Nuclear weapon design


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Nuclear weapon designs are often divided into two classes, based on the
dominant source of the nuclear weapon's energy.

   * Fission bombs derive their power from nuclear fission, where heavy
     nuclei (uranium or plutonium) split into lighter elements when
     bombarded by neutrons (produce more neutrons which bombard other
     nuclei, triggering a chain reaction). These are historically called
     atom bombs or A-bombs, though this name is not precise due to the fact
     that chemical reactions release energy from atomic bonds and fusion is
     no less atomic than fission. Despite this possible confusion, the term
     atom bomb has still been generally accepted to refer specifically to
     nuclear weapons, and most commonly to pure fission devices.

   * Fusion bombs are based on nuclear fusion where light nuclei such as
     hydrogen and helium combine together into heavier elements and release
     large amounts of energy. Weapons which have a fusion stage are also
     referred to as hydrogen bombs or H-bombs because of their primary fuel,
     or thermonuclear weapons because fusion reactions require extremely
     high temperatures for a chain reaction to occur.

The distinction between these two types of weapon is blurred by the fact
that they are combined in nearly all complex modern weapons: a smaller
fission bomb is first used to reach the necessary conditions of high
temperature and pressure to allow fusion to occur. On the other hand, a
fission device is more efficient when a fusion core first boosts the
weapon's energy. Since the distinguishing feature of both fission and fusion
weapons is that they release energy from transformations of the atomic
nucleus, the best general term for all types of these explosive devices is
"nuclear weapon".

Other specific types of nuclear weapon design which are commonly referred to
by name include: neutron bomb, cobalt bomb, enhanced radiation weapon, and
salted bomb.

The simplest nuclear weapons are pure fission bombs. These were the first
types of nuclear weapons built during the Manhattan Project and they are a
building block for all advanced nuclear weapons designs.

A mass of fissile material is called critical when it is capable of a
sustained chain reaction, which depends upon the size, shape and purity of
the material as well as what surrounds the material. A numerical measure of
whether a mass is critical or not is available as the neutron multiplication
factor, k, where

     k = f - l

Where f is the average number of neutrons released per fission event and l
is the average number of neutrons lost by either leaving the system or being
captured in a non-fission event.When k=1 the mass is critical, k<1 is
subcritical and k>1 is supercritical. A fission bomb works by rapidly
changing a subcritical mass of fissile material into a supercritical
assembly, causing a chain reaction which rapidly releases large amounts of
energy. In practice the mass is not made slightly critical, but goes from
slightly subcritical (k=.9) to highly supercritical (k= 2 or 3), so that
each neutron creates several new neutrons and the chain reaction advances
more quickly. The main challenge in producing an efficient explosion using
nuclear fission is to keep the bomb together long enough for a substantial
fraction of the available nuclear energy to be released.

Until detonation is desired, the weapon must consist of a number of separate
pieces each of which is below the critical size either because they are too
small or unfavorably shaped. To produce detonation, the fissile material
must be brought together rapidly. In the course of this assembly process the
chain reaction is likely to start causing the material to heat up and
expand, preventing the material from reaching its most compact (and most
efficient) form. It may turn out that the explosion is so inefficient as to
be practically useless. The majority of the technical difficulties of
designing and manufacturing a fission weapon are based on the need to both
reduce the time of assembly of a supercritical mass to a minimum and reduce
the number of stray (pre-detonation) neutrons to a minimum.

The isotopes desirable for a nuclear weapon are those which have a high
probability of fission reaction, yield a high number of excess neutrons,
have a low probability of absorbing neutrons without a fission reaction, and
do not release a large number of spontaneous neutrons. The primary isotopes
which fit these criteria are U-235, Pu-239 and U-233.

Enriched Materials

Naturally occurring uranium consists mostly of U-238, with a small part
U-235. The U-238 isotope has a high probability of absorbing a neutron
without a fission, and also a higher rate of spontaneous fission. For
weapons uranium is enriched through isotope separation. Uranium which is
more than 80% U-235 is called highly enriched uranium (HEU), and weapons
grade uranium is at least 93.5% U-235. U-235 has a spontaneous fission rate
of 0.16 fissions/sec-kg. which is low enough to make super critical assembly
relatively easy. The critical mass for an unreflected sphere of U-235 is
about 50 kg, which is a sphere with a diameter of 17 cm. This size can be
reduced to about 15 kg with the use of a neutron reflector surrounding the sphere.

Plutonium (atomic number 94, two more than uranium) does not occur in nature
and is manufactured by exposing U-238 to a neutron source (i.e. a nuclear
reactor). When U-238 absorbs a neutron the resulting U-239 isotope then beta
decays twice into Pu-239. The plutonium can then be chemically separated
from the uranium and be isolated for weapons use. Pu-239 has a higher
probability for fission than U-235, and a larger number of neutrons produced
per fission event, resulting in a smaller critical mass. Pure Pu-239 also
has a reasonably low rate of neutron emission due to spontaneous fission (10
fission/sec-kg), making it feasible to assemble a super critical mass before
predetonation. The Pu-239 will be invariably contaminated by Pu-240,
however, due to the fact that the freshly made Pu-239 captures a neutron to
make Pu-240. Pu-240 has a high rate of spontaneous fission events (415,000
fission/sec-kg), making it extremely difficult to assemble a super critical
mass before the neutrons emitted from spontaneous fission start a premature
chain reaction and cause the weapon to fizzle. Weapons grade plutonium must
contain no more than 7% Pu-240 - and is obtained by only exposing U-238
samples to neutron sources for short periods of time to reduce the amount of
Pu-240 made. The critical mass for an unreflected sphere of plutonium is 16
kg, but through the use of a neutron reflecting tamper the pit of plutonium
in a fission bomb is reduced to 10 kg, which is a sphere with a diameter of
10 cm.

Combination Methods

The simplest technical mechanism for assembling a supercritical mass is to
shoot one piece of fissile material as a projectile against a second part as
a target, usually called the gun method. This is how the Little Boy weapon
which was detonated over Hiroshima worked. This method of combination can
only be used for U-235 because of the relatively long amount of time it
takes to combine the materials, making predetonation likely for Pu-239 which
has a higher spontaneous neutron release due to Pu-240 contamination.

The more difficult, but superior, method of combination is referred to as
the implosion method and uses conventional explosives surrounding the
material to rapidly compress the mass to a supercritical state. For Pu-239
assemblies a contamination of only 1% Pu-240 produces so many neutrons that
implosion systems are required to produce efficient bombs. This is the
reason that the more technically difficult implosion method was used on the
plutonium Fat Man weapon which was detonated over Nagasaki.

Weapons assembled with this method also tend to be more efficient than the
weapons employing the gun method of combination. The reason that the
implosion method is more efficient is because it not only combines the
masses, but also increases the density of the mass. The neutron
multiplication factor, k, of a fissionable assembly is proportional to the
density squared, meaning that k goes up by a factor of four if the density
is doubled. Most modern weapons use a hollow plutonium core with an
implosion mechanism for detonation.

This precision compression of the pit creates a need for very precise design
and machining of the pit and explosive lenses. The milling machines used are
so precise that they could cut the polished surfaces of eyeglass lenses.
Machining plutonium is difficult not only because of its toxicity but also
because plutonium has many different metallic phases and changing phases
distorts the metal.

Tamper / Neutron Reflector

In a uranium graphite chain reacting pile the critical size may be
considerably reduced by surrounding the pile with a layer of graphite, since
such an envelope reflects many neutrons back into the pile. A similar
envelope can be used to reduce the critical size of a weapon, but here the
envelope has an additional role: its very inertia delays the expansion of
the reacting material. For this reason such an envelope is often called a
tamper. As has already been remarked, the weapon tends to fly to bits as the
reaction proceeds and this tends to stop the reaction, so the use of a
tamper makes for a longer lasting, more energetic, and more efficient
explosion. The most effective tamper is the one having the highest density;
high tensile strength turns out to be unimportant because no material will
hold together under the extreme pressures of a nuclear weapon. It is a
fortunate coincidence that materials of high density are also excellent as
reflectors of neutrons.

While the effect of a tamper is to increase the efficiency - both by
reflecting neutrons and by delaying the expansion of the bomb, the effect on
the efficiency is not as great as on the critical mass. The reason for this
is that the process of reflection is relatively time consuming and may not
occur extensively before the chain reaction is terminated.

Neutron trigger / Initiator

One of the key elements in the proper operation of a nuclear weapon is
initiation of the fission chain reaction at the proper time. To obtain a
significant nuclear yield of the nuclear explosive, sufficient neutrons must
be present within the supercritical core at the right time. If the chain
reaction starts too soon, the result will be only a 'fizzle yield,' much
below the design specification; if it occurs too late, there may be no yield
whatever. Several ways to produce neutrons at the appropriate moment have
been developed.

Early neutron sources consisted of a highly radioactive isotope of Polonium
(Po-210), which is a strong alpha emitter combined with beryllium which will
absorb alphas and emit neutrons. This isotope of polonium has a half life of
almost 140 days, and a neutron initiator using this material needs to have
the polonium, which is generated in a nuclear reactor, to be replaced
frequently. To supply the initiation pulse of neutrons at the right time,
the polonium and the beryllium need to be kept apart until the appropriate
moment and then thoroughly and rapidly mixed by the implosion of the weapon.
This method of neutron initiation is sufficient for weapons utilizing the
slower gun combination method, but the timing is not precise enough for an
implosion weapon design.

Another method of providing source neutrons, is through a pulsed neutron
emitter which is a small ion accelerator with a metal hydride target. When
the ion source is turned on to create a plasma of deuterium or tritium, a
large voltage is applied across the tube which accelerates the ions into
tritium rich metal (usually scandium). The ions are accelerated so that
there is a high probability of nuclear fusion occurring. The
deuterium-tritium fusion reactions emit a short pulse of 14 MeV neutrons
which will be sufficient to initiate the fission chain reaction. The timing
of the pulse can be precisely controlled making it better suited for an
implosion weapon design.

Practical Limitations of the Fission Bomb

A pure fission bomb is practically limited to a yield of a few hundred
kilotons by the large amounts of fissile material needed to make a large
weapon. It is technically difficult to keep a large amount of fissile
material in a subcritical assembly while waiting for detonation, and it is
also difficult to physically transform the subcritical assembly into a
supercritical one quick enough that the device explodes rather than
prematurely detonating such that a majority of the fuel is unused
(inefficient predetonation). The most efficient pure fission bomb would
still only consume 20% of its fissile material before being blown apart, and
can often be much less efficient (Fat Man only had an efficiency of 1.4%).
Large yield, pure fission weapons are also unattractive due to the weight,
size and cost of using large amounts of highly enriched material.

Thermonuclear weapons (also Hydrogen bomb or Fusion bomb)

The amount of energy released by a weapon can be greatly increased by the
addition of nuclear fusion reactions. Fusion releases even more energy per
reaction than fission, and can also be used as a source for additional
neutrons. The light weight of the elements used as fusion fuel, combined
with the larger energy release, means that fusion is a very efficient fuel
by weight, making it possible to build extremely high yield weapons which
are still portable enough to easily deliver. Fusion is the combination of
two light atoms, usually isotopes of hydrogen, to form a more stable heavy
atom and release excess energy. The fusion reaction requires the atoms
involved to have a high thermal energy, which is why the reaction is called
thermonuclear. The extreme temperatures and densities necessary for a fusion
reaction are easily generated by a fission explosion.

The simplest way to utilize fusion is to put a mixture of deuterium and
tritium inside the hollow core of an implosion style plutonium pit. When the
imploding fission chain reaction brings the fusion fuel to a sufficient
pressure, the fusion reaction occurs fairly quickly and releases a large
number of energetic neutrons into the surrounding fissile material, which
allows the fissile material to burn more efficiently. The efficiency (and
therefore yield) of a pure fission bomb can be doubled through the use of a
fusion boosted core, with very little increase in the size and weight of the
device. The amount of energy released through fusion is very small compared
to the energy from fission, so the fusion chiefly increases the fission
efficiency by providing a burst of additional neutrons. The fusion core of
modern fusion weapons is lithium-7 deuteride.

Staged thermonuclear weapons

The basic principles behind modern thermonuclear weapons were discovered
independently by scientists in different countries. Edward Teller and
Stanislaw Ulam at Los Alamos worked out the idea of staged detonation
coupled with radiation implosion in what is known in the United States as
the Teller-Ulam design. Soviet physicist Andrei Sakharov independently
arrived at the same answer (which he called his Third Idea) a short time
later. A single small fission bomb, the trigger, is placed at the point of a
cone-shaped arrangement of X-ray mirrors. The mirrors focus the X-rays from
the fission explosive on a column of lithium deuteride. The radiation
pressure of the X-rays heats and pressurizes the deuterium enough to fuse
into helium, and emit copious neutrons. The neutrons transmute the lithium
to tritium, which then also fuses and emits large amount of gamma rays. A
heavy, U-238 cone between the fission bomb and the column prevented the
premature collapse of the column by direct X-ray pressure.

Advanced Thermonuclear Weapons Designs

The largest modern fission-fusion-fission weapons include a fissionable
outer shell of U-238, the more inert waste isotope of uranium, or
constructed the X-ray mirrors of polished U-238. This otherwise inert U-238
would be detonated by the intense fast neutrons from the fusion stage,
increasing the yield of the bomb many times. For maximum yield, however,
moderately enriched uranium is preferable as a jacket material. The largest
bomb ever exploded was of this type, a 60 megaton bomb named Tsar Bomba that
was exploded by the Soviet Union in Siberia.

The cobalt bomb uses cobalt in the shell, and the fusion neutrons convert
the cobalt into cobalt-60, a powerful long-term (5 years) emitter of gamma
rays. In general this type of weapon is a salted bomb and variable fallout
effects can be obtained by using different salting isotopes. Gold has been
proposed for short-term fallout (days), tantalum and zinc for fallout of
intermediate duration (months), and cobalt for long term contamination
(years). To be useful for salting, the parent isotopes must be abundant in
the natural element, and the neutron-bred radioactive product must be a
strong emitter of penetrating gamma rays.

The primary purpose of this weapon is to create extremely radioactive
fallout to deny a region to an advancing army, a sort of wind-deployed
mine-field. No cobalt or other salted bomb has ever been atmospherically
tested, and as far as is publicly known none have ever been built. In light
of the ready availability of fission-fusion-fission bombs, it is unlikely
any special-purpose fallout contamination weapon will ever be developed. The
British did test a bomb that incorporated cobalt as an experimental
radiochemical tracer (Antler/Round 1, 14 September 1957). This 1 kt device
was exploded at the Tadje site, Maralinga range, Australia. The experiment
was regarded as a failure and not repeated.

The thought of using cobalt, which has the longest half-life of the feasible
salting materials, caused Leo Szilard to refer to the weapon as a potential
doomsday device. With a 5yr half-life people would have to remain shielded
underground for many years, effectively wiping out humanity. However this
would require a massive (unrealistic) amount of such bombs, yet the public
heard of it and there were numerous stories involving a single bomb wiping
out the planet.

A final variant of the thermonuclear weapons is the enhanced radiation
weapon, or neutron bomb which are small thermonuclear weapons in which the
burst of neutrons generated by the fusion reaction is intentionally not
absorbed inside the weapon, but allowed to escape. The X-ray mirrors and
shell of the weapon are made of chromium or nickel so that the neutrons are
permitted to escape. This intense burst of high-energy neutrons is the
principle destructive mechanism. Neutrons are more penetrating than other
types of radiation so many shielding materials that work well against gamma
rays do not work nearly as well. The term "enhanced radiation" refers only
to the burst of ionizing radiation released at the moment of detonation, not
to any enhancement of residual radiation in fallout (as in the salted bombs
discussed above).

Neutron bombs could be used as strategic anti-missile weapons, and as
tactical weapons intended for use against armored forces. As an anti-missile
weapon ER weapons were developed to protect U.S. ICBM silos from incoming
Soviet warheads by damaging the nuclear components of the incoming warhead
with the intense neutron flux. Tactical neutron bombs are primarily intended
to kill soldiers who are protected by armor. Armored vehicles are extremely
resistant to blast and heat produced by nuclear weapons, so the effective
range of a nuclear weapon against tanks is determined by the lethal range of
the radiation, although this is also reduced by the armor. By emitting large
amounts of lethal radiation of the most penetrating kind, ER warheads
maximize the lethal range of a given yield of nuclear warhead against
armored targets.

One problem with using radiation as a tactical anti-personnel weapon is that
to bring about rapid incapacitation of the target, a radiation dose that is
many times the lethal level must be administered. A radiation dose of 600
rads is normally considered lethal (it will kill at least half of those who
are exposed to it), but no effect is noticeable for several hours. Neutron
bombs were intended to deliver a dose of 8000 rads to produce immediate and
permanent incapacitation. A 1 kt ER warhead can do this to a T-72 tank crew
at a range of 690 m, compared to 360 m for a pure fission bomb. For a "mere"
600 rad dose the distances are 1100 m and 700 m respectively, and for
unprotected soldiers 600 rad exposures occur at 1350 m and 900 m. The lethal
range for tactical neutron bombs exceeds the lethal range for blast and heat
even for unprotected troops.

The neutron flux can induce significant amounts of short lived secondary
radioactivity in the environment in the high flux region near the burst
point. The alloy steels used in armor can develop radioactivity that is
dangerous for 24-48 hours. If a tank exposed to a 1 kt neutron bomb at 690 m
(the effective range for immediate crew incapacitation) is immediately
occupied by a new crew, they will receive a lethal dose of radiation within
24 hours.

Some authorities say that due to the rapid attenuation of neutron energy by
the atmosphere (it drops by a factor of 10 every 500 m in addition to the
effects of spreading) ER weapons are only effective at short ranges, and
thus are practical only in relatively low yields. These ER warheads are said
to be designed to minimize the amount of fission energy and blast effect
produced relative to the neutron yield. The principal reason is said to be
to allow their use close to friendly forces.

These same authorities say that the common perception of the neutron bomb as
a "landlord bomb" that would kill people but leave buildings undamaged is
greatly overstated. At the conventional effective combat range (690 m) the
blast from a 1 kt neutron bomb will destroy or damage to the point of
unusability almost any civilian building. Thus the use of neutron bombs to
stop an enemy attack, which requires exploding large numbers of them to
blanket the enemy forces, would also destroy all buildings in the area.

Another view of the neutron bomb and its tactics exists. The inventor of the
neutron bomb, Samuel Cohen, wrote a book in which he stated that the
effective range of a pure neutron bomb exceeded 10 Km of altitude. Samuel
Cohen stated explicitly that "enhanced radiation" weapons deployed in
Germany during the cold war were political compromises designed to have
substantial blast, with radiation effects deliberately reduced to eliminate
any possibility of surviving structures. He also quoted radiation releases
of 100KRads at the ground from pure neutron weapons exploded at 10Km.

The neutron absorption spectra of air is disputed by some authorities, and
may depend in part on absorption by hydrogen from water vapor. It therefore
might vary exponentially with humidity, making high-altitude neutron bombs
immensely more deadly in desert climates than in humid ones. This effect
also varies with altitude.

According to Samuel Cohen, one possible tactic of using such "true" neutron
bombs is therefore to launch them as defensive weapons against armored
attacks. Civilians enter radiation shelters, and the bomb is exploded 10Km
over the armored attack. Portable armor is said to be unable to shield tank
and aircraft crews. In such an event, a city's trees and grass would have
been killed by radiation, but buildings would remain undamaged for the
emerging civilians.

Such neutron bombs would be very potent anti-ship weapons. A major support
of Cohen's research was the U.S. Navy.