nuclear requirements. As arsenals on each side climbed above ten thousand warheads, the two nations began to negotiate arms control treaties to limit their unwieldy and expensive nuclear arsenals. Both sides, as well as the other nuclear powers, conducted extensive tests first in the atmosphere and then underground.

Although it seems likely that the nuclear arsenals of the superpowers will be reduced, nuclear weapons will continue to play a key role in world politics. If a nation possesses nuclear arms, that nation can deter a second nation from attacking with conventional weapons by threatening retaliation. Planners must worry that the future world will see confrontation among many nations, each armed with at least a handful of nuclear weapons and crude delivery systems.

The question the world must ask is whether the horrifying destructive effects of nuclear weapons can continue to frighten the global community into a refusal to employ them. Concerns over the risk of a global nuclear war first emerged at the start of the Cold War and lingered well into the twenty-first century, as numerous nuclear powers, including the US and Russia, maintained largely tense relations with each other and some long-standing nuclear agreements were either not renewed or withdrawn from by the 2020s. At that time the US and Russia controlled the vast majority of the world's nuclear weapons. The ongoing interest in the acquisition of nuclear weapons by authoritarian governments, particularly North Korea, also prompted global concern.

The technologies developed in manufacturing and studying nuclear weapons generally have applications only to weapons systems. Therefore, the development of nuclear weapons has principally affected the physics community in cultivating the perception that physicists are valuable because they can build bombs. Large-scale federal funding of basic physics research in the United States has resulted, at least in part, from that perception.

Principal terms

BLAST WAVE: the outward-traveling pressure wave created as a nuclear explosion compresses the medium around itself

CRATER: a cavity created in the earth's surface by the detonation of a nuclear explosive

ELECTROMAGNETIC PULSE (EMP): a strong, short burst of long-wavelength electromagnetic radiation created in the atmosphere by a nuclear detonation whose environment is not symmetric

FALLOUT: debris from a nuclear explosion containing radioactive nuclides that may be carried by winds for hundreds of kilometers before radioactive particles settle out of the atmosphere

FIREBALL: the expanding region of extremely hot gases surrounding the point of detonation of a nuclear weapon immediately after the explosion

NUCLEAR WINTER: the global cooling produced by the smoke and dust thrown high into the atmosphere by the detonation of many nuclear weapons

THERMAL RADIATION: electromagnetic radiation from a nuclear fireball that travels ahead of the blast wave from the explosion

YIELD: the total energy produced by a nuclear explosion, usually measured in the tons of the chemical explosive TNT (trinitrotoluene), whose detonation would release the same amount of energy

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