nuclear blast primary effects

he Blast Wave

A fraction of a second after a nuclear explosion, the heat from the fireball causes a high-pressure wave to develop and move outward producing the blast effect. The front of the blast wave, i.e., the shock front, travels rapidly away from the fireball, a moving wall of highly compressed air.

The effects of the blast wave on a typical wood framed house.

The air immediately behind the shock front is accelerated to high velocities and creates a powerful wind. These winds in turn create dynamic pressure against the objects facing the blast. Shock waves cause a virtually instantaneous jump in pressure at the shock front. The combination of the pressure jump (called the overpressure) and the dynamic pressure causes blast damage. Both the overpressure and the dynamic pressure reach to their maximum values upon the arrival of the shock wave. They then decay over a period ranging from a few tenths of a second to several seconds, depending on the blast's strength and the yield.

Overpressure

Blast effects are usually measured by the amount of overpressure, the pressure in excess of the normal atmospheric value, in pounds per square inch (psi).

After 10 seconds, when the fireball of a 1-megaton nuclear weapon has attained its maximum size (5,700 feet across), the shock front is some 3 miles farther ahead. At 50 seconds after the explosion, when the fireball is no longer visible, the blast wave has traveled about 12 miles. It is then traveling at about 784 miles per hour, which is slightly faster than the speed of sound at sea level.

Peak overpressure	Maximum Wind Speed
50 psi	934 mph
20 psi	502 mph
10 psi	294 mph
5 psi	163 mph
2 psi	70 mph

As a general guide, city areas are completely destroyed by overpressures of 5 psi, with heavy damage extending out at least to the 3 psi contour.

These many different effects make it difficult to provide a simple rule of thumb for assessing the magnitude of injury produced by different blast intensities. A general guide is given below:

Overpressure	Physical Effects
20 psi	Heavily built concrete buildings are severely damaged or demolished.
10 psi	Reinforced concrete buildings are severely damaged or demolished. Most people are killed.
5 psi	Most buildings collapse. Injuries are universal, fatalities are widespread.
3 psi	Residential structures collapse. Serious injuries are common, fatalities may occur.
1 psi	Window glass shatters Light injuries from fragments occur. Blast Effects on Humans Blast damage is caused by the arrival of the shock wave created by the nuclear explosion. Humans are actually quite resistant to the direct effect of overpressure. Pressures of over 40 psi are required before lethal effects are noted. The danger from overpressure comes from the collapse of buildings that are generally not as resistant. Urban areas contain many objects that can become airborne, and the destruction of buildings generates many more. The collapse of the structure above can crush or suffocate those caught inside. Serious injury or death can also occur from impact after being thrown through the air. Blast effects on a concrete building at Hiroshima. The blast also magnifies thermal radiation burn injuries by tearing away severely burned skin. This creates raw open wounds that readily become infected. The Mach Stem If the explosion occurs above the ground, when the expanding blast wave strikes the surface of the earth, it is reflected off the ground to form a second shock wave traveling behind the first. This reflected wave travels faster than the first, or incident, shock wave since it is traveling through air already moving at high speed due to the passage of the incident wave. The reflected blast wave merges with the incident shock wave to form a single wave, known as the Mach Stem. The overpressure at the front of the Mach wave is generally about twice as great as that at the direct blast wave front. A diagram of the Mach effect. At first the height of the Mach Stem wave is small, but as the wave front continues to move outward, the height increases steadily. At the same time, however, the overpressure, like that in the incident wave, decreases because of the continuous loss of energy and the ever-increasing area of the advancing front. After about 40 seconds, when the Mach front from a 1-megaton nuclear weapon is 10 miles from ground zero, the overpressure will have decreased to roughly 1 psi. Thermal Radiation A primary form of energy from a nuclear explosion is thermal radiation. Initially, most of this energy goes into heating the bomb materials and the air in the vicinity of the blast. Temperatures of a nuclear explosion reach those in the interior of the sun, about 100,000,000° Celsius, and produce a brilliant fireball. The fireball shortly after detonation. Two pulses of thermal radiation emerge from the fireball. The first pulse, which lasts about a tenth of a second, consists of radiation in the ultraviolet region. The second pulse which may last for several seconds, carries about 99 percent of the total thermal radiation energy. It is this radiation that is the main cause of skin burns and eye injuries suffered by exposed individuals and causes combustible materials to break into flames. Thermal radiation damage depends very strongly on weather conditions. Clouds or smoke in the air can considerably reduce effective damage ranges versus clear air conditions. The Fireball The fireball, an extremely hot and highly luminous spherical mass of air and gaseous weapon residues, occurs within less than one millionth of one second of the weapon's detonation. Immediately after its formation, the fireball begins to grow in size, engulfing the surrounding air. This growth is accompanied by a decrease in temperature because of the accompanying increase in mass. At the same time the fireball rises, like a hot-air balloon. Within seven-tenths of one millisecond from the detonation, the fireball from a 1-megaton weapon is about 440 feet across, and this increases to a maximum value of about 5,700 feet in 10 seconds. It is then rising at a rate of 250 to 350 feet per second. After a minute, the fireball has cooled to such an extent that it no longer emits visible radiation. It has then risen roughly 4.5 miles from the point of burst. Illustrated components of a nuclear explosion. The Mushroom Cloud As the fireball increases in size and cools, the vapors condense to form a cloud containing solid particles of the weapon debris, as well as many small drops of water derived from the air sucked into the rising fireball. The early formation of the mushroom cloud. Depending on the height of burst, a strong updraft with inflowing winds, called "afterwinds," are produced. These afterwinds can cause varying amounts of dirt and debris to be sucked up from the earth's surface into the cloud. In an air burst with a moderate (or small) amount of dirt and debris drawn up into the cloud, only a relatively small proportion become contaminated with radioactivity. For a burst near the ground, however, large amounts of dirt and debris are drawn into the cloud during formation. The color of the cloud is initially red or reddish brown, due to the presence of nitrous acid and oxides of nitrogen. As the fireball cools and condensation occurs, the color changes to white, mainly due to the water droplets (as in an ordinary cloud). The cloud consists chiefly of very small particles of radioactive fission products and weapon residues, water droplets, and larger particles of dirt and debris carried up by the afterwinds. The eventual height reached by the radioactive cloud depends upon the heat energy of the weapon and upon the atmospheric conditions. If the cloud reaches the tropopause, about 6-8 miles above the Earth's surface, there is a tendency for it to spread out. But if sufficient energy remains in the radioactive cloud at this height, a portion of it will ascend into the more stable air of the stratosphere. The mushroom cloud forming at the Nevada Test Site. The cloud attains its maximum height after about 10 minutes and is then said to be "stabilized." It continues to grow laterally, however, to produce the characteristic mushroom shape. The cloud may continue to be visible for about an hour or more before being dispersed by the winds into the surrounding atmosphere where it merges with natural clouds in the sky. Thermal Pulse Effects One of the important differences between a nuclear and conventional weapon is the large proportion of a nuclear explosion's energy that is released in the form of thermal energy. This energy is emitted from the fireball in two pulses. The first is quite short, and carries only about 1 percent of the energy; the second pulse is more significant and is of longer duration (up to 20 seconds). The thermal pulse charring the paint. The energy from the thermal pulse can initiate fires in dry, flammable materials, such as dry leaves, grass, old newspaper, thin dark flammable fabrics, etc. The incendiary effect of the thermal pulse is also substantially affected by the later arrival of the blast wave, which usually blows out any flames that have already been kindled. However, smoldering material can reignite later. The major incendiary effect of nuclear explosions is caused by the blast wave. Collapsed structures are much more vulnerable to fire than intact ones. The blast reduces many structures to piles of kindling, the many gaps opened in roofs and walls act as chimneys, gas lines are broken open, storage tanks for flammable materials are ruptured. The primary ignition sources appear to be flames and pilot lights in heating appliances (furnaces, water heaters, stoves, etc.). Smoldering material from the thermal pulse can be very effective at igniting leaking gas. Thermal radiation damage depends very strongly on weather conditions. Cloud cover, smoke, or other obscuring material in the air can considerably reduce effective damage ranges versus clear air conditions. Effects of the thermal pulse on clothing. Thermal radiation also affects humans both directly - by flash burns on exposed skin - and indirectly - by fires started by the explosion. Firestorms Under some conditions, the many individual fires created by a nuclear explosion can coalesce into one massive fire known as a "firestorm." The combination of many smaller fires heats the air and causes winds of hurricane strength directed inward toward the fire, which in turn fan the flames. For a firestorm to develop: There must be at least 8 pounds of combustibles per square foot. At least one-half of the structures in the area are on fire simultaneously. There is initially a wind of less than 8 miles per hour. The burning area is at least 0.5 square miles. In Hiroshima, a firestorm did develop and about 4.4 square miles were destroyed. Although there was some damage from uncontrolled fires at Nagasaki, a firestorm did not develop. One reason for this was the difference in the terrain. Hiroshima is relatively flat, while Nagasaki has uneven terrain. The firestorm at Hiroshima. Firestorms can also be caused by conventional bombing. During World War II, the cities of Dresden, Hamburg, and Tokyo all suffered the effects of firestorms. Flash Burns Flash burns are one of the serious consequences of a nuclear explosion. Flash burns result from the absorption of radiant energy by the skin of exposed individuals. A distinctive feature of flash burns is the fact they are limited to exposed areas of the skin facing the explosion. The burns are in a pattern corresponding to the dark portions of the kimono she was wearing at the time of the explosion. A 1-megaton explosion can cause first-degree burns (a bad sunburn) at a distance of about 7 miles, second-degree burns (producing blisters and permanent scars) at distances of about 6 miles, and third-degree burns (which destroy skin tissue) at distances up to 5 miles. Third-degree burns over 24 percent of the body, or second-degree burns over 30 percent, will result in serious shock, and will probably prove fatal unless prompt, specialized medical care is available. It has been estimated that burns caused some 50 percent of the deaths at Hiroshima and Nagasaki. Flash blindness Flash blindness is caused by the initial brilliant flash of light produced by the nuclear detonation. The light is received on the retina than can be tolerated, but less than is required for irreversible injury. The retina is particularly susceptible to visible and short wavelength infrared light. The result is a bleaching of visual pigment and temporary blindness. Vision is completely recovered as the pigment is regenerated. During the daylight hours, flash blindness does not persist for more than 2 minutes, but generally lasts a few seconds. At night, when the pupil is dilated, flashblindness will last for a longer period of time. A 1-megaton explosion can cause flash blindness at distances as great as 13 miles on a clear day, or 53 miles on a clear night. If the intensity is great enough, a permanent retinal burn will result. Retinal injury is the most far-reaching injury effect of nuclear explosions, but it is relatively rare since the eye must be looking directly at the detonation. Retinal injury results from burns in the area of the retina where the fireball image is focused. Nuclear Radiation The release of radiation is a phenomenon unique to nuclear explosions. There are several kinds of radiation emitted; these types include gamma, neutron, and ionizing radiation, and are emitted not only at the time of detonation (initial radiation) but also for long periods of time afterward (residual radiation). Initial Nuclear Radiation Initial nuclear radiation is defined as the radiation that arrives during the first minute after an explosion, and is mostly gamma radiation and neutron radiation. The level of initial nuclear radiation decreases rapidly with distance from the fireball to where less than one roentgen may be received five miles from ground zero. In addition, initial radiation lasts only as long as nuclear fission occurs in the fireball. Initial nuclear radiation represents about 3 percent of the total energy in a nuclear explosion. Though people close to ground zero may receive lethal doses of radiation, they are concurrently being killed by the blast wave and thermal pulse. In typical nuclear weapons, only a relatively small proportion of deaths and injuries result from initial radiation. Residual Nuclear Radiation The residual radiation from a nuclear explosion is mostly from the radioactive fallout. This radiation comes from the weapon debris, fission products, and, in the case of a ground burst, radiated soil. 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 particles and gamma radiation. Radiation Effects on Humans Certain body parts are more specifically affected by exposure to different types of radiation sources. Several factors are involved in determining the potential health effects of exposure to radiation. These include: The size of the dose (amount of energy deposited in the body) The ability of the radiation to harm human tissue Which organs are affected The most important factor is the amount of the dose - the amount of energy actually deposited in your body. The more energy absorbed by cells, the greater the biological damage. Health physicists refer to the amount of energy absorbed by the body as the radiation dose. The absorbed dose, the amount of energy absorbed per gram of body tissue, is usually measured in units called rads. Another unit of radation is the rem, or roentgen equivalent in man. To convert rads to rems, the number of rads is multiplied by a number that reflects the potential for damage caused by a type of radiation. For beta, gamma and X-ray radiation, this number is generally one. For some neutrons, protons, or alpha particles, the number is twenty. Hair The losing of hair quickly and in clumps occurs with radiation exposure at 200 rems or higher. Brain Since brain cells do not reproduce, they won't be damaged directly unless the exposure is 5,000 rems or greater. Like the heart, radiation kills nerve cells and small blood vessels, and can cause seizures and immediate death. Thyroid The certain body parts are more specifically affected by exposure to different types of radiation sources. The thyroid gland is susceptible to radioactive iodine. In sufficient amounts, radioactive iodine can destroy all or part of the thyroid. By taking potassium iodide can reduce the effects of exposure. Blood System When a person is exposed to around 100 rems, the blood's lymphocyte cell count will be reduced, leaving the victim more susceptible to infection. This is often refered to as mild radiation sickness. Early symptoms of radiation sickness mimic those of flu and may go unnoticed unless a blood count is done.According to data from Hiroshima and Nagaski, show that symptoms may persist for up to 10 years and may also have an increased long-term risk for leukemia and lymphoma. For more information, visit Radiation Effects Research Foundation. Heart Intense exposure to radioactive material at 1,000 to 5,000 rems would do immediate damage to small blood vessels and probably cause heart failure and death directly. Gastrointestinal Tract Radiation damage to the intestinal tract lining will cause nausea, bloody vomiting and diarrhea. This is occurs when the victim's exposure is 200 rems or more. The radiation will begin to destroy the cells in the body that divide rapidly. These including blood, GI tract, reproductive and hair cells, and harms their DNA and RNA of surviving cells. Reproductive Tract Because reproductive tract cells divide rapidly, these areas of the body can be damaged at rem levels as low as 200. Long-term, some radiation sickness victims will become sterile. Dose-rem Effects 5-20 Possible late effects; possible chromosomal damage. 20-100 Temporary reduction in white blood cells. 100-200 Mild radiation sickness within a few hours: vomiting, diarrhea, fatigue; reduction in resistance to infection. 200-300 Serious radiation sickness effects as in 100-200 rem and hemorrhage; exposure is a Lethal Dose to 10-35% of the population after 30 days (LD 10-35/30). 300-400 Serious radiation sickness; also marrow and intestine destruction; LD 50-70/30. 400-1000 Acute illness, early death; LD 60-95/30. 1000-5000 Acute illness, early death in days; LD 100/10 Long Term Effects on Humans Long after the acute effects of radiation have subsided, radiation damage continues to produce a wide range of physical problems. These effects- including leukemia, cancer, and many others- appear two, three, even ten years later. Blood Disorders According to Japanese data, there was an increase in anemia among persons exposed to the bomb. In some cases, the decrease in white and red blood cells lasted for up to ten years after the bombing. Cataracts There was an increase in cataract rate of the survivors at Hiroshima and Nagasaki, who were partly shielded and suffered partial hair loss. Malignant Tumors All ionizing radiation is carcinogenic, but some tumor types are more readily generated than others. A prevalent type is leukemia. The cancer incidence among survivors of Hiroshima and Nagasaki is significantly larger than that of the general population, and a significant correlation between exposure level and degree of incidence has been reported for thyroid cancer, breast cancer, lung cancer, and cancer of the salivary gland. Often a decade or more passes before radiation-caused malignancies appear. Keloids Beginning in early 1946, scar tissue covering apparently healed burns began to swell and grow abnormally. Mounds of raised and twisted flesh, called keloids, were found in 50 to 60 percent of those burned by direct exposure to the heat rays within 1.2 miles of the hypocenter. Keloids are believed to be related to the effects of radiation. Radioactive Fallout Fallout is the radioactive particles that fall to earth as a result of a nuclear explosion. It consists of weapon debris, fission products, and, in the case of a ground burst, radiated soil. Fallout particles vary in size from thousandths of a millimeter to several millimeters. Much of this material falls directly back down close to ground zero within several minutes after the explosion, but some travels high into the atmosphere. This material will be dispersed over the earth during the following hours, days (and) months. Fallout is defined as one of two types: early fallout, within the first 24 hours after an explosion, or delayed fallout, which occurs days or years later. Most of the radiation hazard from nuclear bursts comes from short-lived radionuclides external to the body; these are generally confined to the locality downwind of the weapon burst point. This radiation hazard comes from radioactive fission fragments with half-lives of seconds to a few months, and from soil and other materials in the vicinity of the burst made radioactive by the intense neutron flux. Most of the particles decay rapidly. Even so, beyond the blast radius of the exploding weapons there would be areas (hot spots) the survivors could not enter because of radioactive contamination from long-lived radioactive isotopes like strontium 90 or cesium 137. For the survivors of a nuclear war, this lingering radiation hazard could represent a grave threat for as long as 1 to 5 years after the attack. Predictions of the amount and levels of the radioactive fallout are difficult because of several factors. These include; the yield and design of the weapon, the height of the explosion, the nature of the surface beneath the point of burst, and the meteorological conditions, such as wind direction and speed. An air burst can produce minimal fallout if the fireball does not touch the ground. On the other hand, a nuclear explosion occurring at or near the earth's surface can result in severe contamination by the radioactive fallout. Radioactive Fallout Fallout is the radioactive particles that fall to earth as a result of a nuclear explosion. It consists of weapon debris, fission products, and, in the case of a ground burst, radiated soil. Fallout particles vary in size from thousandths of a millimeter to several millimeters. Much of this material falls directly back down close to ground zero within several minutes after the explosion, but some travels high into the atmosphere. This material will be dispersed over the earth during the following hours, days (and) months. Fallout is defined as one of two types: early fallout, within the first 24 hours after an explosion, or delayed fallout, which occurs days or years later. Most of the radiation hazard from nuclear bursts comes from short-lived radionuclides external to the body; these are generally confined to the locality downwind of the weapon burst point. This radiation hazard comes from radioactive fission fragments with half-lives of seconds to a few months, and from soil and other materials in the vicinity of the burst made radioactive by the intense neutron flux. Most of the particles decay rapidly. Even so, beyond the blast radius of the exploding weapons there would be areas (hot spots) the survivors could not enter because of radioactive contamination from long-lived radioactive isotopes like strontium 90 or cesium 137. For the survivors of a nuclear war, this lingering radiation hazard could represent a grave threat for as long as 1 to 5 years after the attack. Predictions of the amount and levels of the radioactive fallout are difficult because of several factors. These include; the yield and design of the weapon, the height of the explosion, the nature of the surface beneath the point of burst, and the meteorological conditions, such as wind direction and speed. An air burst can produce minimal fallout if the fireball does not touch the ground. On the other hand, a nuclear explosion occurring at or near the earth's surface can result in severe contamination by the radioactive fallout. The Fallout Pattern The details of the actual fallout pattern depend on wind speed and direction and on the terrain. The fallout will contain about 60 percent of the total radioactivity. The largest particles will fall within a short distance of ground zero. Smaller particles will require many hours to return to earth and may be carried hundreds of miles. This means that a surface burst can produce serious contamination far from the point of detonation. This map shows the total dose contours from early fallout from a surface burst of a 1-megaton fission yield. From the 15-megaton thermonuclear device tested at Bikini Atoll on March 1, 1954 - the BRAVO shot of Operation CASTLE - the fallout caused substantial contamination over an area of more than 7,000 square miles. The contaminated region was roughly cigar-shaped and extended more than 20 miles upwind and over 350 miles downwind. Fallout can also enter into the stratosphere. In this stable region, radioactive particles can remain from 1 to 3 years before returning to the surface. 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. Electromagnetic Pulse Electromagnetic pulse (EMP) is an electromagnetic wave similar to radio waves, which results from secondary reactions occurring when the nuclear gamma radiation is absorbed in the air or ground. It differs from the usual radio waves in two important ways. First, it creates much higher electric field strengths. Whereas a radio signal might produce a thousandth of a volt or less in a receiving antenna, an EMP pulse might produce thousands of volts. Secondly, it is a single pulse of energy that disappears completely in a small fraction of a second. In this sense, it is rather similar to the electrical signal from lightning, but the rise in voltage is typically a hundred times faster. This means that most equipment designed to protect electrical facilities from lightning works too slowly to be effective against EMP. There is no evidence that EMP is a physical threat to humans. However, electrical or electronic systems, particularly those connected to long wires such as power lines or antennas, can undergo damage. There could be actual physical damage to an electrical component or a temporary disruption of operation. The range of the EMP effects of a high altitude burst. An attacker might detonate a few weapons at high altitudes in an effort to destroy or damage the communications and electric power systems. It can be expected that EMP would cause massive disruption for an indeterminable period, and would cause huge economic damages. On July 8, 1962, the EMP from the high altitude (250 miles above Johnston Island) "Starfish Prime" test (1.4 Mt) turned off 300 streetlights in Oahu, Hawaii (740 miles away). Ozone Depletion When a nuclear weapon explodes in the air, the surrounding air is subjected to great heat, followed by relatively rapid cooling. These conditions are ideal for the production of tremendous amounts of nitric oxides. These oxides are carried into the upper atmosphere, where they reduce the concentration of protective ozone. Ozone is necessary to block harmful ultraviolet radiation from reaching the Earth's surface. Oxides of nitrogen form a catalytic cycle to reduce the protective ozone layer. The nitric oxides produced by the weapons could reduce the ozone levels in the Northern Hemisphere by as much as 30 to 70 percent. Such a depletion might produce changes in the Earth's climate, and would allow more ultraviolet radiation from the sun through the atmosphere to the surface of the Earth, where it could produce dangerous burns and a variety of potentially dangerous ecological effects. It has been estimated that as much as 5,000 tons of nitric oxide is produced for each megaton of nuclear explosive power. Nuclear Winter In 1983, R.P. Turco, O.B. Toon, T.P. Ackerman, J.B. Pollack, and Carl Sagan (referred to as TTAPS) published a paper entitled "Global Atmospheric Consequences of Nuclear War" which is the foundation on which the nuclear winter theory is based on. Theory states that nuclear explosions will set off firestorms over many cities and forests within range. Great plumes of smoke, soot, and dust would be sent aloft from these fires, lifted by their own heating to high altitudes where they could drift for weeks before dropping back or being washed out of the atmosphere onto the ground. Several hundred million tons of this smoke and soot would be shepherded by strong west-to-east winds until they would form a uniform belt of particles encircling the Northern Hemisphere. These thick black clouds could block out all but a fraction of the sun's light for a period as long as several weeks. The conditions of semidarkness, killing frosts, and subfreezing temperatures, combined with high doses of radiation from nuclear fallout, would interrupt plant photosynthesis and could thus destroy much of the Earth's vegetation and animal life. The extreme cold, high radiation levels, and the widespread destruction of industrial, medical, and transportation infrastructures along with food supplies and crops would trigger a massive death toll from starvation, exposure, and disease. It is not certain that a nuclear war would produce a nuclear winter effect. However, it remains a possibility and the TTAPS study concluded: "...the possibility of the extinction of Homo Sapiens cannot be excluded."