The immediate phenomena associated with a nuclear explosion, as well as the effects of shock and blast and of thermal and nuclear radiations, vary with the location of the point of burst in relation to the surface of the earth. For descriptive purposes five types of burst are distinguished, although many variations and intermediate situations can arise in practice.
The main types, which will be defined below, are (1) air burst, (2) high-altitude burst, (3) underwater burst, (4) underground burst, and (5) surface burst.
Provided the nuclear explosion takes place at an altitude where there is still an appreciable atmosphere, e.g., below about 100,000 feet, the weapon residues almost immediately incorporate material from the surrounding medium and form an intensely hot and luminous mass, roughly spherical in shape, called the “fireball.”
An “air burst” is defined as one in which the weapon is exploded in the air at an attitude below 100,000 feet, but at such a height that the fireball (at roughly maximum brilliance in its later stages) does not touch the surface of the earth.
For example, in the explosion of a 1-megaton weapon the fireball may grow until it is nearly 5,700 feet (1.1 mile) across at maximum brilliance. This means that, in this particular case, the explosion must occur at least 2,850 feet above the earth’s surface if it is to be called an air burst.
The quantitative aspects of an air burst will be dependent upon its energy yield, but the general phenomena are much the same in all cases. Nearly all of the shock energy that leaves the fireball appears as air blast, although some is generally also transmitted into the ground. The thermal radiation will travel long distances through the air and may be of sufficient intensity to cause moderately severe burns of exposed skin as far away as 12 miles from a 1-megaton explosion, on a fairly clear day.
For air bursts of higher energy yields, the corresponding distances will, of course, be greater. The thermal radiation is largely stopped by ordinary opaque materials; hence, buildings and clothing can provide protection.
The initial nuclear radiation from an air burst will also penetrate a long way in air, although the intensity falls off fairly rapidly at increasing distances from the explosion. The interactions with matter that result in the absorption of energy from gamma rays and from neutrons are quite different.
Different materials are thus required for the most efficient removal of these radiations; but concrete, especially if it incorporates a heavy element, such as iron or barium, represents a reasonable practical compromise for reducing the intensities of both gamma rays and neutrons. A thickness of about 4 feet of ordinary concrete would probably provide adequate protection from the effects of the initial nuclear radiation for people at a distance of about 1 mile from an air burst of a 1-megaton nuclear weapon. However, at this distance the blast effect would be so great that only specially designed blast-resistant structures would survive.
In the event of a moderately high (or high) air burst, the fission products remaining after the nuclear explosion will be dispersed in the atmosphere. The residual nuclear radiation arising from these products will be of minor immediate consequence on the ground.
On the other hand, if the burst occurs nearer the earth’s surface, the fission products may fuse with particles of earth, part of which will soon fall to the ground at points close to the explosion. This dirt and other debris will be contaminated with radioactive material and will, consequently, represent a possible danger to living things.
A “high-altitude burst” is defined as one in which the explosion takes place at an altitude in excess of 100,000 feet. Above this level, the air density is so low that the interaction of the weapon energy with the surroundings is markedly different from that at lower altitudes and, moreover, varies with the altitude. The absence of relatively dense air causes the fireball characteristics in a high-altitude explosion to differ from those of an air burst.
For example, the fraction of the energy converted into blast and shock is less and decreases with increasing altitude. Two factors affect the thermal energy radiated at high altitude.
First, since a shock wave does not form so readily in the less dense air, the fireball is able to radiate thermal energy that would, at lower altitudes, have been used in the production of air blast.
Second, the less dense air allows energy from the exploding weapon to travel much farther than at lower altitudes. Some of this energy simply warms the air at a distance from the fireball and it does not contribute to the energy that can be radiated within a short time.
In general, the first of these factors is effective between 100,000 and 140,000 feet, and a larger proportion of the explosion energy is released in the form of thermal radiation than at lower altitudes. For explosions above about 140,000 feet, the second factor becomes the more important, and the fraction of the energy that appears as thermal radiation at the time of the explosion becomes smaller.
The fraction of the explosion energy emitted from a weapon as nuclear radiations is independent of the height of burst. However, the partition of that energy between gamma rays and neutrons received at a distance will vary since a significant fraction of the gamma rays result from interactions of neutrons with nitrogen atoms in the air at low altitudes.
Furthermore, the attenuation of the initial nuclear radiation with increasing distance from the explosion is determined by the total amount of air through which the radiation travels. This means that, for a given explosion energy yield, more initial nuclear radiation will be received at the same slant range on the earth’s surface from a high-altitude detonation than from a moderately high air burst. In both cases the residual radiation from the fission products and other weapon residues will not be significant on the ground.
Both the initial and the residual nuclear radiations from high-altitude bursts will interact with the constituents of the atmosphere to expel electrons from the atoms and molecules. Since the electron carries a negative electrical charge, the residual part of the atom (or molecule) is positively charged, i.e., it is a positive ion. This process is referred to as “ionization,” and the separated electrons and positive ions are called “ion pairs.”
The existence of large numbers of electrons and ions at high altitudes may have seriously degrading effects on the propagation of radio and radar signals. The free electrons resulting from gamma-ray ionization of the air in a high-altitude explosion may also interact with the earth’s magnetic field to generate strong electromagnetic fields capable of causing damage to unprotected electrical or electronic equipment located in an extensive area below the burst.
The phenomenon known as the “electromagnetic pulse” (or EMP) is described elsewhere in this book. The EMP can also be produced in surface and low air bursts, but a much smaller area around the detonation point is affected.
If a nuclear explosion occurs under such conditions that its center is beneath the ground or under the surface of water, the situation is described as an “underground burst” or an “underwater burst,” respectively. Since some of the effects of these two types of explosions are similar, they will be considered here together as subsurface bursts.
In a subsurface burst, most of the shock energy of the explosion appears as underground or underwater shock, but a certain proportion, which is less the greater the depth of the burst, escapes and produces air blast. Much of the thermal radiation and of the initial nuclear radiation will he absorbed within a short distance of the explosion. The energy of the absorbed radiations will merely contribute to the heating of the ground or body of water.
Depending upon the depth of the explosion, some of the thermal and nuclear radiations will escape, but the intensities will generally be less than for an air burst. However, the residual nuclear radiation, i.e., the radiation emitted after the first minute, now becomes of considerable significance, since large quantities of earth or water in the vicinity of the explosion will be contaminated with radioactive fission products.
A “surface burst” is regarded as one which occurs either at or slightly above the actual surface of the land or water. Provided the distance above the surface is not great, the phenomena are essentially the same as for a burst occurring on the surface. As the height of burst increases up to a point where the fireball (at maximum brilliance in its later stages) no longer touches the land or water, there is a transition zone in which the behavior is intermediate between that of a true surface burst and of an air burst.
In surface bursts, the air blast and ground (or water) shock are produced in varying proportions depending on the energy of the explosion and the height of burst.
Although the five types of burst have been considered as being fairly distinct, there is actually no clear line of demarcation between them. It will be apparent that, as the height of the explosion is decreased, a high-altitude burst will become in air burst, and an air burst will become a surface burst.
Similarly, a surface burst merges into a subsurface explosion at a shallow depth, when part of the fireball actually breaks through the surface of the land or water. It is nevertheless a matter of convenience, as will be seen in later chapters, to divide nuclear explosions into the five general types defined above.