Nuclear Weapons Effects--An Overview

by Wm. Robert Johnston
last updated 8 March 2005

The following discussion will briefly outline the various effects of nuclear detonations. For additional information see Nuclear weapons effects: Some data and The Effects of a Nuclear Attack on the Rio Grande Valley.

A nuclear explosion involves an energy release from nuclear reactions. These nuclear reactions include fission (splitting of plutonium and/or uranium-235 atomic nuclei), possibly fusion (fusing of light nuclei into heavier helium nuclei), and other reactions. The reactions in nuclear explosions release neutrons, gamma rays, and heat (along with some other particle radiations). These energy releases produce some mix of the following effects, depending on where the explosion occurs. (For more information, see these diagrams of a typical fission weapon and a typical thermonuclear weapon.)

Immediate effects (i.e. effects in the first few minutes) of a nuclear explosion in the atmosphere include primarily flash, blast, and prompt radiation (along with some other effects). Delayed effects include primarily fallout, along with other effects important only if many weapons are detonated.

Immediate Effects

A fireball is immediately formed as the gamma rays from the explosion superheat nearby air and/or other material (including the remains of the nuclear device). For an explosion in the atmosphere, the fireball will quickly expand to a maximum size, then continue cooling as it rises through the surrounding cooler air. This becomes the characteristic mushroom cloud.

Flash is the intense light and other thermal radiation given off by the fireball. With a temperature of thousands of degrees, the fireball radiates visible light, infrared light, and ultraviolet light. At close ranges, this light can cause heating and/or ignition of combustible materials and cause burn injuries to people. The damaging quantities of flash energy are released over a period of time that is longer for larger weapons, ranging from a fraction of a second for smaller weapons to a fraction of a minute for the largest weapons.

Flash is only important if the fireball is in the atmosphere. An explosion on (or near) the ground (or water) surface will produce about half as much energy release from flash as compared to an atmospheric explosion. Atmospheric conditions, such as clouds or haze, may reduce the range of flash effects.

Mass fire conditions, such as firestorms, may occur since a nuclear explosion may simultaneously ignite fires across large areas. This requires significant available fuel, such as from cities or forests.

Ground or water shock will result from explosions on (or near) the surface of the ground or water. This ground shock can damage or destroy hardened underground structures. In water this shock will be damaging to nearby vessels and may also produce a surface wave to limited ranges.

A crater is formed by an explosion at (or near) the ground surface. The size of the crater depends on the type of ground material and how close to the ground surface the explosion occurs.

Deep underground explosions will vaporize a cavity. Depending on the strength of the surrounding rock, a "chimney" of material over the cavity will collapse downward. If the explosion is close enough to the surface, the chimney collapse will reach the surface and a crater will form there.

Blast is the shock wave traveling through the atmosphere. This shock wave separates from the expanding fireball and briefly travels faster than sound, but then slows to the speed of sound. The arrival of the blast includes a sudden overpressure and high winds--winds initially outward, then reversing direction briefly before subsiding. At close ranges the blast will damage or destroy buildings and other structures and blow objects through the air. The blast corresponds to the "noise" of a nuclear explosion: until the blast arrives, which is generally after the flash effects, there is no direct noise from the explosion through the air.

Prompt radiation includes neutron radiation and gamma radiation released in the first few seconds of the detonation. These radiations are absorbed by the atmosphere, so they are only important close to the detonation. Prompt radiation is ionizing radiation and causes biological injury at the cellular level and molecular level. For doses over 1000 rem, death is virtually certain within days to weeks from failure of the digestive system or the central nervous system. From 100 rem to 1000 rem, the symptoms known as radiation sickness include injury to the tissues that produce blood. Symptoms may not appear for a few days; if death results, it will occur within one to eight weeks.

Smaller radiation doses may produce genetic defects in reproductive cells (potentially producing birth defects in offspring) or increased risk of cancer (and related diseases) developing years to decades later. The risks in both cases, however, are not dramatically different from (normal) background rates--at least for doses small enough not to cause immediate death.

Electromagnetic pulse (EMP) is important only for high altitude bursts. For such detonations, ionization of the upper atmosphere can produce a brief intense pulse of radio frequency radiation which can damage or disrupt electronic devices. For explosions above most of the atmosphere, EMP can affect large areas.

Ionization of the atmosphere from explosions in the atmosphere can interfere with radar and radio communications for short periods.

Charged particles produced by explosions above the Earth's atmosphere can be captured by the Earth's magnetic field, temporarily creating radiation belts that can damage spacecraft or injure astronauts/cosmonauts in orbit.

Delayed Effects

Fallout or delayed radiation is ionizing radiation from radioactive byproducts of the detonation. If these radioactive atoms are combined with debris from the ground or water droplets in the air, they will slowly settle to the ground downwind. This occurs with surface (or near surface) bursts. Local fallout is that material settling from the troposphere, within hours to a few days. For explosions high enough that the fireball does not touch the ground, the radioactive atoms tend to disperse sufficiently that local fallout is relatively insignificant.

Material lifted into the stratosphere may be dispersed far more widely and is called global fallout, settling to the ground over weeks and months. Global fallout is far less intense due to this dispersion and because the delay in reaching the ground reduces the radioactivity. Basically, global fallout is only important for large numbers of explosions (hundreds or thousands).

Since fallout is deposited by the wind, if and where it falls is very weather dependent. Once the radioactive fallout has been deposited on the ground, it continues giving off radiation. The rate of radiation decays in general by a factor of ten for every factor of seven increase in time after the explosion. Thus, fallout on the ground may be dangerous for days, weeks, or months depending on how much was present initially and whether any is moved (by erosion, for example).

The biological effects of radiation doses from fallout are the same as those for prompt radiation, with some differences. Because the radiation comes from particles, these particles can be inhaled and/or ingested (directly or from contaminated food), causing radiation injury to specific body organs. Radioactive particles on the skin can produce radiation burns to the skin. The biological effects cited for prompt radiation are diminished if the radiation is absorbed over periods of time of more than a week--with the exception of long-term effects such as production of cancer. Because radiation from fallout may be delivered over a longer period of time, these long-term effects may be non-negligible.

A nuclear explosion adjacent to highly radioactive material (such as spent fuel from a nuclear reactor) will vaporize the material and absorb this into the fireball. As a result, the fallout will be more severe and significantly more long lasting (hazardous for months or years, depending on intensity).

Changes in trace stratospheric gases can result for detonations in the atmosphere if the fireball rises into the stratosphere. In this case, the high temperatures of the fireball destroy ozone and create various oxides of nitrogen. For large numbers (hundreds or thousands) of high yield detonations, these changes can exceed natural variations. Such depletion of ozone would increase solar ultraviolet reaching the Earth's surface. Nitrogen oxides tend to produce a global cooling effect. Both consequences would return to normal over periods of years.

Nuclear winter or nuclear autumn is associated with large numbers of explosions (hundreds or thousands) over cities or other sources of combustible material. If smoke (and/or dust) from such explosions reach the stratosphere in large quantities, this material can take months or years to settle out of the atmosphere. By blocking sunlight from reaching the surface and lower atmosphere, this smoke can reduce sunlight and temperatures at the surface. Current understanding is that this phenomenon could have some effect on agricultural outputs but would not cause unnaturally cold temperatures or total crop failures.

Other indirect effects from the collapse of human infrastructure may be more important. Lack of sanitation and other damages from one or more explosions can increase the spread of disease. Widespread destruction poses additional risks in terms of loss of shelter and food supply. Psychological effects of explosions affecting a population include the various traumatic effects along with the general lack of understanding of phenomena such as radiation.

© 2002-2004, 2005 by Wm. Robert Johnston.
Last modified 8 March 2005.
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