A few words about nuclear weapons
Nuclear weapons exploit two principle physical, or more specifically nuclear,
properties of certain substances: fission and fusion.
Fission is possible in a number of heavy elements, but in weapons it is principally
confined to what is termed slow neutron fission in just two particular isotopes: 235U
and 239Pu. These are termed fissile, and are the source of energy in
atomic weapons. An explosive chain reaction can be started with relatively slight energy
input (so-called slow neutrons) in such material.
An actual 239Pu ingot, alloyed with gallium for
improved physical properties
Isotopes are 'varieties' of an element which differ only in their number of neutrons.
For example, hydrogen exists as 1H 2H and 3H -- different
isotopes of the same chemical element, with no, one, and two neutrons respectively. All
the chemical properties, and most of the physical properties, are the same between
isotopes. Nuclear properties may differ significantly, however.
The fission, or 'splitting' of an atom, releases a very large amount of energy per unit
volume -- but a single atom is very small indeed. The key to an uncontrolled or explosive
release of this energy in a mass of fissile material large enough to constitute a weapon
is the establishment of a chain reaction with a short time period and high growth rate.
This is surprisingly easy to do.
Fission of 235U (uranium) or 239Pu (plutonium) starts in most
weapons with an incident source of neutrons. These strike atoms of the fissile material,
which (in most cases) fissions, and each atom in so doing releases, on average, somewhat
more than 2 neutrons. These then strike other atoms in the mass of material, and so on.
If the mass is too small, or has too large a surface area, too many neutrons escape and
a chain reaction is not possible; such a mass is termed subcritical. If the neutrons
generated exactly equal the number consumed in subsequent fissions, the mass is said to be
critical. If the mass is in excess of this, it is termed supercritical.
Fission (atomic) weapons are simply based on assembling a supercritical mass of fissile
material quickly enough to counter disassembly forces.
The majority of the energy release is nearly instantaneous, the mean time from neutron
release to fission can be of the order of 10 nanoseconds, and the chain reaction builds
exponentially. The result is that greater than 99% of the very considerable energy
released in an atomic explosion is generated in the last few (typically 4-5) generations
of fission -- less than a tenth of a microsecond.*
This tremendous energy release in a small space over fantastically short periods of
time creates some unusual phenomena -- physical conditions that have no equal on earth, no
matter how much TNT is stacked up.
Plutonium (239Pu) is the principal fissile material used in today's nuclear
weapons. The actual amount of this fissile material required for a nuclear weapon is
Below is a scale model of the amount of 239Pu required
in a weapon with the force that destroyed the city of Nagasaki in 1945:
In the Fat Man (Nagasaki) weapon design an excess of Pu was provided. Most of the
remaining bulk of the weapon was comprised of two concentric shells of high explosives.
Each of these was carefully fashioned from two types of explosives with differing burn
rates. These, when detonated symmetrically on the outermost layer, caused an implosion
or inward-moving explosion.
The two explosive types were shaped to create a roughly
spherical convergent shockwave which, when it reached the Pu 'pit' in the center of the
device, caused it to collapse.
The Pu pit became denser, underwent a phase change, and
A small neutron source, the initiator, placed in the very
center of this Pu pit, provided an initial burst of neutrons -- final generations of
which, less than a microsecond later, saw the destruction of an entire city and more than
Nearly all the design information for weapons such as these is now in the public
domain; in fact, considering the fact that fission weapons exploit such a simple and
fundamental physical (nuclear) property, it is no surprise that this is so. It is more
surprising that so much stayed secret for so long, at least from the general public.
A neutron reflector, often made of beryllium, is placed outside the central pit to
reflect neutrons back into the pit. A tamper, often made of depleted uranium or 238U
helps control premature disassembly. Modern fission devices use a technique called
'boosting' (referred to in the next section), to control and enhance the yield of the
Today's nuclear threat lies mostly in preventing this fissile special nuclear material
(often referred to as SNM) from falling into the wrong hands: once there, it is a very
short step to construct a working weapon.
What we do now to keep these devices out of
the hands of groups like Al-Qaeda is vital to civilized peoples.
A schematic of a hypothetical 'boosted' fission weapon
The gadget device used in the Trinity test: the world's
first nuclear weapon test.
Note spherical geometry and the HE detonator arrangement. New Mexico, 21KT,
||Typical fission weapon, shortly after detonation at the Nevada
test site, with roughly the same yield as the weapon that destroyed Hiroshima. Reddish
vapor surrounding the plasma toroid includes intensely radioactive fission fragments and
ionized nitrogen oxides from the atmosphere. (Grable, 15KT, 1953)
Fission weapons discussed above are ultimately limited in their destructive capability
by the sheer size a subcritical mass can assume -- and be imploded quickly enough by high
explosives to form a supercritical assembly. The largest known pure fission weapon tested
had a 500 kiloton yield. This is some thirty-eight times the release which destroyed
Hiroshima in 1945. Not satisfied that this was powerful enough, designers developed thermonuclear
Fusion exploits the energy released in the fusing of two atoms to form a new element; e.g.
deuterium atoms fusing to form helium, 2H + 2H = 4He2
, as occurs on the sun. For atoms to fuse, very high temperatures and pressures are
required. Only fusion of the lightest element, hydrogen, has proven practical. And only
the heavy isotopes of hydrogen, 2H (deuterium) and 3H (tritium),
have a low enough threshold for fusion to have been used in weapons successfully thus far.
The first method tried (boosting) involved simply placing 3H in a
void within the center of a fission weapon, where tremendous temperatures and high
pressures were attendant to the fission explosion. This worked; contributing energy to the
overall explosion, and boosting the efficiency of the Pu fissioning as well (fusion
reactions also release neutrons, but with much higher energy).
Because 3H is a
gas at room temperature, it can be easily 'bled' into the central cavity from a storage
bottle prior to an explosion, and impact the final yield of the device. This is still used
today, and allows for what is termed 'dial-a-yield' capability on many stockpiled weapons.
Multistage thermonuclear weapons -- the main component of today's strategic nuclear
forces -- are more complex. These employ a 'primary' fission weapon to serve merely as a trigger.
As mentioned above, the fission weapon is characterized by a tremendous energy release in
a small space over a short period of time. As a result, a very large fraction of the
initial energy release is in the form of thermal X-rays.
These X-rays are channeled to a
'secondary' fusion package. The X-rays travel into a cavity within a
cylindrical radiation container.
pressure from these X-rays either directly, or through an intermediate material often
cited as a polystyrene foam, ablates a cylindrical enclosure containing thermonuclear fuel
(shown in blue at left); this can be Li2H (lithium deuteride).
the central axis of this fuel is a rod of fissile material, termed a 'sparkplug'.
contracting fuel package becomes denser, the sparkplug begins to fission, neutrons from
this transmute the Li2H into 3H that can readily fuse with 2H
(the fusion reaction 3H + 2H has a very high cross-section, or
probability, in typical secondary designs), heat increases greatly, and fusion continues
through the fuel mass.
A final 'tertiary' stage can be added to this in the form of an
exterior blanket of 238U, wrapping the outer surface of the radiation case or
the fuel package. 238U is not fissionable by the slower neutrons which dominate
the fission weapon environment, but fusion releases copious high energy neutrons and this
can fast fission the ordinary uranium.
This is a cheap (and radiologically very dirty) way
to greatly increase yield. The largest weapon ever detonated -- the Soviet Union's 'super
bomb', was some 60 MT in yield, and would have been nearer 100MT had this technique
been used in its tertiary. Again, to control the yield
precisely, 3H may be bled from a separate tank into the core of the primary, as
shown in the hypothetical diagram on the left of a modern thermonuclear weapon.
primary/secondary/tertiary or multistage arrangement can be increased -- unlike the
fission weapon -- to provide insane governments with any arbitrarily large yield.
Rare photo of the actual shrimp device used in Castle
Bravo. Note the cylindrical geometry, and the emergent spherical fission trigger on the
right. Light pipes leading to ceiling are visible near the fission trigger and at two
points along the secondary for transmitting early diagnostic information to remote
collection points, before they themselves are destroyed.
Note the 'danger, no smoking' sign at lower left. 15MT, 1954.
Fusion, or thermonuclear weapons, are not simple to design nor are they likely targets
of construction for would-be terrorists today.
Many aspects of the relevant radiation
transport, X-ray opacities, and ultra-high T and D equations-of-state (EOS) for relevant
materials are still classified to this day (though increasing dissemination of
weapons-adaptable information from the inertially-confined fusion (ICF) area may change
this in time). Keeping such information classified makes good sense.
Typical appearance of a thermonuclear weapon
detonation -- from many miles away.
(Castle Romeo, 7MT, 1954)
were required to record the fleeting moments of a
weapon's initial detonation. One such method was the Rapatronic camera, developed by Dr.
Harold Edgerton. The images it created are bizarre. Check out our collection of