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Effects of Nuclear Weapons
A nuclear explosion is a complex event. A huge amount of energy is released with the total energy distributed across a spectrum of outputs: blast, heat, and several forms of radiation. Depending on the altitude of the detonation, the size of the explosion, and distance from the center, the effects vary from instant vaporization to survivable impacts. Longer term effects linger in the form of residual radiation that can harm or kill over days, months or years, varying with the exposure dosage.
Categories of Nuclear Explosions and Effects
There are basically three categories of nuclear explosions. They differ in their effects based on the altitude of the burst:
An air burst is a nuclear explosion where the fireball does not touch the ground, a surface burst involves a fireball on the ground, and a sub-surface burst has the weapon detonated and the fireball formed under the surface of the earth. When the target is a broad area or military formations in the open, an air burst would be most effective. Deeply fortified positions or hardened installations might call for a sub-surface mode of attack. The Hiroshima and Nagasaki attacks were air bursts with detonation 1,900 feet and 1,650 feet above the city respectively.
There are three major effects from a nuclear explosion: blast, heat and radiation. The latter, radiation, may be directly from the fireball or from fallout of material made radioactive by the fireball. Near the center of the explosion the blast and heat are so great that solid materials are vaporized and no protection is possible. As distance from the center increases, the effects diminish and it is possible to talk about protective measures.
Blast Effect of Nuclear Weapons
The blast from a nuclear weapon is no different than from a chemical explosion (e.g. TNT) but is much larger, even thousands of times greater. The pressure wave (overpressure) is a violent wind-like effect moving outward from the center of the explosion that collapses fortifications, walls or buildings and can roll over heavy tanks or other massive objects (and ships if at sea). Direct blast effects on a human are less harmful than the indirect effect of being hit by an object propelled by the blast. That is, the human body can withstand perhaps 150 psi of overpressure with only minor injury such as ear damage, while 25 to 30 psi will collapse a building and injure or kill people within it.
Blast from an air burst or surface burst will be much greater than from a sub-surface burst where the ground will absorb much of the energy. While blast is the primary mode of damage from conventional weapons, it is the least damaging mode for nuclear weapons.
Heat and Light Effect of Nuclear Weapons
Heat is thermal radiation which everyone is familiar with and small, regulated amounts are beneficial, not harmful. In the first one or two seconds of the detonation of a nuclear weapon, the energy released causes the formation of a huge fireball, a hot and highly luminous, spherical mass of air and vaporized materials. The heat radiated from the fireball will ignite combustibles generating widespread fires or firestorms of great magnitude. Exposed materials from blast damaged structures are likely to add to the available fuel, especially in urban areas. The intense heat will burn exposed personnel at great distances from ground zero, even where the effects of blast and initial nuclear radiation are insignificant. The degree of injury from thermal radiation increases with the size of the weapon, but can be mitigated by weather and terrain. Thermal radiation is primarily a line-of-sight phenomena, so terrain masking can help reduce its effects.
The fireball is also a source of extremely bright visible light. The fireball of a nuclear weapon will appear to be many times more brilliant than the sun at noon, even at a great distance from ground zero. Observers who are not protected by dark lenses can be blinded, usually temporarily but permanently if burned.
Heat and light effects will be greatest from an air burst attack while a sub-surface explosion will have most heat and light absorbed by the Earth.
Prompt Radiation Effect of Nuclear Weapons
Prompt nuclear radiation is emitted in the first seconds after detonation, consisting primarily of neutrons and gamma rays. Gamma rays are invisible electromagnetic energy similar to medical X-rays. Gamma rays penetrate materials and interact with the human body causing damage to tissues and the blood-forming cells, altering and destroying cells and causing radiation sickness. Neutrons are nuclear particles with similar effects to those of gamma rays. When emitted at high speed by a nuclear explosion, they are highly penetrating and are easily absorbed by human tissue, producing extensive tissue damage within the body. Since the prompt radiation effects happen so quickly, a lethal or incapacitating dose of radiation can be received before it is possible to take any protective action.
Prompt radiation effects will be greatest from an air burst attack while a sub-surface explosion will have most prompt radiation absorbed by the Earth.
Residual Radiation Effect of Nuclear Weapons
Residual nuclear radiation lasts after the first minute and consists primarily of fallout and neutron-induced radiation. The primary hazard comes from the creation of fallout, produced when material from the Earth is drawn into the fireball, vaporized, combined with radioactive material, and condensed to particles which then fall back to the Earth. While larger particles fall back immediately in the vicinity of ground zero, smaller particles are carried by the winds until they gradually settle on the Earth's surface. Contaminated areas created by fallout may be very small or may extend over thousands of square miles. The dose rate may vary from an insignificant level to an extremely dangerous one for all personnel not taking protective measures.
Another residual radiation hazard arises from neutron-induced radioactivity on the Earth's surface in the immediate vicinity of ground zero. The intensity and extent of the induced radiation field will depend on the type of soil in the area around ground zero, the height of the burst, and the type and yield of the weapon.
The only significant source of residual radiation from an air burst weapon is induced activity in the soil of a limited circular pattern directly beneath the point of burst. A sub-surface burst, in contrast, will create significant hazards from fallout and residual radiation, if the explosion breaches the surface.
Energy Distribution in Nuclear Explosions
A typical nuclear explosion of a tactical-scale weapon will have its energy expended 50% as blast, 35% as thermal radiation (infrared, visible, and ultraviolet light and some soft x-rays), and 15% as nuclear radiation. Of the latter, 5% will be initial neutrons and gamma rays plus 10% as residual nuclear radiation.
Enhanced Radiation Weapons
Enhanced radiation (ER) devices were developed at Lawrence Livermore Laboratory (California) in 1958 and the first experimental weapon was detonated in spring 1963, according to published accounts. ER uses a fission-fusion thermonuclear reaction that maximizes the proportion of fast neutron emission compared to other nuclear effects. In other words, the blast and heat are lesser and the neutron radiation flux is greater. Heat and blast effects are limited to the immediate explosion area, while the neutrons are lethal over a wide area, and are capable of penetrating even armor plate or concrete walls. This type of weapon would tend to kill or sicken people and animals while generally sparing structures and equipment (except electronics that would sustain radiation damage).
Work on ER weaponry was associated with development of the antiballistic missile (ABM) system, but it may have other strategic or tactical uses. There was considerable controversy about their potential use when the idea became public.
Electromagnetic Pulse from a Nuclear Air Burst
A high-altitude nuclear explosion produces an intense electromagnetic pulse (EMP) that can affect land, sea, and air systems across continent-size areas. A one megaton or larger nuclear device detonated above the atmosphere over the middle of the U.S. would affect the entire nation. Unless equipment is specially designed to avoid EMP effects, the electric and magnetic fields from EMP will interact with electrical/electronic systems to produce damaging current and voltage surges. Modern electronic components that are common in widely used computers and communications equipment are very sensitive to EMP effects and would likely be destroyed by such an event. EMP can also be produced by non-nuclear means.
The book, The Effects of Nuclear Weapons , by S. Glasstone and P. Dolan is the classic text on this subject.
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