Nuclear Strike

Outline of the attack

Two mininuclear devices were placed close to the center core of each 110-storey Twin Tower. One mininuke was placed below the 20th floor, and a second one was placed somewhere between the 20th and 70th floor. The bombs were thermonuclear devices and had a combined yield of about 0.5 kton (500,000 kg of TNT-equivalent) for each tower. The two bombs in the tower were detonated close in time to one another.

The bombs had a fission trigger that delivered 4% of the total yield, fusion energy providing the remaining 96% (minimal residual radiation devices). This setup had the purpose of minimizing the release of radiation and thereby reducing the chances of detection.

Cutting charges and exploding charges (RDX and C4) were planted throughout the building from the ground level to the top with even vertical spacing between the charges (see the demolition squibs [10]). Some additional charges were placed in the top portion of the building above the intended fracture point (where the planes hit).

The thermonuclear devices were triggered at 2 seconds and 10 seconds into the demolition cascade in the case of the North Tower (see the video below). The smaller higher-placed bomb was the demolition bomb (it destroyed the resistant steel core first), and the second bomb was the ‘clean-up’ bomb; it vaporized remaining material including steel and airplane black boxes. It also shattered human bodies into micron-sized particles.

Conventional bombs destroyed the top portion of the tower that was above the plane crash point. They were also used to demolish the building below the fracture point – they were detonated in a certain sequence to produce a top-down pattern of destruction. More micro- or mininuclear devices may have been used other than the two per tower mentioned so far. If they were, the total yield of the nuclear bombs was kept well under the 1 kton range.

The three destructive main effects of a nuclear explosion are blast effects, thermal energy effects and radiation effects (prompt and delayed). All three effects were present in the WTC Twin Towers explosions.

fas – Physical Effects of Nuclear Explosions…/fm8-9/1ch3.htm

302. General Effects of Nuclear Explosions.

a. While the destructive action of conventional explosions is due almost entirely to the transmission of energy in the form of a blast wave with resultant mechanical damage, the energy of a nuclear explosion is transferred to the surrounding medium in three distinct forms: blast; thermal radiation; and nuclear radiation. The distribution of energy among these three forms will depend on the yield of the weapon, the location of the burst, and the characteristics of the environment. For a low altitude atmospheric detonation of a moderate sized weapon in the kiloton range, the energy is distributed roughly as follows:

(1) 50% as blast;

(2) 35% as thermal radiation; made up of a wide range of the electromagnetic spectrum, including infrared, visible, and ultraviolet light and some soft x-ray emitted at the time of the explosion; and

(3) 15% as nuclear radiation; including 5% as initial ionizing radiation consisting chiefly of neutrons and gamma rays emitted within the first minute after detonation, and 10% as residual nuclear radiation. Residual nuclear radiation is the hazard in fallout.

b. Considerable variation from this distribution will occur with changes in yield or location of the detonation. This is best shown by comparing the ranges of damage due to these effects of weapons of different size yields (Table 3-I).

c. The distribution of weapon energy yield is altered significantly by the enhanced radiation nuclear warhead. In simplest terms an enhanced radiation warhead is designed specifically to reduce the percentage of energy that is dissipated as blast and heat with a consequent increase in the percentage yield of initial radiation. Approximate percentage energies are 30% blast; 20% thermal; 45% initial radiation; and 5% residual radiation.

[Pic 52]


Above: Fireball in a nuclear bomb test

303. Initial Energy Transfer and Formation of Fireball.

a. Because of the tremendous amounts of energy liberated per unit mass in a nuclear detonation, temperatures of several tens of million degrees centigrade develop in the immediate area of the detonation. This is in marked contrast to the few thousand degrees of a conventional explosion. At these very high temperatures the nonfissioned parts of the nuclear weapon are vaporized. The atoms do not release the energy as kinetic energy but release it in the form of large amounts of electromagnetic radiation. In an atmospheric detonation, this electromagnetic radiation, consisting chiefly of soft x-ray, is absorbed within a few meters of the point of detonation by the surrounding atmosphere, heating it to extremely high temperatures and forming a brilliantly hot sphere of air and gaseous weapon residues, the so-called fireball. Immediately upon formation, the fireball begins to grow rapidly and rise like a hot air balloon. Within a millisecond after detonation, the diameter of the fireball from a 1 megaton (Mt) air burst is 150 m. This increases to a maximum of 2200 m within 10 seconds, at which time the fireball is also rising at the rate of 100 m/sec. The initial rapid expansion of the fireball severely compresses the surrounding atmosphere, producing a powerful blast wave, discussed below.

Fireball. Trinity Test July 16, 1945.

b. The fireball itself emits enormous amounts of electromagnetic radiation, similar in its spectrum to sunlight. This is usually termed thermal radiation. The visible light component accounts for the blinding flash seen upon detonation as well as the subsequent brightness of the fireball, while the infrared component results in widespread burns and incendiary effects.

c. As it expands toward its maximum diameter, the fireball cools, and after about a minute its temperature has decreased to such an extent that it no longer emits significant amounts of thermal radiation. The combination of the upward movement and the cooling of the fireball gives rise to the formation of the characteristic mushroom-shaped cloud.

Nuclear mushroom cloud of a hydrogen bomb

View of North Tower just after leveling [911 Research].

“The color of the radioactive cloud is initially red or reddish brown, due to the presence of various colored compounds (nitrous acid and oxides of nitrogen) at the surface of the fireball. These result from chemical interaction of nitrogen, oxygen, and water vapor in the air at the existing high temperatures and under the influence of the nuclear radiation. As the fireball cools and condensation occurs, the color of the cloud changes to white, mainly due to the water droplets as in an ordinary cloud.” [61 Trinity]

As the fireball cools, the vaporized materials in it condense to form a cloud of solid particles. Following an air burst, condensed droplets of water give it a typical white cloudlike appearance.

Video of mushroom cloud forming

VIDEO: Mushroomcloud  Dailymotion | Atomicarchive

View of South Tower immediately after disintegration [9/11 Research].

South Tower’s mushroom cloud [9/11 Research].


Nagasaki atomic bomb mushroom cloud on left and South Tower explosion mushroom cloud on right.

The stages of a mushroom cloud forming halfway through the collapse of the South Tower [9/11 Research].


Caption: Demolition of tower blocks in Glasgow


Caption: Mitchellhill Flat Demolition, Castlemilk (Glasgow)


Two successive stages of the formation of the North Tower’s mushroom cloud [9/11 Research].

Another sequence of photos showing a mushroom cloud forming during the collapse of the North Tower [Pictures – 9/11 Research].

Video: Squibs and streamers

VIDEO: “Squibs_and_Streamers” Dailymotion

Mushroom Cloud sequence [GIF from Wikipedia].

“Depending on the height of burst of the nuclear weapon and the nature of the terrain below, a strong updraft with inflowing winds, called “afterwinds,” is produced in the immediate vicinity. These afterwinds can cause varying amounts of dirt and debris to be sucked up from the earth’s surface into the radioactive cloud” [Trinity]

Air, dirt and vaporized matter gets sucked up into the mushroom in a surface burst.

Low air burst showing toroidal fireball and dirt cloud [61 Trinity]

Formation of dirt cloud in surface burst.

“Depending on the height of burst of the nuclear weapon and the nature of the terrain below, a strong updraft with inflowing winds, called “afterwinds,” is produced in the immediate vicinity. These afterwinds can cause varying amounts of dirt and debris to be sucked up from the earth’s surface into the radioactive cloud” [Trinity]

“.. In a surface burst, large quantities of earth or water enter the fireball at an early stage and are fused or vaporized.” [61 Trinity Atomic Website]

In the case of a surface burst, this cloud will also contain large quantities of dirt and other debris which are vaporized when the fireball touches the earth’s surface or are sucked up by the strong updrafts afterwards, giving the cloud a dirty brown appearance. The dirt and debris become contaminated with the radioisotopes generated by the explosion or activated by neutron radiation and fall to earth as fallout.

White cloud raised by the nuclear explosion turns brown. [41, Arsenal of Hypocrisy] Banberry 10kton underground nuclear detonation on right.

[The Hiroshima Meteorological Observatory reported that just after the flash, black smoke rose from the ground up to the sky reaching an altitude of several thousand meters, and covered the whole city. When the fireball disappeared, the angry clouds, like grey smoke, rose and reached an altitude of 8,000 meters in 5 minutes after the explosion. [47]

One of the Enola Gay crew recorded in his flight diary, “9:00a.m…..Clouds were observed. Altitude of 12,000 meters or more.” From a distance the cloud formation looked like a mushroom growing out of the ground, with white cloud at the top and yellowish clouds enveloping reddish-black clouds, creating a color that cannot be described as while, black, red or yellow. [47]

In Nagasaki .. just after the flash it appeared that a huge fireball covered the city, as if it were suppressing the city from the sky. Around the fireball there was a doughnut-shaped ring from the midst of which black smoke and flames rose up to the sky in an instant …. When the fireball scattered with a flash, the city was covered with darkness. The smoke rising from the midst of the ring, glittering in colors of red, white and yellow, reached an altitude of 8,000 meters in only 3 or 4 seconds. [47]

Notice the brown, reddish, yellow and black hues of the cloud that covers New York City [Pic: Aman Zafar].

View of pyroclastic cloud from across Hudson River [41].

On 9 September 1945, the New York Times published an eyewitness account of the Nagasaki, Japan bombing, written by William L. Laurence, the official newspaper correspondent of the Manhattan Project, who accompanied one of the three aircraft that made the bombing run. He wrote of the bomb producing a “pillar of purple fire”, out of the top of which came “a giant mushroom that increased the height of the pillar to a total of 45,000 feet.” [49 Wikipedia: Mushroom Cloud]

The pillar or stem of the ordnance cloud is visible here. Notice the different shades in this ordnance plume of the North Tower [Pic 50 Why Indeed ..? ].

d. The cloud rises for a period of approximately 10 minutes to a stabilized height which depends on the thermal output of the weapon and atmospheric conditions. It will continue to grow laterally assuming the familiar mushroom shape and may remain visible for an hour or more under favorable conditions. For example, the nuclear cloud from a 1 Mt surface burst will stabilize at an altitude of over 20 kilometers (km) and will have a mean lateral diameter of 35 km.

Fig. 4 The 100 KT Sedan nuclear explosion, one of the Plowshares excavation tests, was buried at a depth of 635 feet. The main cloud and base surge are typical of shallow-buried nuclear explosions. The cloud is highly contaminated with radioactive dust particles and produces an intense local fallout. [57 FAS Public Interest Report]

Cloud spreading sideways over Manhattan – notice the cauliflower configuration of the cloud that is similar to the cloud in the Sedan test above. Both blasts were surface or close to the surface blasts. Drag loading, a force that is horizontal in direction, spreads the cloud laterally in these types of ground bursts. [Pic: 44 Molecular Dissociation]

The dust cloud of the nuclear detonation(s) spread over a wide radius. Look at the smaller cloud arising on the left. This cloud was the concomitant explosion of one of the other WTC buildings. Its explosion was timed to coincide with the explosion of a Twin Tower so that the smaller explosion of the smaller WTC building would be masked. [Pic: Arsenal of Hypocrisy].


The bomber, piloted by the commander of the 509th Composite Group, Colonel Paul Tibbets, flew at low altitude on automatic pilot before climbing to 31,000 feet as it neared the target area. At approximately 8:15 a.m. Hiroshima time the Enola Gay released “Little Boy,” …. After a second shock wave (reflected from the ground) hit the plane, the crew looked back at Hiroshima. “The city was hidden by that awful cloud . . . boiling up, mushrooming, terrible and incredibly tall,” Tibbets recalled. [47, mbe.doe, The Atomic Bombing of Hiroshima]

Hiroshima: ‘The next moment, “I was blinded for a moment by a piercing flash of bright light, and the air filled with yellow smoke like poison gas. Momentarily, it got so dark I couldn’t see anything. There was a loud, dull, thunderous noise.’ [11 America’s Reaction]

Ground Zero Hiroshima [Picture: 47 Richard Seaman – damage to city]. “Several photographs were taken from the ground after the blast, by residents of adjacent valleys which were shielded from the direct effects of the explosion.”

Hiroshima: ‘Yukiharu did not realize at first that a bomb had exploded. He thought there had been an electrical accident at the plant building he was working in. Then it suddenly became dark and “I heard a huge explosion.”‘ [11. America’s Reaction]

Cloud spreading over Manhattan [43, Dirt to Molecular dissociation].

Hiroshima: ‘Eventually a Japanese staff officer was dispatched by plane to survey the city from overhead, and while he was still nearly 100 miles away from the city he began to report on a huge cloud of smoke that hung over it.’ [47 mbe.doe]

Sky darkened by cloud from air burst in Hiroshima [Pic: 47 Richard Seaman].

The clouds block out the light and the area that is covered by cloud becomes dark. Hiroshima was bombed at 8.15 on a sunny clear morning. Seconds after the atomic bombing, the sky became dark. Likewise, the clouds that developed after the WTC bombing also blocked out the sunlight.

[43 Aman Zafar]

After reaching an altitude of 8,000 meters, the smoke ascended more slowly and took about 30 seconds to reach an altitude of 12,000 meters. Then, the mass of smoke gradually discolored and scattered in wads of white clouds.] [48 Gensuikin]

Cloud from the ground (Nagasaki) [Pic: 46 Dulce Et Decorum Est].

Black clouds tinged with brown reddish hues block out the sunlight and turn day into night. [Pic 44 Dirt to Molecular Dissociation ]

The Darkness Immediately after the Explosion — Why did it get pitch-dark? by Tsutomu IGARASHI

In Chiyoko KUWABARA’s testimony, she mentioned that “it got pitch-dark” right after the explosion. In others’ atomic-bomb testimonies on record, we often find descriptions of “a shroud of darkness”. What’s the cause of it? [..] Could only dust cause that darkness? What caused the light to be completely blocked for as long a few dozen minutes?

The air that rushed in contained all sorts of burnt objects you can think of, including soot, dirt, dust, and the wreckage of buildings, blowing back towards the point of detonation 580 meters above the ground. Inorganic matter didn’t change color much but organic matter got charred and rushed back in. Trees, grass, particles in the air, skins of creatures, clothes, charred human skin and hair, all were absorbed and concentrated. Those which heated to a higher temperature went up further from the central point, forming a huge umbrella which was the lower part of the mushroom cloud. The reason that the lower part of the cloud was dark was that it contained incinerated objects. It was not just due to dirt or debris.”[Yami English]

304. Types of Bursts.

The relative effects of blast, heat, and nuclear radiation will largely be determined by the altitude at which the weapon is detonated. Nuclear explosions are generally classified as air bursts, surface bursts, subsurface bursts, or high altitude bursts.

a. Air Bursts. An air burst is an explosion in which a weapon is detonated in air at an altitude below 30 km but at sufficient height that the fireball does not contact the surface of the earth. After such a burst, blast may cause considerable damage and injury. The altitude of an air burst can be varied to obtain maximum blast effects, maximum thermal effects, desired radiation effects, or a balanced combination of these effects. Burns to exposed skin may be produced over many square kilometers and eye injuries over a still larger area. Initial nuclear radiation will be a significant hazard with smaller weapons, but the fallout hazard can be ignored as there is essentially no local fallout from an air burst. The fission products are generally dispersed over a large area of the globe unless there is local rainfall resulting in localized fallout. In the vicinity of ground zero, there may be a small area of neutron-induced activity which could be hazardous to troops required to pass through the area. Tactically, air bursts are the most likely to be used against ground forces.

b. Surface Burst. A surface burst is an explosion in which a weapon is detonated on or slightly above the surface of the earth so that the fireball actually touches the land or water surface. Under these conditions, the area affected by blast, thermal radiation, and initial nuclear radiation will be less extensive than for an air burst of similar yield, except in the region of ground zero where destruction is concentrated. In contrast with air bursts, local fallout can be a hazard over a much larger downwind area than that which is affected by blast and thermal radiation.

1. Surface burst or atmospheric burst 2. subsurface or underground burst 3. High altitude or air or upper-atmospheric burst 4. underwater burst [Diagram 54 Wikimedia Commons: Types of nuclear testing]

c. Subsurface Burst. A subsurface burst is an explosion in which the point of the detonation is beneath the surface of land or water. Cratering will generally result from an underground burst, just as for a surface burst. If the burst does not penetrate the surface, the only other hazard will be from ground or water shock. If the burst is shallow enough to penetrate the surface, blast, thermal, and initial nuclear radiation effects will be present, but will be less than for a surface burst of comparable yield. Local fallout will be very heavy if penetration occurs.

d. High Altitude Burst. A high altitude burst is one in which the weapon is exploded at such an altitude (above 30 km) that initial soft x-rays generated by the detonation dissipate energy as heat in a much larger volume of air molecules. There the fireball is much larger and expands much more rapidly. The ionizing radiation from the high altitude burst can travel for hundreds of miles before being absorbed. Significant ionization of the upper atmosphere (ionosphere) can occur. Severe disruption in communications can occur following high altitude bursts. They also lead to generation of an intense electromagnetic pulse (EMP) which can significantly degrade performance of or destroy sophisticated electronic equipment. There are no known biological effects of EMP; however, indirect effects may result from failure of critical medical equipment.


305. Formation of Blast Wave.

a. As a result of the very high temperatures and pressures at the point of detonation, the hot gaseous residues move outward radially from the center of the explosion with very high velocities.

Trinity Test July 16, 1945 [PDF 1 Glaser]

The test was code-named Trinity, supposedly after a poem by John Donne which begins: “Batter my heart, three-person’d God” [53 WePlanet].

Most of this material is contained within a relatively thin, dense shell known as the hydrodynamic front. Acting much like a piston that pushes against and compresses the surrounding medium, the front transfers energy to the atmosphere by impulse and generates a steep-fronted, spherically expanding blast or shock wave. At first, this shock wave lags behind the surface of the developing fireball. However, within a fraction of a second after detonation, the rate of expansion of the fireball decreases to such an extent that the shock catches up with and then begins to move ahead of the fireball. For a fraction of a second, the dense shock front will obscure the fireball, accounting for the characteristic double peak of light seen with a nuclear detonation.

b. As it expands, the peak pressures of the blast wave diminish and the speed of propagation decreases from the initial supersonic velocity to that of sound in the transmitting medium. However, upon reflection from the earth’s surface, the pressure in the wave will be reinforced by the fusion of the incident and the reflected wave (the Mach effect) described below.

Mach stem

VIDEO: “MACHSTEM” Dailymotion

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. This reflected blast wave can merge with the incident shock wave. This merging phenomenon is called the “Mach effect”. The overpressure at the front of the Mach wave is generally about twice as great as that at the direct blast wave front. This footage is from a conventional explosion. [55]
c. A large part of the destruction caused by a nuclear explosion is due to blast effects. Objects within the path of the blast wave are subjected to severe, sharp increases in atmospheric pressure and to extraordinarily severe transient winds. Most buildings, with the exception of reinforced or blast- resistant structures, will suffer moderate to severe damage when subjected to overpressures of only 35.5 kiloPascals (kPa) (0.35 Atm). The velocity of the accompanying blast wind may exceed several hundred km/hr. Most materiel targets are drag- or wind-sensitive.

The following four drawings show what level of blast damage (at different psi overpressure) and fire ignition from the thermal pulse might be expected for different strength nuclear explosions (both ground and air bursts) at different distances from ground zero. Take note of the damage range distances from GZ – ground zero. (Courtesy of Nuclear Attack Environment Handbook, FEMA – August, 1990) [Fallout Shelters]

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. [55 Atomic Archive]

This photo sequence shows a wood-frame house exposed to a nuclear blast at the Nevada Test Site. The test was Upshot-Knothole Annie, a 16 Kt tower shot, on March 17, 1953. The house is 1,100 meters from ground zero. The exposure to thermal radiation was 25 cal/cm2, about one-quarter of that experienced at ground zero in Hiroshima. The blast overpressure was 5 psi, and the blast wave created surface winds of 160 mph (257 kpm). [55 Atomic Archive]

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. [55 Atomic Archive]

GIF of the same house [64 Fallout Shelters]

Peace Dome

Hiroshima: The building was the former Hiroshima Prefecture Industrial Promotion Hall where special products of Hiroshima were exhibited and various gatherings were held until the A-bomb was dropped. Since it was located just under thehypocenter, blast pressure was vertically exerted on the bulding and only the dome-shaped framework and part of the outer wall remained. It has come to be called “the A-bomb Dome”, and it has come to symbolize to the people of the world “No More Hiroshimas”. [Gensuikin] [Pic: Atomic Archive]

Hiroshima: The Red Cross building was very close to the hypocenter. Note the depressed roof caused by the explosion occuring overhead. [Atomic Archive]

Hiroshima: The foreground shows the ruins of the Hiroshima Gas Company Building (800 feet from the hypocenter). In the center are the ruins of the Honkawa Elementary School. [Atomic Archive]

Hiroshima: View of the bank district, east from Shima Hospital, the hypocenter. [Atomic Archive]

QUOTE: The steel framework of this factory was broken and bent in a mess as if it were made from a pliable material. The concrete base supporting the steel frame was shoved by the blast. This is testament to how frightful the blast pressure was. It is estimated that this factory was subjected to a wind velocity of 200 meters per second arid a wind pressure of 10 tons per square meter.

Until the Very moment of the explosion, there was an array of machine tools in the factory, and a number of overhead cranes were busily operating. Most of the workers were crushed to death.

On August 9, it is recorded that 1,721 persons came to work, among whom 1,019 died and 149 were seriously injured. The rate of casualties was 68%.

A worker who miraculously kept his life said: “I was talking about work with my colleague, but in a moment he was killed instantly by a crane which crushed half of his body. It was a shocking sight and a horrible way to die –his head was smashed, his belly torn and his bowels ballooned.” This photograph shows where many such tragedies took place. [Gensuikin]

Nagasaki: Mitsubishi Arms Factory Ohashi Plant
About 1.3 kilometers north of the hypocenter. [Nagasaki Museum] Notice how the walls have been stripped bare and all that is left are the steel rebars of the building.

Like Hiroshima, the immediate aftermath in Nagasaki was a nightmare. More than forty percent of the city was destroyed. Major hospitals had been utterly flattened and care for the injured was impossible. Schools, churches, and homes had simply disappeared. Transportation was impossible.

Many of the survivors “Hibakusha” have recorded their memories of those days.

Fujie Urata Matsumoto, relates this scene: “The pumpkin field in front of the house was blown clean. Nothing was left of the whole thick crop, except that in place of the pumpkins there was a woman’s head. I looked at the face to see if I knew her. It was a woman of about forty. She must have been from another part of town – I had never seen her around here. A gold tooth gleamed in the wide-open mouth. A handful of singed hair hung down from the left temple over her cheek, dangling in her mouth. Her eyelids were drawn up, showing black holes where the eyes had been burned out…She had probably looked square into the flash and gotten her eyeballs burned.” [Atomic Archive]

The Chinzei school from the east bank of the Urakami River. Although it appears that the building is intact, the entire roof had caved in. The city did not suffer from a firestorm as had Hiroshima. However, the blast effects were more destructive, because of the bowl-shaped topography and the greater yield of the bomb. [Atomic Archive]

The Urakami Cathedral, one of Nagasaki’s prominent landmarks, stands on a hill amid the rubble of a residential district east of ground zero. [Atomic Archive]

Ruins in the foreground are of the Mitsubishi shipbuilding factory. The white building housed the sawmill. The double smoke stacks are a part of the steel mill. This factory is about 4,750 feet (1,450 meters) south of the hypocenter. [Atomic Archive]

[Stories of material found on Banker’s Trust building at WTC]

d. The range for blast effects increases significantly with the explosive yield of the weapon. In a typical air burst, these values of overpressure and wind velocity noted above will prevail at a range of 0.7 km for 1 kiloton (Kt) yield; 3.2 km for 100Kt; and 15.0 km for 10 Mt.

Table of blast winds speeds and yield
Pictures of complete destruction

306. Propagation of Blast Wave in Air.

During the time the blast wave is passing through the superheated atmosphere in the fireball, it travels at supersonic velocities. After it leaves the vicinity of the fireball, it slows down to the normal speed of sound in the atmosphere. As long as the blast wave is expanding radially, its intensity decreases approximately as the square of the distance. When the expanding blast wave from a nuclear air burst strikes the surface of the earth, however, it is reflected (Figure 3-I), and the reflected wave reinforces and intensifies the primary wave.

a. Targets in the vicinity of ground zero may actually be subjected to two blast waves: the initial or incident wave, followed slightly later by a secondary reflected wave. This limited region close to ground zero in which the incident and reflected waves are separate is known as the region of regular reflection.

Description: Chronological sequence of the Mach-reflected detonation wave of a 20 kiloton air burst [56 Wikimedia Commons. Author: SiriusB ]

b. Beyond the area of regular reflection as it travels through air which is already heated and compressed by the incident blast wave, the reflected wave will move much more rapidly and will very quickly catch up with the incident wave. The two then fuse to form a combined wave front known as the Mach stem. The height of the Mach stem increases as the blast wave moves outward and becomes a nearly vertical blast front. As a result, blast pressures on the surface will not decrease as the square of the distance, and most direct blast damage will be horizontally directed, e.g., on the walls of a building rather than on the roof.

c. As the height of burst for an explosion of given yield is decreased, or as the yield of the explosion for a given height of burst is increased, Mach reflection commences nearer to ground zero and the overpressure near ground zero becomes larger. However, as the height of burst is decreased, the total area of coverage for blast effects is also markedly reduced. The choice of height of burst is largely dependent on the nature of the target. Relatively resistant targets require the concentrated blast of a low altitude or surface burst, while sensitive targets may be damaged by the less severe blast wave from an explosion at a higher altitude. In the latter case a larger area and, therefore, a larger number of targets can be damaged.

d. A surface burst results in the highest possible overpressures near ground zero. In such a burst, the shock front is hemispherical in form, and essentially all objects are subjected to a blast front similar to that in the Mach region described above. A subsurface burst produces the least air blast, since most of the energy is dissipated in the formation of a crater and the production of a ground shock wave.


307. Static Overpressure and Dynamic Pressure.

a. Two distinct though simultaneous phenomena are associated with the blast wave in air:

(1) Static overpressure, i.e., the sharp increases in pressure due to compression of the atmosphere. These pressures are those which are exerted by the dense wall of air that comprises the wave front. The magnitude of the overpressure at any given point is directly proportional to the density of the air in the wave.

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

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.

[Chart: Atomic Archive]
2) Dynamic pressures, i.e., drag forces exerted by the strong transient blast winds associated with the movement of air required to form the blast wave. These forces are termed dynamic because they tend to push, tumble, and tear apart objects and cause their violent displacement.

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

b. In general, the static overpressure rises very abruptly from normal atmospheric in the unaffected air in front of the blast wave to a sharp peak (Figure 3-II). It then decreases behind the front. As the blast wave moves out from ground zero, the peak overpressure of the front diminishes while the decay of overpressure behind the front becomes more gradual. After traveling a sufficient distance from the fireball, the pressure behind the front actually drops below normal atmospheric pressure, the so-called negative phase of the blast wave.

c. In passing through the atmosphere, the blast wave imparts its energy to the molecules of the surrounding air, setting them into motion in the direction of the advancing shock front. The motion of these air molecules is manifested as severe transient winds, known as “blast winds,” which accompany the blast wave. The destructive force associated with these winds is proportional to the square of their velocity and is measured in terms of dynamic pressure. These winds constitute decay forces which produce a large number of missiles and tumbling of objects. These dynamic forces are highly destructive.

d. Most of the material damage caused by a nuclear air burst is caused by a combination of the high static overpressures and the dynamic or blast wind pressures. The relatively long duration of the compression phase of the blast wave (Figure 3-II) is also significant in that structures weakened by the initial impact of the wave front are literally torn apart by the forces and pressures which follow. The compression and drag force phases together may last several seconds or longer, during which forces many times greater than those in the strongest hurricane are present. These persist even through the negative phase of a blast wave when a partial vacuum is present because of the violent displacement of air.

e. It is of practical value to examine the variation in pressure at a fixed location as a function of time. For a short period of time after a nuclear air burst, there will be no increase in pressure since it takes a finite time for the shock front to reach a given point. This arrival time, which may range from a few seconds to minutes, will depend primarily on the distance of the location from the center of burst and to a lesser extent on the yield of the explosion. Initially, the speed of the shock front is many times the speed of sound because it is traveling through superheated air, but as it travels away from the fireball it slows down to the speed of sound, 330 m/sec, in normal atmospheres. With high yield detonations, the early velocity of the shock front and the distance traveled through superheated air is greater. Therefore, time is somewhat less. Upon arrival of the shock front, both the static overpressure and the dynamic pressure increase almost immediately from zero to their maximum values. The peak values of pressure will, of course, depend on the distance from ground zero, the height of burst, and the yield and will be further modified by differences in terrain and meteorological conditions.

With passage of the blast front, both the static and dynamic pressures decay, though at slightly different rates. Most blast damage will be experienced during the positive or compression phase of the wave. The duration of this positive phase increases with yield and distance from ground zero and ranges from 0.2 to 0.5 sec for a 1 KT nuclear air burst to 4 to 10 sec for a 10 Mt explosion. This compares with only a few hundredths of a second for the duration of a blast wave from a conventional high-explosive detonation.

f. Because of the much longer duration of the blast wave from a nuclear explosion, structures are subjected to maximum loading for correspondingly longer periods of time, and damage will be much more extensive for a given peak overpressure than might otherwise be expected. During the negative phase, which is generally of even longer duration, the static pressure will drop below normal atmospheric pressure and the blast winds will actually reverse direction and blow back towards ground zero. Damage sustained during the negative phase is generally minor, however, because the peak values of underpressure and wind velocity are relatively low.

Kitchen window standard [Nukefix]

The kitchen window standard is this: A 1 psi blast wave is equivalent to the kitchen window being hit with 1,920 pounds of force when all doors and windows of the house are closed [1,920 = 30x64x1]. Similarly, a 3 psi blast is equivalent to the window being hit with 5,760 pounds of force. So you can see why not only windows break, but walls can come tumbling down with such overpressures. [Nukefix]

Exceptions at especially high blast pressures (15-29 psi):

17 psi,
32,640 lbs on the kitchen window

‘As Glasstone [co-author of The Effects of Nuclear Weapons] states, this building near ground zero in Hiroshima “suffered remarkably little damage externally” …, “however there was considerable destruction of the interior and contents due to entry of blast through doors and window openings and to subsequent fires. “‘ [Nukefix]

Some exceptions at especially high blast pressures (15 to 29 psi)

15 psi,
28,000 lbs on the kitchen windo

“Here the smokestacks are narrow, and thus the blast wave so rapidly passed around them that the pressure was exerted for an insufficient time for destruction. The surface area evidently was sufficiently low relative to the structural strength of the stacks. Moreover, the smokestacks were open at the top. The blast wave may have traveled down the stacks exerting equalizing pressures on interior and exterior surfaces.”

High blast pressures (5 – 6.1 psi)
6.1 psi,
11,712 lbs on the kitchen window

“As can be seen at the lower portion of the telephone pole, compression due to blast overpressure had little effect [rather like squeezing a block of wood], but the vast change in air pressure created high winds, which broke the upper section of the pole. In the background we see a trolley car which despite very high pressures remained intact, although its windows were blown out.”

5 psi,
9,600 lbs on the kitchen window

“Looking at the “Before” and “After,” obviously the home was totally destroyed at 5 psi. But an identical building at 1.7 psi was, as Glasstone states, “damaged to a considerable extent … , but could be made available for habitation by shoring and some fairly inexpensive repairs.” The most evident exterior damage at 1.7 psi was windows being blown out; the exterior otherwise appeared intact. The destruction occurred during tests at a Nevada test site in 1955.”

Wood-framed homes at high blast pressures and at risk of burning (4.3 to 5 psi)

5 psi,
9,600 lbs on the kitchen window

1 second after the burst, intense heat caused this house to be covered with a thick black smoke that ceased within 2 seconds without igniting the house. The house had been given, as Glasstone states, “a white exterior finish in order to reflect the thermal radiation and minimize the chance of fire.” [..] Several seconds after detonation, the blast wave arrived and totally demolished the house, as recorded by a high speed camera.

4.3 psi,
8,256 lbs. on the kitchen window

Such a large proportion of wood-framed structures were destroyed by fire at Hiroshima that, as Glasstone states, “very little detailed information concerning blast damage was obtained.” This building, which did not succumb to fire at Hiroshima, illustrates the effect of high blast pressure.

Wood-framed home at greater distance from blast (1.7 psi)

1.7 psi,
3,264 lbs on the kitchen window

The most apparent external damage was that the doors and windows were blown out and the roof damaged. As Glasstone states, this house “was badly damaged both internally and externally, but it remained standing. … [..] People would have tended to experience cuts from glass fragments and, as Glasstone states, “possible fatal injuries from flying debris or as a result of translational displacement of the body as a whole.” Shelters in the basement were intact. At Hiroshima 2% of the population were killed and 25% injured in the 2 to 0.75 psi region.

Survival Town house

“A 29-kiloton device named “Apple II” was detonated from a 500-foot tower on Yucca Flat.
It was the second nationally televised nuclear test associated with an extensive civil effects program. This “Survival Town” house, some 7,500 feet from a 29-kiloton nuclear detonation, remained essentially intact. Survival Town consisted of houses, office buildings, fallout shelters, power systems, communications equipment, radio broadcasting station, and trailer homes. The town was built for a Civil Defense exercise and to test not previously subjected to a nuclear blast.” [74. Shundahai]

300-Year Old Camphor Tree

Hiroshima: In the precincts of the Kokutaiji Temple, the big camphor tree, said to be over 300 years old was designated as a natural monument. Its branches and thick leaves provided a place of comfort for the passers-by during summertime. Its roots spread out in all directions for 300 meters, and the street car lines shown on the left in the photograph had to avoid the tree, which formed an archway over the sidewalk. By a blast pressure of 19 tons per square meter, the tree was uprooted. Also, hundreds of tombstones were knocked in all directions by the complex flow of wind from the blast. The white building seen on the extreme right is the Hiroshima branch of the Bank of Japan. Because it was built of strong ferro-concrete and stonework, the exterior remained uncollapsed but the interior burned. [Gensuikin]

Damaged windows of the WFC (World Financial Center, headquarters of American Express. Even though the building was able to stand despite the blast effects, the overpressure from the blast blew out the windows. There was widespread damage to windows in the 9/11 attack. [Pic from Dust to Dirt 3 Judy Wood]

Another view of the WFC [Pic: Dust to Dirt 3]

Caption reads: “A downtown Las Vegas window, showing how the glass was sucked out by the rarefaction wave rather than pushed in by the compression wave resulting from the November 1, 1953, nuclear test at the Nevada Test Site.” [Pic from Glasstone Blogspot]

“As the blast wave moves out from ground zero, the peak overpressure of the front diminishes while the decay of overpressure behind the front becomes more gradual. After traveling a sufficient distance from the fireball, the pressure behind the front actually drops below normal atmospheric pressure, the so-called negative phase of the blast wave. [..] During the negative phase, which is generally of even longer duration, the static pressure will drop below normal atmospheric pressure and the blast winds will actually reverse direction and blow back towards ground zero. [FAS]

308. Blast Loading.

When a blast wave strikes the surface of a hard target, such as a building, the reflected wave will reinforce the incident wave, and the face of the building will be subjected to overpressures 2 to 8 times that of the incident wave alone. The severity of this additional stress depends on many factors, including the peak overpressure of the incident blast wave, as well as the angle at which the wave strikes the building. As the shock front advances, it bends or diffracts around the building, and the pressure on the front wall decreases rapidly. However, during the brief interval in which the blast wave has not yet engulfed the entire structure, a considerable pressure gradient exists from front to rear that places a severe stress on the building. For small objects, this period of so-called diffraction loading is so small that no significant stress is encountered. For large buildings, however, the stress of diffraction loading will be considerable. Even after the shock front has passed across the building, the structure will still be subjected to a severe compression force and to severe drag forces from the transient winds. The actual overpressures required to produce severe damage to diffraction sensitive targets are actually quite low. Table 3-II depicts failure of sensitive structural elements when exposed to overpressure blast loading.

Large pieces of steel called tridents recovered from the World Trade Center site, and once a structural part of the ground level exterior arches of the twin towers, are preserved in Hangar 17 of Kennedy International Airport. Three are about 1,350 pieces of steel, many weighing over 30 tons.[am New York]

A trident bent by a force capable of bending and twisting 30 tons of metal as if it were a paper clip [Pic: am New York]

Another trident bent into a U-shape. [Pic: am New York]

Another trident bent into a similar shape [Pic: am New York]

Beams twisted into various shapes [am New York]

Pieces of steel recovered from the World Trade Center site are preserved in Hangar 17 of Kennedy International Airport. Three are about 1,350 pieces of steel, many weighing over 30 tons.

Pieces of steel recovered from the World Trade Center site are preserved in Hangar 17 of Kennedy International Airport. Three are about 1,350 pieces of steel, many weighing over 30 tons. [Pic: am New York]

Angle iron from a factory
Part of Mitsubishi Nagasaki Arms Factory Mori-machi Plant
located about 1.3 kilometers from the hypocenter. [Nagasaki Museum]

Pictures of wreckage in Japan

309. Drag Loading.

All objects in the path of the blast wave, regardless of size or structure, will be subject to the dynamic pressure loading or drag forces of the blast winds. Drag loading is influenced to a moderate degree by the shape of the target. Round objects are relatively unaffected by the winds, while flat or recessed surfaces offer great resistance and hence are subjected to increased impact pressure and probability of damage. The effect of dynamic pressure is generally dependent on the peak value of dynamic pressure and its duration. While the dynamic pressure at the face of a building is generally less than the peak overpressure due to the blast wave and its reflection, the period of dynamic loading is much longer than that of diffraction loading, and hence the damage to frame-type buildings, bridges, and other structures will be considerable.

75. Pic – WTC 9/11 Untold Story

75. Pic – WTC 9/11 Untold Story

Notice the shattered windows. 75. Pic – WTC 9/11 Untold Story

  • Thick dust clouds spewed from towers in all directions, at around 50 feet/second.
  • Solid objects were thrown ahead of the dust – a feature of explosive demolition.
  • The steel was thoroughly cleansed of its spray-on insulation.
  • Some pieces of the perimeter wall were thrown laterally 500 feet.
  • Aluminum cladding was blown 500 feet in all directions.
  • Blast waves broke hundreds of windows in buildings over 400 feet away.

75. WTC 9/11 Untold Story

A man stands amidst the rubble of what was once Hiroshima, Japan, a month after the detonation of the atomic bomb. [Photo: AP/Wide World Photo. From 77 Bookrags: Hiroshima]

[ 77]

Equipment and personnel are relatively resistant to static overpressures but highly vulnerable to dynamic pressure. For example, military vehicles, from jeeps to tanks, are most likely to suffer damage when pushed, overturned, and thrown about by the blast winds.

When the atomic bomb exploded, seventy streetcars were operating in Hiroshima. Near the hypocenter, several were burned black. Occupants died where they sat. [77]

Streetcar completely gutted by fire (Model 150)
Approx. 320m from the hypocenter, Moto-machi
The streetcar packed with passengers was headed towards Hiroshima Station about 70 meters east of the Kamiya-cho intersection when the atomic bomb consumed it in fire. [77]

Overturned cars and wrecked twisted cars

Twisted trident

Twisted metal in Japan

Likewise, blast winds are the cause of most blast injuries. Because of the violence of the winds associated with even low values of overpressure, mechanical injuries due to missiles sent into motion by the winds or to violent bodily translation will far outnumber direct blast injuries due to actual compression of the organism.

“The explosion created a supersonic shock wave which was responsible for destroying most of the buildings in the blast zone. Fully half of the bomb’s released energy was released in the form of this wind, which spread out at 440 meters per second (1600 km/hr or 1000 miles/hr; the speed of sound is 330 meters per second). It not only knocked things down, it also filled the air with debris. The section of concrete wall below has numerous glass shards embedded in it, even though it was 2200 meters (one and a half miles) from the hypocenter, and sheltered from the blast by a low hill.” [Richard Seaman: Atomic Bomb Museum]

Story of Ondrovic thrown about

310. Shock Waves in Other Media.

a. In surface and subsurface bursts, a sizable portion of the yield is transmitted in the form of ground or water shock waves. In the case of a surface burst on land, a crater is formed at ground zero, the size of which depends primarily upon yield. Relatively little damage beyond a distance of approximately three crater radii will occur due to ground shock. Most damage will be due to the accompanying air blast wave. In subsurface bursts the crater will be formed either by ejection of material as in a shallow explosion or by the collapse of ground into the cavity formed by a deeper explosion. Since the overpressure in a ground shock wave decreases very rapidly with distance, shock damage will again be confined to a region close to the point of detonation.

Crater diagram

Judy Wood picture of crater at WTC

Crater pictures of nuclear explosions-Wikipedia-nuclear testing

b. Ground shock waves will also be induced as a result of an air burst. If the overpressure in the blast wave is very large, the ground shock will penetrate some distance into the ground and may damage underground structures and buried utilities, etc.

[c. Because of the density and relative incompressibility of water, shock waves in that medium have very high peak overpressures and velocities of propagation. The peak overpressure at a distance of 1 km from a 10 Kt underwater burst is approximately 6080 kPa (60 atm (atmospheres of pressure)), while the peak overpressure in air at the same distance from an air burst is only 111.4 kPa (1.1 atm). The resulting surface waves at this distance will be approximately 10 m in height. The shock front will also travel at approximately five times the speed of the blast wave in air. Severe damage to naval vessels may result from the shock wave produced by an underwater or water surface burst. Although the major portion of the shock energy is propagated in the water, a significant amount is also transferred through the surface as a typical air blast. This blast wave could probably be the principal source of damage to land targets if the explosion occurred in a coastal area.]

Measurement of blast – calculation, calculator, equations

Maps and diagrams and photos of damage

The Scorched Plain of Hiroshima
The T-shaped Aioi Bridge in the center of the photo is the target of the bombing. The land mass lying behind the bridge and sandwiched between two rivers is Nakajima-cho (the present Peace Memorial Park). [76]

Mechanical Injuries – Report by The Manhattan Engineer District, June 29, 1946

The mechanical injuries included fractures, lacerations, contusions, abrasions, and other effects to be expected from falling roofs, crumbling walls, flying debris and glass, and other indirect blast effects. The appearance of these various types of mechanical injuries was not remarkable to the medical authorities who studied them.

It was estimated that patients with lacerations at Hiroshima were less than 10,600 feet from X, whereas at Nagasaki they extended as far as 12,200 feet.

The tremendous drag of wind, even as far as 1 mile from X, must have resulted in many injuries and deaths. Some large pieces of a prison wall, for example, were flung 80 feet, and many have gone 30 feet high before falling. The same fate must have befallen many persons, and the chances of a human being surviving such treatment are probably small.

Shielding discussion

Diagrams of shielding effect

Bomb shelters – pictures

Aerial map with giant question mark?


311. Formation of Thermal Radiation.

Large amounts of electromagnetic radiation in the visible, infrared, and ultraviolet regions of the electromagnetic spectrum are emitted from the surface of the fireball within the first minute or less after detonation. This thermal radiation travels outward from the fireball at the speed of light, 300,000 km/sec. The chief hazard of thermal radiation is the production of burns and eye injuries in exposed personnel. Such thermal injuries may occur even at distances where blast and initial nuclear radiation effects are minimal. Absorption of thermal radiation will also cause the ignition of combustible materials and may lead to fires which then spread rapidly among the debris left by the blast. The range of thermal effects increases markedly with weapon yield.

“Approximately 35 percent of the energy from a nuclear explosion is an intense burst of thermal radiation, i.e., heat. The effects are similar to the effect of a two-second flash from an enormous sunlamp. Since the thermal radiation travels at roughly the speed of light, the flash of light and heat precedes the blast wave by several seconds, just as lightning is seen before thunder is heard.

The visible light will produce “flashblindness” in people who are looking in the direction of the explosion. Flashblindness can last for several minutes, after which recovery is total. If the flash is focused through the lens of the eye, a permanent retinal burn will result. At Hiroshima and Nagasaki, there were many cases of flashblindness, but only one case of retinal burn, among the survivors.” [Atomic Archives]

Before nuclear detonation: a screen capture from CNN news report of WTC 1 exploding. [40][911 research] Watch clip below.

An instant later the screen fills up with an intense blinding light. This still is taken from the same CNN news report. [40, 9/11 Research]. The news clip is below.

Video: Intense blinding light

VIDEO: CNNvideoflashshorter Dailymotion

Clip of CNN news report showing the blinding flash. The nuclear flash has been described as more brilliant than the light from “thousands of suns”.

QUOTE: At 05:29:45 local time (Mountain War Time), (11:29:45 GMT) the device exploded with an energy equivalent to around 20 kilotons of TNT (87.5 TJ). It left a crater of radioactive glass in the desert 10 feet (3 meters) deep and 1,100 ft (330 meters) wide. At the time of detonation, the surrounding mountains were illuminated brighter than daytime for one to two seconds …. In the official report on the test, General Farrell wrote, “The lighting effects beggared description. The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, gray, and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined…” [62 Wikipedia Trinity Test]

“This is usually termed thermal radiation. The visible light component accounts for the blinding flash seen upon detonation as well as the subsequent brightness of the fireball.” [fas]

QUOTE: Orange [an airburst nuclear test in 1958 near Johnston Island] produced a thermal flash exposure of 3.0 cal/sq cm at ground zero, not enough to cause fires, but enough to cause serious eye retina heating and permanent damage for anyone watching the fireball at night without welder’s goggles. [58 Glasstone Blogspot]

Orange at 1.0 second

Orange at 2.0 seconds

Orange at 3.0 seconds

GIF of the flashing of the Orange nuclear test explosion

QUOTE: People farther from the point of detonation experienced first the flash and heat, followed seconds later by a deafening boom and the blast wave. [mbe.doe]

312. Propagation of Thermal Energy.

a. Most of the energy released in the fission or fusion processes is initially in the form of the kinetic energy of the products of the reactions (e.g., fission fragments, etc.). Within millionths of a second after detonation, numerous inelastic collisions of these vaporized atoms give rise to a plasma of intensely hot weapon residues. Since the temperature of this system is of several tens of million degrees centigrade, it emits enormous quantities of energy in the form of electromagnetic radiation. This radiation is subsequently absorbed by the surrounding atmosphere, which is heated to extremely high temperatures, causing it to emit additional radiation of slightly lower energy. This complex process of radiative transfer of energy is the basic mechanism by which the fireball is formed and expands.

b. Because this thermal radiation travels at the speed of light, and its mean free path (distance between point of emission and point of absorption) is relatively long, the initial expansion of the fireball is extremely rapid, much more so than the outward motion of gaseous material from the center of the burst responsible for production of the blast wave. Consequently, the blast wave front at first lags behind the radiative front (surface of the fireball).

c. However, as the fireball expands and its energy is deposited in an ever-increasing volume its temperature decreases and the transfer of energy by thermal radiation becomes less rapid. At this point, the blast wave front begins to catch up with the surface of the fireball and then moves ahead of it, a process called hydrodynamic separation. Due to the tremendous compression of the atmosphere by the blast wave, the air in front of the fireball is heated to incandescence. Thus, after hydrodynamic separation, the fireball actually consists of two concentric regions: the hot inner core known as the isothermal sphere; and an outer layer of luminous shock-heated air.

d. The outer layer initially absorbs much of the radiation from theisothermal sphere and hence the apparent surface temperature of the fireball and the amount of radiation emitted from it decreases after separation. But, as the shock front advances still farther, the temperature of the shocked air diminishes and it becomes increasingly transparent. This results in an unmasking of the still incandescent isothermal region and an increase in the apparent surface temperature of the fireball. This phenomena is referred to as breakaway.

313. Rate of Thermal Emission.

a. The rate of thermal emission from the fireball is governed by its apparent surface temperature. From the foregoing discussion, it should be apparent that the thermal output of a nuclear air burst will then occur in two pulses (Figure 3-IV), an initial pulse, consisting primarily of ultraviolet radiation, which contains only about 1% of the total radiant energy of the explosion and is terminated as the shock front moves ahead of the fireball, and a second pulse which occurs after breakaway.

b. The thermal radiation emitted from the fireball surface during the second thermal pulse is responsible for most of the thermal effects. It consists chiefly of radiation in the infrared, visible, and ultraviolet regions of the electromagnetic spectrum. Thermal exposure (measured in joules per unit area of exposed surface) will be less farther from the center of the explosion because the radiation is spread over a greater area and is attenuated in passing through the intervening air. Since the fireball is very close to a point source of thermal radiation, the quantity of thermal radiation at any given point varies approximately with the square of the distance from the explosion. The inverse square law does not apply exactly because thermal radiation, particularly ultraviolet, will also be absorbed and scattered by the atmosphere. The degree of atmospheric visibility affects the attenuation of thermal energy with distance to a limited degree, but less than would be expected from the purely absorptive properties of the atmosphere, because the decrease in transmission is largely compensated by an increase in scattered radiation.

Hiroshima as seen from the edge of the Kanda Bridge about three hours after the explosion. Note the two-toned plume: black smoke from the firestorm and the white cloud of the nuclear explosion above it. [Pic from Atomic Archive]

314. Shielding.

Since thermal radiation travels in straight lines from the fireball (unless scattered) any opaque object interposed between the fireball and the target will act as a shield and provide significant protection from thermal radiation. If a significant amount of scattering is present, as is the case when visibility is poor, thermal radiation will be received from all directions and shielding will be less effective.

315. Yield and Altitude.

a. Yield. The total amount of thermal radiation, the period of time during which it is emitted, and the range for thermal effects increase with the yield of the nuclear explosion (Figure 3-V).

b. Altitude Effects. The thermal radiation intensity at a given point will depend on the altitude and the type of burst. In general, the thermal hazard is greatest in the case of a low altitude air burst. General thermal effects will be less for surface bursts and frequently nonexistent for subsurface bursts. In surface bursts a large part of the thermal energy is absorbed by the ground or water around ground zero. In addition, shielding due to terrain irregularities of dust, moisture, and various gases in the air near the surface of the earth will tend to reduce the amount of thermal energy reaching a target. In subsurface bursts without appreciable penetration, most of the thermal energy is absorbed and dissipated in heating and vaporizing soil and water below the surface.

c. High Altitude Effects. In high altitude air bursts (above 30 km), the low density of the atmosphere alters the nature of the thermal radiation process because the primary thermal radiation is absorbed in a much larger volume of air, and the temperature of the system is correspondingly less. While a greater percentage of the yield of the explosion appears in the form of thermal radiation, much of the radiation is emitted so slowly that it is ineffective. About 25-35% of the total yield is emitted in a single pulse of very short duration. Moreover, because of the relatively great distance between the center of the burst and the earth’s surface, the intensity of thermal radiation at ground level is generally low.

316. Thermal Effects.

a. When thermal radiation strikes an object, part will be reflected, part will be transmitted, and the rest will be absorbed. The fraction of the incident radiation that is absorbed depends on the nature and color of the material. A thin material may transmit a large part of the radiant energy striking it. A light colored object may reflect much of the incident radiation and thus escape damage. Thermal damage and injury is due to the absorption of large amounts of thermal energy within relatively short periods of time. The absorbed thermal radiation raises the temperature of the absorbing surface and results in scorching, charring, and possible ignition of combustible organic materials, such as wood, paper, fabrics, etc. If the target material is a poor thermal conductor, the absorbed energy is largely confined to a superficial layer of the material.

b. The radiation exposure (# Joules/sq/cm) required for the ignition of materials and other thermal effects increases with the yield of the weapon (Table 3-III). This is so because increased thermal energy is required to compensate for energy lost via conduction and convection during the longer thermal pulse of higher yield weapons. For lower yield weapons, the thermal pulse is so short that there is not much time for these processes to cool the exposed surface. Hence, a much higher percentage of the deposited thermal energy is effective in producing thermal damage. This increased thermal requirement does not mean that the thermal hazard is less significant for higher yields. On the contrary, the total thermal energy released during a nuclear explosion increases markedly with yield, and the effects extend over much greater distances. Therefore, although more thermal energy is required to produce a given thermal response for a large yield explosion, the effective range to which this level extends is very much greater.

c. Actual ignition of materials exposed to thermal radiation is highly dependent on the width of the thermal pulse (which is dependent on weapon yield) and the nature of the material, particularly its thickness and moisture content. At locations close to ground zero where the radiant thermal exposure exceeds 125 Joules/sq cm, almost all ignitable materials will flame, although burning may not be sustained (Table 3-III). On the other hand, at greater distances only the most easily ignited materials will flame, although charring of exposed surfaces may occur. The probability of significant fires following a nuclear explosion depends on the density of ignition points, the availability and condition of combustible material (whether hot, dry, wet), wind, humidity, and the character of the surrounding area. Incendiary effects are compounded by secondary fires started by the blast wave effects such as from upset stoves and furnaces, broken gas lines, etc. In Hiroshima, a tremendous fire storm developed within 20 minutes after detonation. A fire storm burns in upon itself with great ferocity and is characterized by gale force winds blowing in towards the center of the fire from all points of the compass. It is not, however, a phenomenon peculiar to nuclear explosions, having been observed frequently in large forest fires and following incendiary raids during World War II.


The Scorched Plain of Hiroshima
The T-shaped Aioi Bridge in the center of the photo is the target of the bombing. The land mass lying behind the bridge and sandwiched between two rivers is Nakajima-cho (the present Peace Memorial Park). [76]


317. Sources of Nuclear Radiation.

Blast and thermal effects occur to some extent in all types of explosions, whether conventional or nuclear. The release of ionizing radiation, however, is a phenomenon unique to nuclear explosions and is an additional casualty producing mechanism superimposed on blast and thermal effects. This radiation is basically of two kinds, electromagnetic and particulate, and is emitted not only at the time of detonation (initial radiation) but also for long periods of time afterward (residual radiation). Initial or prompt nuclear radiation is that ionizing radiation emitted within the first minute after detonation and results almost entirely from the nuclear processes occurring at detonation. Residual radiation is defined as that radiation which is emitted later than 1 minute after detonation and arises principally from the decay of radioisotopes produced during the explosion.

318. Initial Radiation.

About 5% of the energy released in a nuclear air burst is transmitted in the form of initial neutron and gamma radiation. The neutrons result almost exclusively from the energy producing fission and fusion reactions, while the initial gamma radiation includes that arising from these reactions as well as that resulting from the decay of short-lived fission products. The intensity of initial nuclear radiation decreases rapidly with distance from the point of burst due to the spread of radiation over a larger area as it travels away from the explosion, and to absorption, scattering, and capture by the atmosphere. The character of the radiation received at a given location also varies with distance from the explosion. Near the point of the explosion, the neutron intensity is greater than the gamma intensity, but with increasing distance the neutron-gamma ratio decreases. Ultimately, the neutron component of initial radiation becomes negligible in comparison with the gamma component. The range for significant levels of initial radiation does not increase markedly with weapon yield and, as a result, the initial radiation becomes less of a hazard with increasing yield. With larger weapons, above 50 Kt, blast and thermal effects are so much greater in importance that prompt radiation effects can be ignored.

319. Residual Radiation.

The residual radiation hazard from a nuclear explosion is in the form of radioactive fallout and neutron-induced activity. Residual ionizing radiation arises from:

a. Fission Products. These are intermediate weight isotopes which are formed when a heavy uranium or plutonium nucleus is split in a fission reaction. 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 and gamma radiation. Approximately 60 grams of fission products are formed per kiloton of yield. The estimated activity of this quantity of fission products 1 minute after detonation is equal to that of 1.1 x 1021 Bq (30 million kilograms of radium) in equilibrium with its decay products.

b. Unfissioned Nuclear Material. Nuclear weapons are relatively inefficient in their use of fissionable material, and much of the uranium and plutonium is dispersed by the explosion without undergoing fission. Such unfissioned nuclear material decays by the emission of alpha particles and is of relatively minor importance.

c. Neutron-Induced Activity. If atomic nuclei capture neutrons when exposed to a flux of neutron radiation, they will, as a rule, become radioactive (neutron-induced activity) and then decay by emission of beta and gamma radiation over an extended period of time. Neutrons emitted as part of the initial nuclear radiation will cause activation of the weapon residues. In addition, atoms of environmental material, such as soil, air, and water, may be activated, depending on their composition and distance from the burst. For example, a small area around ground zero may become hazardous as a result of exposure of the minerals in the soil to initial neutron radiation. This is due principally to neutron capture by sodium (Na), manganese, aluminum, and silicon in the soil. This is a negligible hazard because of the limited area involved.

320. Fallout.

a. Worldwide Fallout. After an air burst the fission products, unfissioned nuclear material, and weapon residues which have been vaporized by the heat of the fireball will condense into a fine suspension of very small particles 0.01 to 20 micrometers in diameter. These particles may be quickly drawn up into the stratosphere, particularly so if the explosive yield exceeds 10 Kt. They will then be dispersed by atmospheric winds and will gradually settle to the earth’s surface after weeks, months, and even years as worldwide fallout. The radiobiological hazard of worldwide fallout is essentially a long-term one due to the potential accumulation of long-lived radioisotopes, such as strontium-90 and cesium-137, in the body as a result of ingestion of foods which had incorporated these radioactive materials. This hazard is much less serious than those which are associated with local fallout and, therefore, is not discussed at length in this publication. Local fallout is of much greater immediate operational concern.

b. Local Fallout. In a land or water surface burst, large amounts of earth or water will be vaporized by the heat of the fireball and drawn up into the radioactive cloud. This material will become radioactive when it condenses with fission products and other radiocontaminants or has become neutron-activated. There will be large amounts of particles of less than 0.1 micrometer to several millimeters in diameter generated in a surface burst in addition to the very fine particles which contribute to worldwide fallout. The larger particles will not rise into the stratosphere and consequently will settle to earth within about 24 hours as local fallout. Severe local fallout contamination can extend far beyond the blast and thermal effects, particularly in the case of high yield surface detonations. Whenever individuals remain in a radiologically contaminated area, such contamination will lead to an immediate external radiation exposure as well as a possible later internal hazard due to inhalation and ingestion of radiocontaminants. In severe cases of fallout contamination, lethal doses of external radiation may be incurred if protective or evasive measures are not undertaken. In cases of water surface (and shallow underwater) bursts, the particles tend to be rather lighter and smaller and so produce less local fallout but will extend over a greater area. The particles contain mostly sea salts with some water; these can have a cloud seeding affect causing local rainout and areas of high local fallout. For subsurface bursts, there is an additional phenomenon present called “base surge.” The base surge is a cloud that rolls outward from the bottom of the column produced by a subsurface explosion. For underwater bursts the visible surge is, in effect, a cloud of liquid (water) droplets with the property of flowing almost as if it were a homogeneous fluid. After the water evaporates, an invisible base surge of small radioactive particles may persist. For subsurface land bursts, the surge is made up of small solid particles, but it still behaves like a fluid. A soil earth medium favors base surge formation in an underground burst.

c. Meteorological Effects. Meteorological conditions will greatly influence fallout, particularly local fallout. Atmospheric winds are able to distribute fallout over large areas. For example, as a result of a surface burst of a 15 Mt thermonuclear device at Bikini Atoll on March 1, 1954, a roughly cigar-shaped area of the Pacific extending over 500 km downwind and varying in width to a maximum of 100 km was severely contaminated. Snow and rain, especially if they come from considerable heights, will accelerate local fallout. Under special meteorological conditions, such as a local rain shower that originates above the radioactive cloud, limited areas of heavy contamination may be formed.


Some links to investigate: – Nuclear Weapon Underground Testing – Underground Nuclear Weapons Testing – Nuclear Weapon Testing – Nuclear Weapon Effects – Radiation Effects – Thermal Effects – Blast Effects  –  EMP Effectside