4. Nuclear Blast Effects and Thermal Radiation
What exactly happens when a nuclear weapon explodes? After having previously discussed how the physics package is designed, it’s the natural question to ask. Over the next few articles in this series, I’ll cover the direct effects of a nuclear detonation over a city. There are other important situations that I will not cover, but which briefly deserve to be mentioned which include detonations underground (mainly for testing) and underwater (mainly for testing and naval warfare).
The direct effects are the ones that garner military consideration as they are comparatively predictable, controllable, and occur on short time scales. For civilians, often the indirect effects matter more in the hours, days, weeks, and years afterward. These indirect effects include ozone depletion, the potential for nuclear winter, humanitarian disruption, and radioactive toxic fallout.
Blast Effects of a Nuclear Weapon
For attacking cities and land based military installations, there’s two basic ways to drop a nuke: surface bursts and air bursts. Nuclear weapons are famous for their deadly light and heat as is befitting the brief life of a synthetic terrestrial star, but from the military’s perspective, it’s the blast pressure that does most of the direct damage. The other harmful effects are of particular interest to ordinary people, but they become important relatively long after the roughly thirty seconds it would take for a Nagasaki size explosive’s energy to dissipate. A one megaton bomb would take more than fifty seconds to dissipate.
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The direct effects of an atomic blast are a large fireball, wide area blast pressure, ionizing radiation, a flash of thermal radiation, EMP, chemical changes to atmospheric composition and radioactive fallout. The direct threats to humans consist in being blown to bits, cooking internally, flash burns, barometric injury, temporary or permanent blinding, crushing, lacerations, radiation sickness, and later on cancer. About 50% of the bomb’s energy is invested in the air blast, 35% into thermal radiation, and 15% into nuclear radiation.
Surface bursts, where the fireball touches the ground, are useful for attacking hardened missile silos and command and control centers because it exerts maximum pressure on the ground (greater than 10,000 psi). This results in a smaller more intense ring of blast damage, a large crater, and lots of radioactive dirt and debris scattered to the wind as fallout.
Air bursts, where the fireball does not touch the ground, are more useful for leveling cities. It only takes between 5–20 psi overpressure to heavily damage or destroy most civilian structures by crushing them like empty soda cans. An air burst also applies its destructive power more evenly over a wider area than a surface burst. Because the fireball does not touch the ground and kick up debris, air bursts can be considered radiologically cleaner in their after effects.
“In the immediate neighborhood of a ground burst a target suffers extremely high pressures. For large charges this is, however, not an advantage because it means that the immediate neighborhood would be destroyed more radically than necessary, and the energy so wasted would not be available elsewhere. In addition to avoiding overdestruction, an air burst at sufficient height minimizes the shielding of remote structures by those near the detonation.
For atomic bombs there is a further reason for detonation at an altitude. It is not desirable to let the ball of fire with its enormous temperatures, approaching 1,000,000°C., immediately around the explosion get in contact with matter in bulk, in particular with the ground. Such contact would result in the loss of much energy in evaporating the earth. There is a further advantage in an air burst, for such a burst is accompanied by certain forms of blast reflection which would not occur in the case of a surface burst.”
— “The Effects of Atomic Weapons” p. 61–62 (1950)
The damage done by the blast wave can be divided into static overpressure and dynamic pressure. Static overpressure is an increase in ambient air pressure, the same effect that gives divers the bends or crushes the aforementioned empty cans. Dynamic pressure is the spreading out of a ball of high pressure into areas of low pressure, or in other words high winds like a hurricane. According to “The Effects of Nuclear War”, a publication by the now defunct Congressional Office of Technology Assessment: “In general, large buildings are destroyed by the overpressure, while people and objects such as trees and utility poles are destroyed by the wind.” (page 16)
Crushing Pressure
The large surface areas of building walls allow enormous pressures to be applied for even small increases in the ambient air pressure. Objects like humans have a smaller surface area, but are easier to blow away like Mary Poppins. Telephone poles are solid, unlike hollow buildings, so crushing pressure affects them less. According to “The Effects of Nuclear War”, a one megaton air burst can exert greater than 5 psi overpressure, enough to crush a house, up to about four miles away from ground zero alongside hurricane force winds of 160 mph.
Human bodies are fairly resistant to barotrauma, or air pressure damage. Amusingly, it appears that over the decades each new research group attempted to barely differentiate itself by changing one word in their report’s title culminating in the 1977 report “The Effects of Nuclear Weapons” by Glasstone and Dolan. They found that the threshold of eardrum rupture is around 5 psi, lung damage between 8–15 psi, and 50% lethality between 30–50 psi. What this means is that human beings and buildings very close to the bomb are killed, but there exists a wide region around the blast where buildings are much more vulnerable than humans. Building collapses are a large factor in raising the death toll from atomic warfare. This is important for civil defense as it means a well constructed shelter can afford significant protection. For a contemporary take on civil defense, check out this article by AV Flox and Yonatan Zunger.
High Winds
While overpressure is the effect that demolishes buildings, the wind is quite deadly as well. It can be illustrative to compare the wind speed at various distances from the blast to hurricanes and tornados. Hurricane and tornado scales relate speed to expected damage, so they cut off above level five because nearly everything is already destroyed by that point.
For a one megaton nuclear explosive, at distances of up to three miles, wind speeds are so high that they vastly exceed the rating boundary of the most catastrophic hurricane and tornados. Category 5 hurricanes can leave the area “uninhabitable for weeks or months” and EF5 tornados will cause “incredible phenomena” to occur like leveling everything in their path such that nothing recognizable remains, twisting skyscrapers, launching tree missiles, and at last gifting us flying cars. At distances of up to about six miles, nuclear winds are still strong enough to strip the walls off buildings and kill people out in the open. Around eleven miles out, windows still shatter and can lacerate people standing near them. That said, the winds don’t include the potential of flooding or hail and are of relatively short duration compared with a storm that can linger in place for minutes, hours, or days as they did in the case of Hurricane Harvey.
Nonetheless, those brief moments are enough. A picture of the aftermath in Nagasaki is featured below. Haunting before and after pictures of Hiroshima can be seen in “Hiroshima: Before and After the Atomic Bombing” by Alan Taylor. The below picture is not shown to celebrate the damage, but rather to illustrate the kinds of crimes these devices have committed and to ground your mental model in the real world.
Area Damage
The area damage effect a weapon has is a bit counter intuitive to understand. While bigger is certainly badder, a number of smaller weapons can destroy more territory than a single big explosive. Explosive blasts disperse energy roughly in a sphere around the origin of the detonation, but the damage is applied to a roughly circular area on the ground. Additionally, the blast radius increases with an inverse-cube law. This is different from the inverse square laws you may be familiar with from gravitation and electromagnetism. The inverse-cube arises because while radiative spherical wavefronts disperse with the increasing size of the spherical shell, blasts rely on the pressure of the volume of air behind them to power the wavefront.
When these effects are taken together, much of the blast energy is wasted (especially the upper half that blows skyward). If you’re a bomb designer that wants to double the damaged area, you’ll need about three times higher yield. If you want to double the affected radial distance, you’ll need a bomb about eight times bigger. (These numbers come from solving 2 = x^(2/3) and 2 = x^(1/3) respectively.)
There’s a point of diminishing returns for making bigger bomb yields and it informs the different choices militaries make about their nuclear force compositions. For instance, the U.S. has retired most of its designs that have greater than about a megaton yield. However, this is compensated for by a war plan that uses a number of accurate smaller bombs to achieve about the same effect. For countries with the capability to deploy smaller yield missiles, the use of very high yield weapons (greater than one megaton yield) could be interpreted as a cost saving measure to avoid building and maintaining additional equipment. High yield weapons are also used to compensate for low accuracy missiles, but at least in the United States, our missiles have become very accurate.
Thermal Effects of a Nuclear Weapon
The blinding light, thermal energy, ionizing and fallout radiation make up the other half of the bomb’s energy. The widest area direct effect of an atom bomb is the release of thermal energy. The core of the bomb initiates its explosion by emitting very high energy radiation which is absorbed by the air around it and reemits it at lower energies. This heating of the air to over a million degrees centigrade is what creates the enormous luminous ball of fire at the center of the explosion. However, as discussed above, air bursts are typically set to such an altitude that the ball of fire does not touch the ground.
Within about three seconds of detonation (The Effects of Atomic Weapons p. 175), the bomb releases a large flux of thermal radiation that creates a similar effect to microwaving a large area using many frequencies of light. Very close to ground zero, this creates very high temperatures.
Nagasaki at ground zero was estimated to be heated to between 3000–4000 degrees centigrade. This very short but intense energy flux bleached exposed materials, leaving the negative imprints of unfortunate people’s shadows etched in stone and wood. People this close to the blast were concurrently killed by several different effects and while they were probably not vaporized, they certainly were intensely heated and blown away violently.
Farther away from the blast, the thermal flux produces third, second, and first degree burns to exposed skin at increasing distance from the center.
“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.”
— Atomicarchive.com, “Flash Burns”
Since the length of the thermal flux is so short, only a few seconds, nearly any covering like a wall, blanket, or lightly colored (i.e. more reflective) clothing can help protect you.
In the aftermath of Hiroshima’s bombing, the U.S. government sponsored filming of the consequences of the bombing including the medical effects of the bomb. The films were classified and were not released to the public until the 1980s in part due to concerns on the effect on public opinion. You can view the effects of flash burns here, but it is quite graphic. The video contains images of severely burned men, women and children with much of the healed tissue resulting in raised keloid scarring.
At the moment of explosion the bomb is so bright, many times brighter than the sun, that even if you are not looking at it, it can induce temporary flash blindness. Looking directly at the fireball will blind you permanently.
“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…. 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.”
— Atomicarchive.com, “Flash blindness”
Temporary blindness doesn’t sound so bad, but it happens at the same time that buildings are crumbling, your clothes are burning, and while you or people around you may be seriously injured. Those seconds to minutes when you’re unable to see could prove decisive for survival.
The heat can also set kindling on fire. If the fuel is dense enough, a firestorm can result. A firestorm occurs when many small individual fires combine into a single large fire that can sustain its own wind system and thereby continue to feed itself oxygen. A firestorm did result in Hiroshima, but not in Nagasaki. Fires can also be started from the broken gas and electrical lines of demolished buildings.
Unlike for high energy gamma and x-ray radiation, the atmosphere is mostly transparent to thermal radiation, so most of the attenuation with distance is due to a reduction in energy density as the spherical shell of light expands. Most of the energy emitted by a nuclear blast is within or adjacent to the visible spectrum, so the extent of thermal burns will depend on visibility. A beautiful clear day is much more dangerous than a hazy day for someone placed relatively far from the explosion.
I’ve made a MacOS Grapher file that lets you play with different visibility conditions given a 20 kT A-bomb scenario. The equations are taken from The Effects of Atomic Weapons p. 192. Note that these graphs apply only to thermal radiation, not X-rays and gamma rays. (download)
Next time, I’ll be covering the effects of ionizing radiation. If you’re not a trained radiation worker, I think, similarly to this article, you’ll find that there’s a lot of counter intuitive and striking information to understand. See you then. ☀
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NEXT TIME
Alpha, Beta, Gamma, Neutron: Nuclear Ionizing Radiation and Cancer
This article is part of a series on nuclear war. Read more at Insane Before the Sun.
A special thanks to Chris at Google Books for granting my request for full public access to The Effects of Atomic Weapons. It was instrumental for writing this article.
