Understanding Radiation
Radiation and Fallout
 Fallout is simply the dirt and dust which
falls to the ground following a nuclear explosion. It
will "charged" with radiation and will eventually "burn"
itself out - a process that will take several days.
Radioactive fallout will fall in a manner
similar to that following a volcanic eruption. It will
be flaky in appearance and its size may reduce to dust
particles or smaller. Expect it to be thicker near the
detonation site and thinner as it travels down wind.
The bad news about fallout is that it's
radioactivity can penetrate thick surfaces (including steel,
wood, and earth) even though the actual dust from the
fallout can not. In short, if you are exposed outside
a shelter to an amount of just 400 R/hr, you will be dead
within a few hours. The good news is that fallout's
radioactive properties reduce to near normal levels around
about 48 hours.
This is where an underground bomb shelter
comes into play. Ideally following a nuclear explosion
in which you have survived the initial blast, you would
simply collect your family inside your "by-the-book"
constructed shelter and wait it out. Four days later,
you come out and start rebuilding your lives.
Understanding fallout and what you need do
to be near 100% free of its dangers is critical. The
construction of your underground bomb shelter should satisfy
the amount of barrier you need to survive. Basically,
you should build you shelter so its roof is at least 48
inches underground (36 for undisturbed soil). Whether
you use steel-reinforced concrete or a layer of lead, the 3
to 4 feet of soil will provide effective protection and act
as the first barrier for keeping radioactive elements from
entering your body.
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.
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.
he 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.
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:
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.
Un-fissioned 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.
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.
Fallout
Worldwide Fallout
After an air burst the fission products,
un-fissioned 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.
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 radio contaminants 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.
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.
General Principles of Nuclear Explosions
An explosion, in general,
results from the very rapid release of a large amount of
energy within a limited space. This is true for a
conventional “high explosive,” such as TNT, as well as for a
nuclear (or atomic) explosion,1 although the energy is
produced in quite different ways. The sudden liberation of
energy causes a considerable increase of temperature and
pressure, so that all the materials present are converted
into hot, compressed gases.
Since these gases are at very high temperatures and
pressures, they expand rapidly and thus initiate a pressure
wave, called a “shock wave,” in the surrounding medium air,
water, or earth. The characteristic of a shock wave is that
there is (ideally) a sudden increase of pressure at the
front, with a gradual decrease behind it. A shock wave in
air is generally referred to as a “blast wave” because it
resembles and is accompanied by a very strong wind. In water
or in the ground, however, the term “shock” is used, because
the effect is like that of a sudden impact.
Nuclear weapons are similar to those of more conventional
types insofar as their destructive action is due mainly to
blast or shock. On the other hand, there are several basic
differences between nuclear and high-explosive weapons.
In the first place, nuclear explosions can be many thousands
(or millions) of times more powerful than the largest
conventional detonations.
Second, for the release of a given amount of energy, the
mass of a nuclear explosive would be much less than that of
a conventional high explosive. Consequently, in the former
case, there is a much smaller amount of material available
in the weapon itself that is converted into the hot,
compressed gases mentioned above. This results in somewhat
different mechanisms for the initiation of the blast wave.
Third, the temperatures reached in a nuclear explosion are
very much higher than in a conventional explosion, and a
fairly large proportion of the energy in a nuclear explosion
is emitted in the form of light and heat, generally referred
to as “thermal radiation.” This is capable of causing skin
burns and of starting fires at considerable distances.
Fourth, the nuclear explosion is accompanied by
highly-penetrating and harmful invisible rays, called the
“initial nuclear radiation.”
Finally the substances remaining after a nuclear explosion
are radioactive, emitting similar radiations over an
extended period of time. This is known as the “residual
nuclear radiation” or “residual radioactivity” (Figure 6-2).
It is because of these fundamental differences between a
nuclear and a conventional explosion, including the
tremendously greater power of the former, that the effects
of nuclear weapons require special consideration. In this
connection, a knowledge and understanding of the mechanical
and the various radiation phenomena associated with a
nuclear explosion are of vital importance.
The purpose here is to describe the different forms in which
the energy of a nuclear explosion are released, to explain
how they are propagated, and to show how they may affect
people (and other living organisms) and materials. Where
numerical values are given for specific observed effects, it
should be kept in mind that there are inevitable
uncertainties associated with the data, for at least two
reasons.
In the first place, there are inherent difficulties in
making exact measurements of weapons effects. The results
are often dependent on circumstances which are difficult, if
not impossible, to control, even in a test and certainly
cannot be predicted in the event of an attack.
Furthermore, two weapons producing the same amount of
explosive energy may have different quantitative effects
because of differences in composition and design.
It is hoped, nevertheless, that the information contained in
this volume, which is the best available, may be of
assistance to those responsible for defense planning and in
making preparations to deal with the emergencies that may
arise from nuclear warfare. In addition, architects and
engineers may be able to utilize the data in the design of
structures having increased resistance to damage by blast,
shock, and fire, and which provide shielding against nuclear
radiations.
Atomic Structure and
Isotopes
All
substances are made up from one or more of about 90
different kinds of simple materials known as “elements.”
Among the common elements are the gases hydrogen, oxygen,
and nitrogen; the solid nonmetals carbon, sulfur, and
phosphorus; and various metals, such as iron, copper, and
zinc. A less familiar element, which has attained prominence
in recent years because of its use as a source of nuclear
energy, is uranium, normally a solid metal.
The smallest part of any element that can exist, while still
retaining the characteristics of the element, is called an
“atom” of that element. Thus, there are atoms of hydrogen,
of iron, of uranium, and so on, for all the elements. The
hydrogen atom is the lightest of all atoms, whereas the
atoms of uranium are the heaviest of those found on earth.
Heavier atoms, such as those of plutonium, also important
for the release of nuclear energy, have been made
artificially. Frequently, two or more atoms of the same or
of different elements join together to form a “molecule.”
Every atom consists of a relatively heavy central region or
“nucleus, “ surrounded by a number of very light particles
known as “electrons.” Further, the atomic nucleus is itself
made up of a definite number of fundamental particles,
referred to as “protons” and “neutrons.”
These two particles have almost the same mass, but they
differ in the respect that the proton carries a unit charge
of positive electricity whereas the neutron, as its name
implies, is uncharged electrically, i.e., it is neutral.
Because of the protons present in the nucleus, the latter
has a positive electrical charge, but in the normal atom
this is exactly balanced by the negative charge carried by
the electrons surrounding the nucleus.
The essential difference between atoms of different elements
lies in the number of protons (or positive charges) in the
nucleus; this is called the “atomic number” of the element.
Hydrogen atoms, for example, contain only one proton, helium
atoms have two protons, uranium atoms have 92 protons, and
plutonium atoms 94 protons.
Although all the nuclei of a (liven element contain the same
number of protons, they may have different numbers of
neutrons. The resulting atomic species, which have identical
atomic numbers but which differ in their masses, are called
“isotopes” of the particular element. All but about 20 of
the elements occur in nature in two or more isotopic forms,
and many other isotopes, which are unstable, i.e.,
radioactive, have been obtained in various ways.
Each isotope of a given element is identified by its “mass
number,” which is the sum of the numbers of protons and
neutrons in the nucleus. For example, the element uranium,
as found in nature, consists mainly of two isotopes with
mass numbers of 235 and 238; they are consequently referred
to as uranium-235 and uranium-238, respectively.
The nuclei of both isotopes contain 92 protons-as do the
nuclei of all uranium isotopes-but the former have in
addition 143 neutrons and the latter 146 neutrons. The
general term “nuclide” is used to describe any atomic
species distinguished by the composition of its nucleus,
i.e., by the number of protons and the number of neutrons.
Isotopes of a given element are nuclides having the same
number of protons but different numbers of neutrons in their
nuclei.
In a conventional explosion, the energy released arises from
chemical reactions; these involve a rearrangement among the
atoms, e.g., of hydrogen, carbon, oxygen, and nitrogen,
present in the chemical high-explosive material. In a
nuclear explosion, on the other hand, the energy is produced
as a result of the formation of different atomic nuclei by
the redistribution of the protons and neutrons within the
interacting nuclei.
What is sometimes referred to as atomic energy is thus
actually nuclear energy, since it results from particular
nuclear interactions. It is for the same reason, too, that
atomic weapons are preferably called “nuclear weapons.” The
forces between the protons and neutrons within atomic nuclei
are tremendously greater than those between the atoms;
consequently, nuclear energy is of a much higher order of
magnitude than conventional (or chemical) energy when equal
masses are considered.
Many nuclear processes are known, but not all are
accompanied by the release of energy. There is a definite
equivalence between mass and energy, and when a decrease of
mass occurs in a nuclear reaction there is an accompanying
release of a certain amount of energy related to the
decrease in mass.
These mass changes are really a reflection of the difference
in the internal forces in the various nuclei. It is a basic
law of nature that the conversion of any system in which the
constituents are held together by weaker forces into one in
which the forces are stronger must be accompanied by the
release of energy, and a corresponding decrease in mass.
In addition to the necessity for the nuclear process to be
one in which there is a net decrease in mass, the release of
nuclear energy in amounts sufficient to cause an explosion
requires that the reaction should be able to reproduce
itself once it has been started.
Two kinds of nuclear interactions can satisfy the conditions
for the production of large amounts of energy in a short
time. They are known as “fission” (splitting) and “fusion”
(joining together). The former process takes place with some
of the heaviest (high atomic number) nuclei; whereas the
latter, at the other extreme, involves some of the lightest
(low atomic number) nuclei.
The materials used to produce nuclear explosions by fission
are certain isotopes of the elements uranium and plutonium.
As noted above, uranium in nature consists mainly of two
isotopes, namely, uranium-235 (about 0.7 percent), and
uranium-238 (about 99.3 percent).
The less abundant of these isotopes, i.e., uranium-235, is
the readily fissionable species that is commonly used in
nuclear weapons. Another isotope, uranium-233, does not
occur naturally, but it is also readily fissionable and it
can be made artificially starting with thorium-232. Since
only insignificant amounts of the element plutonium are
found in nature, the fissionable isotope used in nuclear
weapons, plutonium-239, is made artificially from
uranium-238.
When a free (or unattached) neutron enters the nucleus of a
fissionable atom, it can cause the nucleus to split into two
smaller parts. This is the fission process, which is
accompanied by the release of a large amount of energy. The
smaller (or lighter) nuclei which result are called the
“fission products.” The complete fission of 1 pound of
uranium or plutonium releases as much explosive energy as
does the explosion of about 8,000 (short) tons of TNT.
In nuclear fusion, a pair of light nuclei unite (or fuse)
together to form a nucleus of a heavier atom. An example is
the fusion of the hydrogen isotope known as deuterium or
“heavy hydrogen.” Under suitable conditions, two deuterium
nuclei may combine to form the nucleus of a heavier element,
helium, with the release of energy.
Nuclear fusion reactions can be brought about by means of
very high temperatures, and they are thus referred to as
“thermonuclear processes.” The actual quantity of energy
liberated, for a given mass of material, depends on the
particular isotope (or isotopes) involved in the nuclear
fusion reaction.
As an example, the fusion of all the nuclei present in 1
pound of the hydrogen isotope deuterium would release
roughly the same amount of energy as the explosion of 26,000
tons of TNT.
In certain fusion processes, between nuclei of the hydrogen
isotopes, neutrons of high energy are liberated. These can
cause fission in the most abundant isotope (uranium-238) in
ordinary uranium as well as in uranium-235 and plutonium239.
Consequently, association of the appropriate fusion
reactions with natural uranium can result in an extensive
utilization of the latter for the release of energy.
A device in which fission and fusion (thermonuclear)
reactions are combined can therefore produce an explosion of
great power. Such weapons might typically release about
equal amounts of explosive energy from fission and from
fusion.
A distinction has sometimes been made between atomic
weapons, in which the energy arises from fission, on the one
hand, and hydrogen (or thermonuclear) weapons, involving
fusion, on the other hand. In each case, however, the
explosive energy results from nuclear reactions, so that
they are both correctly described as nuclear weapons. In
this chapter, therefore, the general terms “nuclear bomb”
and “nuclear weapon” will be used, irrespective of the type
of nuclear reaction producing the energy of the explosion.
Energy Yield of a Nuclear Explosion
The “yield” of a nuclear weapon is a measure of the amount
of explosive energy it can produce. It is the usual practice
to state the yield in terms of the quantity of TNT that
would generate the same amount of energy when it explodes.
Thus, a 1-kiloton nuclear weapon is one which produces the
same amount of energy in an explosion as does 1 kiloton (or
1,000 tons) of TNT. Similarly, a 1-megaton weapon would have
the energy equivalent of 1 million tons (or 1,000 kilotons)
of TNT.
The earliest nuclear bombs, such as were dropped over Japan
in 1945 and used in the tests at Bikini in 1946, released
very roughly the same quantity of energy as 20,000 tons (or
20 kilotons) of TNT. Since that time, much more powerful
weapons, with energy yields in the megaton range, have been
developed.
From the statement earlier that the fission of 1 pound of
uranium or plutonium will release the same amount of
explosive energy as about 8,000 tons of TNT, it is evident
that in a 20-kiloton nuclear weapon 2.5 pounds of material
undergo fission. However, the actual weight of uranium or
plutonium in such a weapon is greater than this amount.
In other words, in a fission weapon, only part of the
nuclear material suffers fission. The efficiency is thus
said to be less than 100 percent. The material that has not
undergone fission remains in the weapon residues after the
explosion.
Distribution of Energy in Nuclear Explosions
It has been mentioned that one important difference between
nuclear and conventional (or chemical) explosions is the
appearance of an appreciable proportion of the energy as
thermal radiation in the former case. The basic reason for
this difference is that, weight for weight, the energy
produced by a nuclear explosive is millions of times as
great as that produced by a chemical explosive.
Consequently, the temperatures reached in the former case
are very much higher than in the latter, namely, tens of
millions of degrees in a nuclear explosion compared with a
few thousands in a conventional explosion. As a result of
this great difference in temperature, the distribution of
the explosion energy is quite different in the two cases.
Broadly speaking, the energy may be divided into three
categories kinetic (or external) energy, i.e. , energy of
motion of electrons, atoms, and molecules as a whole;
internal energy of these particles and thermal radiation
energy. The proportion of thermal radiation energy increases
rapidly with increasing temperature.
At the moderate temperatures attained in a chemical
explosion, the amount of thermal radiation is comparatively
small, and so essentially all the energy released at the
time of the explosion appears as kinetic and internal
energy. This is almost entirely converted into blast and
shock, in the manner described earlier. Because of the very
much higher temperatures in a nuclear explosion, however, a
considerable proportion of the energy is re leased as
thermal radiation. The manner in which this takes place is
described later.
The fraction of the explosion energy received at a distance
from the burst point in each of the forms depicted in Figure
6-2 depends on the nature and yield of the weapon and
particularly on the environment of the explosion.
For a nuclear detonation in the atmosphere below an altitude
of about 100,000 feet, from 35 to 45 percent of the
explosion energy is received as thermal energy in the
visible and infrared portions of the spectrum.
In addition, below an altitude of about 40,000 feet, about
50 percent of the explosive energy is used in the production
of air shock. At somewhat higher altitudes, where there is
less air with which the energy of the exploding, nuclear
weapon can interact, the proportion of energy converted into
shock is decreased whereas that emitted as thermal radiation
is correspondingly increased.
The expect distribution of energy between air shock and
thermal radiation is related in a complex manner to the
explosive energy yield, the burst altitude, and, to some
extent, to the weapon design, as will be seen in this and
later chapters.
However, an approximate rule of thumb for a fission weapon
exploded in the air at an altitude of less than about 40,000
feet is that 35 percent of the explosion energy is in the
form of thermal radiation and 50 percent produces air shock.
Thus, for a burst at moderately low altitudes, the air shock
energy from a fission weapon will be about half of that from
a conventional high explosive with the same total energy
release; in the latter, essentially all of the explosive
energy is in the form of air blast.
This means that if a 20-kiloton fission weapon, for example,
is exploded in the air below 40,000 feet or so, the energy
used in the production of blast would be roughly equivalent
to that from 10 kilotons of TNT.
Regardless of the height of burst, approximately 85 percent
of the explosive energy of a nuclear fission weapon produces
air blast (and shock), thermal radiation, and heat. The
remaining 15 percent of the energy is released as various
nuclear radiations. Of this, 5 percent constitutes the
initial nuclear radiation, defined as that produced within a
minute or so of the explosion.
The final 10 percent of the total fission energy represents
that of the residual (or delayed) nuclear radiation which is
emitted over a period of time. This is largely due to the
radioactivity of the fission products present in the weapon
residues (or debris) after the explosion. In a thermonuclear
device, in which only about half of the total energy a rises
from fission, the residual nuclear radiation carries only 5
percent of the energy released in the explosion.
It should be noted that there are no nuclear radiations from
a conventional explosion since the nuclei are unaffected in
the chemical reactions which take place.
Because about 10 percent of the total fission energy is
released in the form of residual nuclear radiation some time
after the detonation, this is not included when the energy
yield of a nuclear explosion is stated, e.g., in terms of
the TNT equivalent.
Hence, in a pure fission weapon the explosion energy is
about 90 percent of the total fission energy, and in a
thermonuclear device it is, on the average, about 95 percent
of the total energy of the fission and fusion reactions.
This common convention will be adhered to in subsequent
chapters.
For example, when the yield of a nuclear weapon is quoted or
used in equations, figures, etc., it will represent that
portion of the energy delivered within a minute or so, and
will exclude the contribution of the residual nuclear
radiation.
The initial nuclear radiation consists mainly of “gamma
rays,” which are electromagnetic radiations of high energy
originating in atomic nuclei, and neutrons. These
radiations, especially gamma rays, can travel great
distances through air and can penetrate considerable
thicknesses of material.
Although they can neither be seen nor felt by human beings,
except at very high intensities which cause a tingling
sensation, gamma rays and neutrons can produce harmful
effects even at a distance from their source. Consequently,
the initial nuclear radiation is an important aspect of
nuclear explosions.
The delayed nuclear radiation arises mainly front the
fission products which, in the course of their radioactive
decay, emit gamma rays and another type of nuclear radiation
called “beta particles.” The latter are electrons, i.e.,
particles carrying a negative electric charge, moving with
high speed; they are formed by a change (neutron proton +
electron) within the nuclei of the radioactive atoms. Beta
particles, which are also invisible, are much less
penetrating than gamma rays, but like the latter they
represent a potential hazard.
The spontaneous emission of beta particles and gamma rays
from radioactive substances, i.e., a radioactive nuclide (or
radionuclide), such as the fission products, is a gradual
process. It takes place over a period of time, at a rate
depending upon the nature of the material and upon the
amount present.
Because of the continuous decay, the quantity of the
radionuclide and the rate of emission of radiation decrease
steadily. This means that the residual nuclear radiation,
due mainly to the fission products, is most intense soon
after the explosion but diminishes in the course of time.
Types of Nuclear Explosions
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.
Scientific Basis of Nuclear
Explosions
Fission Energy
The significant point about the fission of a uranium (or
plutonium) nucleus by means of a neutron, in addition to the
release of a large quantity of energy, is that the process
is accompanied by the instantaneous emission of two or more
neutrons; thus,
__________________________________________________________
Table 6-1.
Neutron + uranium-235 = fission fragments +
(or uraniurn-233) 2 or 3 neutrons + energy.
(or plutonium-239)
__________________________________________________________
The neutrons liberated in this manner are able to induce
fission of additional uranium (or plutonium) nuclei, each
such process resulting in the emission of more neutrons
which can produce further fission, and so on.
Thus, in principle, a single neutron could start off a chain
of nuclear fissions, the number of nuclei suffering fission,
and the energy liberated, increasing at a tremendous rate.
as will be seen shortly.
There are many different ways in which the nuclei of a given
fissionable species can split tip into two fission fragments
(initial fission products), but the total amount of energy
liberated per fission does not vary greatly. A satisfactory
average value of this energy is 200 million electron volts.
The million electron volt (or 1 MeV) unit has been found
convenient for expressing the energy released in nuclear
reactions; it is equivalent to 1.6 x 10-6 erg or 1.6 x 10-13
joule. The manner in which this energy is distributed among
the fission fragments and the various radiations associated
with fission is shown in Table 3.
__________________________________________________________
Table 6-2: Distribution of Fission
Energy
MeV__
Kinetic energy of fission fragments 165 ± 5
Instantaneous gamma-ray energy 7 ± 1
Kinetic energy of fission neutrons 5 ± 0.5
Beta particles front fission products 7 ± 1
Gamma rays from fission products 6 ± 1
Neutrinos from fission products 10____
Total energy per fission 200 ± 6
__________________________________________________________
The results in Table 6-2 may he taken as being approximately
applicable to either uranium-233, uranium-235, or
plutonium-239. These are the only three known substances,
which are reasonably stable so that they can be stored
without appreciable decay, that are capable of undergoing
fission by neutrons of all energies. Hence, they are the
only materials that can be used to sustain a fission chain.
Uranium-238, the most abundant isotope in natural uranium,
and thorium-232 will suffer fission by neutrons of high
energy only, but not by those of lower energy. For this
reason these substances cannot sustain a chain reaction.
However, when fission does occur in these elements, the
energy distribution is quite similar to that shown in the
table.
Only part of the fission energy is immediately available in
a nuclear explosion; this includes the kinetic energy of the
fission fragments, most of the energy of the instantaneous
gamma rays, which is converted into other forms of energy
within the exploding weapon, and also most of the neutron
kinetic energy, but only a small fraction of the decay
energy of the fission products.
There is some compensation from energy released in reactions
in which neutrons are captured by the weapon debris, and so
it is usually accepted that about 180 MeV of energy are
immediately available per fission. There are 6.02 x 1023
nuclei in 235 grams of uranium-235 (or 239 grams of
plutonium-239), and by making use of familiar conversion
factors the results quoted in Table 1.45 may be obtained for
the energy (and other) equivalents of 1 kiloton of TNT. The
calculations are based on an accepted, although somewhat
arbitrary, figure of 1012 calories as the energy released in
the explosion of this amount of TNT3.
Table 1.45 Equivalents of 1 Kiloton of
TNT
Complete fission of 0.057 kg (57 grams or
2 ounces) fissionable material
Fission of 1.45 x 1025 nuclei
1012 calories
2.6 x 1025 million electron volts
4.48 x 1019 ergs (4.18 x 1012 joules)
1.16 x 106 kilowatt-hours
3.97 x 109 British thermal units
Critical Mass for a Fission Chain
Although two to three neutrons are produced in the fission
reaction for every nucleus that undergoes fission, not all
of these neutrons are available for causing further
fissions. Some of the fission neutrons are lost by escape,
whereas others are lost in various nonfission reactions.
In order to sustain a fission chain reaction, with
continuous release of energy, at least one fission neutron
must be available to cause further fission for each neutron
previously absorbed in fission. If the conditions arc such
that the neutrons are lost at a faster rate than they are
formed by fission, the chain reaction would not be
self-sustaining.
Some energy would be produced, but the amount would not be
large enough, and the rate of liberation would not be
sufficiently fast, to cause an effective explosion. It is
necessary, therefore, in order to achieve a nuclear
explosion, to establish conditions under which the loss of
neutrons is minimized. in this connection, it is especially
important to consider the neutrons which escape from the
substance undergoing fission.
The escape of neutrons occurs at the exterior of the uranium
(or plutonium) material. The rate of loss by escape will
thus be determined by the surface area. On the other hand,
the fission process, which results in the formation of more
neutrons, takes place throughout the whole of the material
and its rate is, therefore, dependent upon the mass.
By increasing the mass of the fissionable material, at
constant density, the ratio of the surface area to the mass
is decreased; consequently, the loss of neutrons by escape
relative to their formation by fission is decreased. The
same result can also be achieved by having a constant mass
but compressing it to a smaller volume (higher density), so
that the surface area is decreased.
The situation may be understood by reference to Fig. 1.48
showing two spherical masses, one larger than the other, of
fissionable material of the same density. Fission is
initiated by a neutron represented by a dot within a small
circle.
It is supposed that in each act of fission three neutrons
are emitted; in other words, one neutron is captured and
three are expelled. The removal of a neutron from the system
is indicated by the head of an arrow. Thus, an arrowhead
within the sphere means that fission has occurred and extra
neutrons are produced, whereas an arrowhead outside the
sphere implies the loss of a neutron. It is evident from
Fig. 1.48 that a much greater fraction of the neutrons is
lost from the smaller than from the larger mass.
Figure 1.48. Effect of increased mass of fissionable
material in reducing the proportion of neutrons lost by
escape.
If the quantity of a fissionable isotope of uranium (or
plutonium) is such that the ratio of the surface area to the
mass is large, the proportion of neutrons lost by escape
will be so great that the propagation of a nuclear fission
chain, and hence the production of an explosion, will not be
possible. Such a quantity of material is said to be “subcritical.”
But as the mass of the piece of uranium (or plutonium) is
increased (or the volume is decreased by compression) and
the relative loss of neutrons is thereby decreased, a point
is reached at which the chain reaction can become
self-sustaining. This is referred to as the “critical mass”
of the fissionable material under the existing conditions.
For a nuclear explosion to take place, the weapon must thus
contain a sufficient amount of a fissionable uranium (or
plutonium) isotope for the critical mass to be exceeded.
Actually, the critical mass depends, among other things, on
the shape of the material, its composition and density (or
compression), and the presence of impurities which can
remove neutrons in nonfission reactions.
By surrounding the fissionable material with a suitable
neutron “reflector,” the loss of neutrons by escape can be
reduced, and the critical mass can thus be decreased.
Moreover, elements of high density, which make good
reflectors for neutrons of high energy, provide inertia,
thereby delaying expansion of the exploding material. The
action of the reflector is then like the familiar tamping in
blasting operations. As a consequence of its neutron
reflecting and inertial properties, the “tamper” permits the
fissionable material in a nuclear weapon to be used more
efficiently.
Attainment of Critical Mass in a Weapon
Because of the presence of stray neutrons in the atmosphere
or the possibility of their being generated in various ways,
a quantity of a suitable isotope of uranium (or plutonium)
exceeding the critical mass would be likely to melt or
possibly explode. It is necessary, therefore, that before
detonation, a nuclear weapon should contain no piece of
fissionable material that is as large as the critical mass
for the given conditions. In order to produce an explosion,
the material must then be made “supercritical,” i.e., larger
than the critical mass, in a time so short as to preclude a
subexplosive change in the configuration, such as by
melting.
Two general methods have been described for bringing about a
nuclear explosion, that is to say, for quickly converting a
subcritical system into a supercritical one. In the first
method, two or more pieces of fissionable material, each
less than a critical mass, are brought together very rapidly
in order to form one piece that exceeds the critical mass
(Fig. 1.52).
This may be achieved in some kind of gun-barrel device, in
which an explosive propellant is used to blow one
subcritical piece of fissionable material from the breech
end of the gun into another subcritical piece firmly held in
the muzzle end.
The second method makes use of the fact that when a
subcritical quantity of an appropriate isotope of uranium
(or plutonium) is strongly compressed, it can become
critical or supercritical as indicated above. The
compression may be achieved by means of a spherical
arrangement of specially fabricated shapes (lenses) of
ordinary high explosive.
In a hole in the center of this system is placed a
subcritical sphere of fissionable material. When the
highexplosive lens system is set off, by means of a
detonator on the outside of each lens, an inwardly-directed
spherical “implosion” wave is produced. A similar wave can
be realized without lenses by detonating a large number of
points distributed over a spherical surface. When the
implosion wave reaches the sphere of uranium (or plutonium),
it causes the latter to be compressed and become
supercritical (Fig. 1.53). The introduction of neutrons from
a suitable source can then initiate a chain reaction leading
to an explosion.
Time Scale of a Fission Explosion
An interesting insight into the rate at which the energy is
released in a fission explosion can be obtained by treating
the fission chain as a series of “generations.” Suppose that
a certain number of neutrons are present initially and that
these are captured by fissionable nuclei; then, in the
fission process other neutrons are released.
These neutrons, are, in turn, captured by fissionable nuclei
and produce more neutrons, and so on. Each stage of the
fission chain is regarded as a generation, and the
“generation time” is the average time interval between
successive generations. The time required for the actual
fission of a nucleus is extremely short and most of the
neutrons are emitted promptly. Consequently, the generation
time is essentially equal to the average time elapsing
between the release of a neutron and its subsequent capture
by a fissionable nucleus.
This time depends, among other things, on the energy (or
speed) of the neutron, and if most of the neutrons are of
fairly high energy, usually referred to as “fast neutrons,”
the generation time is about a one-hundred-millionth part
(10-8) of a second, i.e., 0.01 microseconds.4
It was mentioned earlier that not all the fission neutrons
are available for maintaining the fission chain because some
are lost by escape and by removal in nonfission reactions.
Suppose that when a nucleus captures a neutron and suffers
fission f neutrons are released; let l be the average number
of neutrons lost, in one way or another, for each fission.
There will thus be f - l neutrons available to carry on the
fission chain. If there are N neutrons present at any
instant, then as a result of their capture by fissionable
nuclei N(f - l) neutrons will be produced at the end of one
generation; hence, the increase in the number of neutrons
per generation is N(f - l) - N or N(f - l - 1). For
convenience, the quantity f - l - 1 , that is, the increase
in neutrons per fission, will be represented by x. If g is
the generation time, then the rate at which the number of
neutrons increases is given by
Rate of neutron increase
dN/dt = Nx/g.
The solution of this equation is
N = N0ext/g,
where N0 is the number of neutrons present initially and N
is the number at a time t later. The fraction t/g is the
number of generations which have elapsed during the time t,
and if this is represented by n, it follows that
N = N0exn.
If the value of x is known, equation can be used to
calculate either the neutron population after any prescribed
number of generations in the fission chain, or,
alternatively, the generations required to attain a
particular number of neutrons. For uranium235, f is about
2.5, l may be taken to be roughly 0.5, so that x, which is
equal to f - l - 1, is close to unity; hence, equation may
be written as
N ~= N0en or N ~= N010n/2.3. (1.56.1)
According to the data in Table 1.45, it would need 1.45 x
1022 fissions, and hence the same number of neutrons, to
produce 0.1 kiloton equivalent of energy. If the fission
chain is initiated by one neutron, so that N0 is 1, it
follows from equation (1.56.1) that it would take
approximately 51 generations to produce the necessary number
of neutrons.
Similarly, to release 100 kilotons of energy would require
1.45 x 1025 neutrons and this number would be attained in
about 58 generations. It is seen, therefore, that 99.9
percent of the energy of a 100-kiloton fission explosion is
released during the last 7 generations, that is, in a period
of roughly 0.07 microsecond. Clearly, most of the fission
energy is released in an extremely short time period. The
same conclusion is reached for any value of the fission
explosion energy.
In 50 generations or so, i.e., roughly half microsecond,
after the initiation of the fission chain, so much energy
will have been released-about 1011 calories-that extremely
high temperatures will be attained. Consequently, in spite
of the restraining effect of the tamper and the weapon
casing, the mass of fissionable material will begin to
expand rapidly.
The time at which this expansion commences is called the
“explosion time.” Since the expansion permits neutrons to
escape more readily, the mass becomes subcritical and the
self-sustaining chain reaction soon ends. An appreciable
proportion of the fissionable material remains unchanged and
some fissions will continue as a result of neutron capture,
but the amount of energy released at this stage is
relatively small.
To summarize the foregoing discussion, it may be stated that
because the fission process is accompanied by the
instantaneous liberation of neutrons, it is possible, in
principle to produce a self-sustaining chain reaction
accompanied by the rapid release of large amounts of energy.
As a result, a few pounds of fissionable material can be
made to liberate, within a very small fraction of a second,
as much energy as the explosion of many thousands of tons of
TNT. This is the basic principle of nuclear fission weapons.
Fission Products
Many different initial fission product nuclei, i.e., fission
fragments, are formed when uranium or plutonium nuclei
capture neutrons and suffer fission. There are 40 or so
different ways in which the nuclei can split up when fission
occurs; hence about 80 different fragments are produced. The
nature and proportions of the fission fragment nuclei vary
to some extent, depending on the particular substance
undergoing fission and on the energy of the neutrons causing
fission.
For example, when uranium-238 undergoes fission as a result
of the capture of neutrons of very high energy released in
certain fusion reactions, the products are somewhat
different, especially in their relative amounts, from those
formed from uranium-235 by ordinary fission neutrons.
Regardless of their origin, most, if not all, of the
approximately 80 fission fragments are the nuclei of
radioactive forms (radioisotopes) of wellknown, lighter
elements. The radioactivity is usually manifested by the
emission of negatively charged beta particles. This is
frequently, although not always, accompanied by gamma
radiation, which serves to carry off excess energy. In a few
special cases, gamma radiation only is emitted.
As a result of the expulsion of a beta particle, the nucleus
of a radioactive substance is changed into that of another
element, sometimes called the “decay product.” In the case
of the fission fragments, the decay products are generally
also radioactive, and these in turn may decay with the
emission of beta particles and gamma rays. On the average
there are about four stages of radioactivity for each
fission fragment before a stable (nonradioactive) nucleus is
formed. Because of the large number of different ways in
which fission can occur and the several stages of decay
involved, the fission product mixture becomes very complex.5
More than 300 different isotopes of 36 light elements, from
zinc to terbium, have been identified among the fission
products.
The rate of radioactive change, i.e., the rate of emission
of beta particles and gamma radiation, is usually expressed
by means of the “half-life” of the radionuclide involved.
This is defined as the time required for the radioactivity
of a given quantity of a particular nuclide to decrease (or
decay) to half of its original value.
Each individual radionuclide has a definite half-life which
is independent of its state or its amount. The half-lives of
the fission products have been found to range from a small
fraction of a second to something like a million years.
Although every radionuclide present among the fission
products is known to have a definite half-life, the mixture
formed after a nuclear explosion is so complex that it is
not possible to represent the decay as a whole in terms of a
half-life. Nevertheless, it has been found that the decrease
in the total radiation intensity from the fission products
can be calculated approximately by means of a fairly simple
formula.
This will be given and discussed in Chapter IX, but the
general nature of the decay rate of fission products, based
on this formula, will be apparent from Fig. 1.64. The
residual radioactivity from the fission products at 1 hour
after a nuclear detonation is taken as 100 and the
subsequent decrease with time is indicated by the curve.
It is seen that at 7 hours after the explosion, the fission
product activity will have decreased to about one-tenth (10
percent) of its amount at 1 hour. Within approximately 2
days, the activity will have decreased to 1 percent of the
1hour value.
In addition to the beta-particle and gamma-ray activity due
to the fission products, there is another kind of residual
radioactivity that should be mentioned. This is the activity
of the fissionable material, part of which remains after the
explosion.
The fissionable uranium and plutonium isotopes are
radioactive, and their activity consists in the emission of
what are called “alpha particles.” These are a form of
nuclear radiation, since they are expelled from atomic
nuclei; but they differ from the beta particles arising from
the fission products in being much heavier and carrying a
positive electrical charge. Alpha particles are, in fact,
identical with the nuclei of helium atoms.
Because of their greater mass and charge, alpha particles
are much less penetrating than beta particles or gamma rays
of the same energy. Thus, very few alpha particles from
radioactive sources can travel more than 1 to 3 inches in
air before being stopped.
It is doubtful that these particles can get through the
unbroken skin, and they certainly cannot penetrate clothing.
Consequently, the uranium (or plutonium) present in the
weapon residues does not constitute a hazard if the latter
are outside the body. However, if plutonium enters the body
by ingestion, through skin abrasions, or particularly
through inhalation, the effects may be serious.
Fusion (Thermonuclear) Reactions
Energy production in the sun and stars is undoubtedly due to
fusion reactions involving the nuclei of various light (low
atomic weight) atoms. From experiments made in laboratories
with charged-particle accelerators, it was concluded that
the fusion of isotopes of hydrogen was possible. This
element is known to exist in three isotopic forms, in which
the nuclei have mass numbers of 1, 2, and 3, respectively.
These are generally referred to as hydrogen (1H), deuterium
(2H or D), and tritium (3H or T). All the nuclei carry a
single positive charge, i.e., they all contain one proton,
but they differ in the number of neutrons. The lightest (1H)
nuclei (or protons) contain no neutrons; deuterium (D)
nuclei contain one neutron, and tritium (T) nuclei contain
two neutrons.
Several different fusion reactions have been observed
between the nuclei of the three hydrogen isotopes, involving
either two similar or two different nuclei. In order to make
these reactions occur to an appreciable extent, the nuclei
must have high energies. One way in which this energy can be
supplied is to raise the temperature to very high levels. In
these circumstances the fusion processes are referred to as
“thermonuclear reactions”.
Four thermonuclear fusion reactions appear to be of interest
for the production of energy because they are expected to
occur sufficiently rapidly at realizable temperatures; these
are:
D + D = 3He + n + 3.2 MeV
D + D = T + 1H + 4.0 MeV
T + D = 4He + n + 17.6 MeV
T + T = 4He + 2n + 11.3 MeV,
where He is the symbol for helium and n (mass = 1)
represents a neutron. The energy liberated in each case is
given in million electron volt (MeV) units.
The first two of these reactions occur with almost equal
probability at the temperatures associated with nuclear
explosions (several tens of million degrees Kelvin), whereas
the third reaction has a much higher probability and the
fourth a much lower probability. Thus, a valid comparison of
the energy released in fusion reactions with that produced
in fission can be made by noting that, as a result of the
first three reactions given above, five deuterium nuclei,
with a total mass of 10 units, will liberate 24.8 MeV upon
fusion. On the other hand, in the fission process, e.g., of
uranium-235, a mass of 235 units will produce a total of
about 200 MeV of energy. Weight for weight, therefore, the
fusion of deuterium nuclei would produce nearly three times
as much energy as the fission of uranium or plutonium.
Another reaction of thermonuclear weapons interest, with
tritium as a product, is
6Li + n = 4He + 3T + 4.8 MeV,
where 6Li represents the lithium-6 isotope, which makes up
about 7.4 percent of natural lithium. Other reactions can
occur with lithium-6 or the more abundant isotope lithium-7
and various particles produced in the weapon. However, the
reaction shown above is of most interest for two reasons:
(1) it has a high probability of occurrence, and
(2) if the lithium is placed in the weapon in the form of
the compound lithium deuteride (LiD), the tritium formed in
the reaction has a high probability of interacting with the
deuterium.
Large amounts of energy are thus released by the third
reaction and additional neutrons are produced to react with
lithium-6.
In order to make the nuclear fusion reactions take place at
the required rate, temperatures of the order of several tens
of million degrees are necessary. The only practical way in
which such temperatures can be obtained on earth is by means
of a fission explosion. Consequently, by combining a
quantity of deuterium or lithium deuteride (or a mixture of
deuterium and tritium) with a fission device, it should be
possible to initiate one or more of the thermonuclear fusion
reactions given above.
If these reactions, accompanied by energy evolution, can be
propagated rapidly through a volume of the hydrogen isotope
(or isotopes) a thermonuclear explosion may be realized.
It will be observed that in three of the fusion reactions,
neutrons are produced. Because of their small mass, these
neutrons carry off most of the reaction energy;
consequently, they have sufficient energy to cause fission
of uranium-238 nuclei. As stated earlier, this process
requires neutrons of high energy.
It is possible, therefore, to make use of the thermonuclear
neutrons by surrounding the fusion weapon with a blanket of
ordinary uranium. The high-energy neutrons are then captured
by uranium-238 nuclei; the latter undergo fission, thereby
contributing to the overall energy yield of the explosion,
and also to the residual nuclear radiation arising from the
fission products.
On the average, the energy released in the explosion of a
thermonuclear weapon originates in roughly equal amounts
from fission and fusion processes, although there may be
variations in individual cases. In “boosted” fission
weapons, thermonuclear neutrons serve to enhance the fission
process; energy released in the thermonuclear reaction is
then a small fraction of the total energy yield.
Thermal Radiation
The observed phenomena associated with a nuclear explosion
and the effects on people and materials are largely
determined by the thermal radiation and its interaction with
the surroundings. It is desirable, therefore, to consider
the nature of these radiations somewhat further. Thermal
radiations belong in the broad category of what are known as
“electromagnetic radiations.”
These are a kind of wave motion resulting from oscillating
electric charges and their associated magnetic fields.
Ordinary visible light is the most familiar kind of
electromagnetic radiation, and all such radiations travel
through the air (or, more exactly, a vacuum) at the same
velocity, namely, the velocity of light, 186,000 miles per
second.
Electromagnetic radiations range from the very short
wavelength (or very high frequency) gamma rays and X rays,
through the invisible ultraviolet to the visible region, and
then to the infrared and radar and radio waves of relatively
long wavelength (and low frequency).
The approximate wavelength and frequency regions occupied by
the different kinds of electromagnetic radiations are
indicated in Fig. 1.74. The wavelengthl in centimeters and
the frequencyv in hertz, i.e., in waves (or cycles) per
second, are related bylv = c, where c is the velocity of
light, 3.00 x 1010 cm per second. According to Planck’s
theory, the energy of the corresponding “quantum” (or unit)
of energy, carried by the “photon,” i.e., the postulated
particle (or atom) of radiation, is given by
E (ergs) = hv = hc / l
= 1.99 x 10-16 /l (cm) (1.74.1)
where h is a universal constant equal to 6.62 x 10-27
erg-second. The energy quantum values for the various
electromagnetic radiations are included in Fig. 1.74; the
results are expressed either in MeV, i.e., million electron
volt, in keV, i.e., kilo (or thousand) electron volt, or in
eV, i.e., electron volt, units. These are obtained from
equation (1.74.1) by writing it in the form
E (MeV) = 1.24 x 10-10 /l (cm) (1.74.2)
It is seen that the energy of the radiations decreases from
left to right in the figure, i.e., as the wavelength
increases and the frequency decreases.
The (thermal) radiation energy density for matter in
temperature equilibrium is given by
E (radiation) = 7.6 x 10-15 T4 ergs/cm3,
where T is the temperature in degrees Kelvin. At the
temperature of a conventional chemical explosion, e.g.,
5,000K, the radiation energy density is then less than 1
erg/cm3, compared with roughly 108 ergs/cm3 for the material
energy, i.e., kinetic energy and internal (electronic,
vibrational, and rotational) energy. Hence, the radiation
energy is a very small proportion of the total energy.
In a nuclear explosion, on the other hand, where
temperatures of several tens of million degrees are reached,
the radiation energy density will be of the order of 1016
ergs/cm3, whereas the material energy is in the range of
1014 to 1015 ergs/cm3. It has been estimated that in a
nuclear explosion some 80 percent of the total energy may be
present initially as thermal radiation energy.
Not only does the radiation energy density increase with
temperature but the rate of its emission as thermal
radiation increases correspondingly. For materials at
temperatures of a few thousand degrees Kelvin, the energy is
radiated slowly, with the greatest part in the ultraviolet,
visible, and infrared regions of the electromagnetic
spectrum (Fig. 1.74). At the temperatures of a nuclear
explosion, however, not only is the radiation energy emitted
very rapidly, but most of this energy is in the spectral
region with wavelengths shorter than the ultraviolet.
When a nuclear weapon explodes, temperature equilibrium is
rapidly established in the residual material. Within about
one microsecond after the explosion, some 70 to 80 percent
of the explosion energy is emitted as primary thermal
radiation, most of which consists of soft X rays.6
Almost all of the rest of the energy is in the form of
kinetic energy of the weapon debris at this time. The
interaction of the primary thermal radiation and the debris
particles with the surroundings will vary with the altitude
of burst and will determine the ultimate partition of energy
between the thermal radiation received at a distance and
shock.
When a nuclear detonation occurs in the air, where the
atmospheric pressure (and density) is near to sealevel
conditions, the soft X rays in the primary thermal radiation
are completely absorbed within a distance of a few feet.
Some of the radiations are degraded to lower energies, e.g.,
into the ultraviolet region, but most of the energy of the
primary thermal radiation serves to heat the air immediately
surrounding the nuclear burst.
It is in
this manner that the fireball is formed. Part of the energy
is then reradiated at a lower temperature from the fireball
and the remainder is converted into shock (or blast) energy.
This explains why only about 35 to 45 percent of the fission
energy from an air burst is received as thermal radiation
energy at a distance, although the primary thermal radiation
may constitute as much as 70 to 80 percent of the total.
Furthermore, because the secondary thermal radiation is
emitted at a lower temperature, it lies mainly in the region
of the spectrum with longer wavelengths (lower photon
energies), i.e., ultraviolet, visible, and infrared.7
In the event of a burst at
high altitudes, where the air density is low, the soft X
rays travel long distances before they are degraded and
absorbed. At this stage, the available energy is spread
throughout such a large volume (and mass) that most of the
atoms and molecules in the air cannot get very hot.
Although the total energy emitted as thermal radiation in a
high-altitude explosion is greater than for an air burst
closer to sea level, about half is reradiated so slowly by
the heated air that it has no great significance as a cause
of damage. The remainder, however, is radiated very much
more rapidly, i.e., in a shorter time interval, than is the
case at lower altitudes.
A shock wave is generated from a high-altitude burst, but at
distances of normal practical interest it produces a smaller
pressure increase than from an air burst of the same yield.
These matters are treated more fully in Chapter II.
FOOTNOTES
1 The Terms “nuclear” and “atomic” may he used
interchangeably so far as weapons, explosions, and energy
are concerned, but “nuclear” is preferred for the reason
given in ¤ 1.11.
2 The remaining (more technical sections of this chapter may
he omitted without loss of continuity.
3 The majority of the experimental and theoretical values of
the explosive energy released by TNT range from 900 to 1,100
calories per gram. At one time, there was some uncertainty
as to whether the term “kiloton” of TNT referred to a short
kiloton (2 x 106 pounds), a metric kiloton (2.205 x 106
pounds), or a long kiloton (2.24 x 106 pounds). In order to
avoid ambiguity, it was agreed that the term “kiloton” would
refer to the release of 1012 calories of explosive energy.
This is equivalent to 1 short kiloton of TNT if the energy
release is 1,102 calories per gram or to 1 long kiloton if
the energy is 984 calories per gram of TNT.
4 A microsecond is a one-millionth part of a second, i.e.,
10-6 second; a hundredth of a microsecond, i.e., 10-8
second, is often called a “shake.” The generation time in
fission by fast neutrons is thus roughly 1 shake.
5 The general term “fission products” is used to describe
this complex mixture.
6 X rays are frequently distinguished as “hard” or “soft.”
The latter have longer wavelengths and lower energies, and
they are more easily absorbed than hard X rays. They are,
nevertheless, radiations of high energy compared with
ultraviolet or visible light.
7 It is sometimes referred to as the “prompt thermal
radiation” because only that which is received within a few
seconds of the explosion is significant as a hazard.
|
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Bomb Shelter Planning
Location,
Underground Bomb Shelter Plans, Blast/Fallout, Radiation |
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Build
Your Bomb Shelter
First Steps, Materials Required, Costs |
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Stocking Your Bomb Shelter
Nuclear
Emergency Kit (NEK), Emergency Supply Kit, Food, Water, Medical,
Etc. |
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Bomb Shelter FAQ's
Complete List of Essential Nuclear Blast
and Underground Bomb Shelter FAQs |
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Understanding Radiation
Overview of
Radioactive Fallout and How to Protect Yourself From
It |
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Nuclear Bomb Facts
Kiloton, Blast Wave, Damage |
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BOMB
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