Most nuclear weapons today—including those
in the U.S., Russian, British, French, and Chinese arsenals—are
two-stage thermonuclear weapons that derive their explosive energy from
the combined power of nuclear fission and fusion. An initial fission
reaction generates the high temperatures needed to trigger a
secondary—and much more powerful—fusion reaction (hence the term
“thermonuclear”). Israel, India, and Pakistan are generally believed to
possess nuclear weapons that utilize only nuclear fission, although some
of these nations may also have some thermonuclear weapons. North Korea
first tested a fission-based weapon in 2006, and tested another in 2009.
Nuclear Fission and Atomic Weapons
The nuclei of atoms consist of two types of
particles: positively charged protons and neutrons with no electric
charge. (All atoms of the same element have the same number of protons,
but the number of neutrons can vary.) The nuclei of some radioactive
elements can split—or fission—if bombarded with fast-moving neutrons.
The by-products of this fission are two lighter nuclei, one or more free
neutrons, and energy in the form of heat and light.
Certain isotopes of radioactive elements
(i.e., variations of the same element with different numbers of neutrons
in the nucleus) such as plutonium-239 or uranium-235 can emit two
neutrons when they fission. These secondary neutrons then collide with
other nearby nuclei, causing them to fission and release two more
neutrons. Each fission reaction doubles the amount of neutrons and
energy released, causing a chain reaction. After only a few
microseconds, this chain reaction can produce an explosion equivalent to
the detonation of many thousands of tons (or kilotons) of TNT. The
so-called atomic (or A-) bombs dropped on Hiroshima and Nag asaki,
Japan, in 1945 were fission-based, and had explosive yields equivalent
to about 15 and 20 kilotons of TNT, respectively. Similar fission
processes (though controlled) generate the energy in nuclear reactors.
Thermonuclear Weapons
Thermonuclear weapons can produce much larger explosions than fission
weapons; the first thermonuclear test explosion had a yield of about
10,000 kilotons (or 10 megatons). Today, U.S. warheads commonly have
explosive yields of several hundred kilotons.
Essentially, the destructive energy produced by such weapons is the
result of three separate but nearly simultaneous explosions. The first
is the detonation of chemical explosives that surround a hollow sphere
(or "pit") of plutonium-239 metal. The force from this blast is directed
inward, compressing the pit and bringing its atoms closer together.
When the plutonium pit becomes dense enough to sustain a fission chain
reaction (a condition termed "supercritical"), a neutron generator
injects neutrons into the pit to initiate the fission chain reaction.
Together, these chemical and fission explosions are known as the nuclear
"primary."
The primary produces the high temperatures and pressures required to
ignite fusion reactions in the "secondary," which actually produces the
third explosion. In fusion, two or more atomic nuclei fuse into one
heavier nucleus and, in the process, release a great deal of energy. In a
thermonuclear weapon, isotopes of hydrogen undergo fusion, which is why
these weapons are commonly called hydrogen or H-bombs.
In practice, a thermonuclear weapon (such as that illustrated in the
diagram) is even more complicated than the description above suggests.
First, a pure fission primary is inefficient since the plutonium pit
will blow itself apart before much of the available plutonium-239
fissions. To reduce the amount of plutonium needed, the fission reaction
can be "boosted" so that a higher fraction of the plutonium fissions.
For boosted primaries, hydrogen gas (consisting of the isotopes
deuterium and tritium, which have one and two neutrons, respectively, in
addition to the one proton that all hydrogen atoms have) is placed
inside the hollow center of the pit. As the plutonium fissions, enough
heat is produced to cause the "boost" gas to undergo fusion, releasing a
burst of high-energy neutrons that, in turn, induce additional fissions
in the pit.
The fusion fuel in the secondary takes the form of lithium deuteride
(a solid compound of lithium and deuterium). Inside the layer of fusion
fuel is a fission "spark plug" consisting of either plutonium-239 or
uranium-235. As the primary explosion compresses the fusion fuel from
the outside, the spark plug material becomes supercritical and fissions,
heating the fusion fuel from the inside and helping to initiate the
fusion reactions. Finally, a layer of uranium that surrounds the fusion
fuel undergoes fission in response to the neutrons released by the
fusion reactions, generally contributing more than half of the total
explosive yield of a thermonuclear weapon.
In addition to the primary and secondary nuclear components that
constitute the "nuclear explosive package," a thermonuclear weapon
typically has thousands of non-nuclear components. These perform a
variety of functions such as preventing the accidental detonation and
unauthorized use of the weapon, arming the weapon by removing these
barriers to detonation, determining the altitude of the weapon during
its delivery so that it detonates at the correct location, and
initiating the detonation by setting off the chemical explosives.
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