There are two basic types of nuclear weapons: those that derive the majority of their energy from nuclear fission reactions alone, and those that use fission reactions to begin
nuclear fusion reactions that produce a large amount of the total energy output.
The two basic
All existing nuclear weapons derive some of their explosive energy from nuclear fission reactions. Weapons whose explosive output is exclusively from fission reactions are commonly referred to as atomic bombs or atom bombs (abbreviated as A-bombs). This has long been noted as something of a
misnomer, as their energy comes from the nucleus of the atom, just as it does with fusion weapons.
In fission weapons, a mass of
fissile material (
enriched uranium or
plutonium) is forced into
exponential growth of
nuclear chain reactions—either by shooting one piece of sub-critical material into another (the "gun" method) or by compressing using
explosive lenses a sub-critical sphere of material using
chemical explosives to many times its original density (the "implosion" method). The latter approach is considered more sophisticated than the former, and only the latter approach can be used if the fissile material is plutonium.
A major challenge in all nuclear weapon designs is to ensure that a significant fraction of the fuel is consumed before the weapon destroys itself. The amount of energy released by fission bombs can range from the equivalent of just under a ton to upwards of 500,000 tons (500
TNT (4.2 to 2.1×108 GJ).
All fission reactions generate
fission products, the remains of the split atomic nuclei. Many fission products are either highly radioactive (but short-lived) or moderately radioactive (but long-lived), and as such, they are a serious form of
radioactive contamination if not fully contained. Fission products are the principal radioactive component of
The most commonly used fissile materials for nuclear weapons applications have been
plutonium-239. Less commonly used has been
Neptunium-237 and some isotopes of
americium may be usable for nuclear explosives as well, but it is not clear that this has ever been implemented, and their plausible use in nuclear weapons is a matter of dispute.
The basics of the
for a hydrogen bomb: a fission bomb uses radiation to compress and heat a separate section of fusion fuel.
The other basic type of nuclear weapon produces a large proportion of its energy in nuclear fusion reactions. Such fusion weapons are generally referred to as thermonuclear weapons or more colloquially as hydrogen bombs (abbreviated as H-bombs), as they rely on fusion reactions between isotopes of
tritium). All such weapons derive a significant portion of their energy from fission reactions used to "trigger" fusion reactions, and fusion reactions can themselves trigger additional fission reactions.
Only six countries—United States, Russia, United Kingdom, China, France, and India—have conducted thermonuclear weapon tests. (Whether India has detonated a "true" multi-staged
thermonuclear weapon is controversial.)
North Korea claims to have tested a fusion weapon as of January 2016, though this claim is disputed.
 Thermonuclear weapons are considered much more difficult to successfully design and execute than primitive fission weapons. Almost all of the nuclear weapons deployed today use the thermonuclear design because it is more efficient.
Thermonuclear bombs work by using the energy of a fission bomb to compress and heat fusion fuel. In the
Teller-Ulam design, which accounts for all multi-megaton yield hydrogen bombs, this is accomplished by placing a fission bomb and fusion fuel (
lithium deuteride) in proximity within a special, radiation-reflecting container. When the fission bomb is detonated,
gamma rays and
X-rays emitted first compress the fusion fuel, then heat it to thermonuclear temperatures. The ensuing fusion reaction creates enormous numbers of high-speed
neutrons, which can then induce fission in materials not normally prone to it, such as
depleted uranium. Each of these components is known as a "stage", with the fission bomb as the "primary" and the fusion capsule as the "secondary". In large, megaton-range hydrogen bombs, about half of the yield comes from the final fissioning of depleted uranium.
Virtually all thermonuclear weapons deployed today use the "two-stage" design described above, but it is possible to add additional fusion stages—each stage igniting a larger amount of fusion fuel in the next stage. This technique can be used to construct thermonuclear weapons of arbitrarily large yield, in contrast to fission bombs, which are limited in their explosive force. The largest nuclear weapon ever detonated, the
Tsar Bomba of the USSR, which released an energy equivalent of over 50 megatons of TNT (210 PJ), was a three-stage weapon. Most thermonuclear weapons are considerably smaller than this, due to practical constraints from missile warhead space and weight requirements.
, often referred to as the "father of the hydrogen bomb"
Fusion reactions do not create fission products, and thus contribute far less to the creation of nuclear fallout than fission reactions, but because all thermonuclear weapons contain at least one fission stage, and many high-yield thermonuclear devices have a final fission stage, thermonuclear weapons can generate at least as much nuclear fallout as fission-only weapons.
There are other types of nuclear weapons as well. For example, a
boosted fission weapon is a fission bomb that increases its explosive yield through a small number of fusion reactions, but it is not a fusion bomb. In the boosted bomb, the neutrons produced by the fusion reactions serve primarily to increase the efficiency of the fission bomb. There are two types of boosted fission bomb: internally boosted, in which a deuterium-tritium mixture is injected into the bomb core, and externally boosted, in which concentric shells of lithium-deuteride and depleted uranium are layered on the outside of the fission bomb core.
Some nuclear weapons are designed for special purposes; a
neutron bomb is a thermonuclear weapon that yields a relatively small explosion but a relatively large amount of neutron
radiation; such a device could theoretically be used to cause massive casualties while leaving infrastructure mostly intact and creating a minimal amount of fallout. The detonation of any nuclear weapon is accompanied by a blast of
neutron radiation. Surrounding a nuclear weapon with suitable materials (such as
gold) creates a weapon known as a
salted bomb. This device can produce exceptionally large quantities of long-lived
radioactive contamination. It has been conjectured that such a device could serve as a "doomsday weapon" because such a large quantity of radioactivities with half-lives of decades, lifted into the stratosphere where winds would distribute it around the globe, would make all life on the planet extinct.
In connection with the
Strategic Defense Initiative, research into the
nuclear pumped laser was conducted under the DOD program
Project Excalibur but this did not result in a working weapon. The concept involves the tapping of the energy of an exploding nuclear bomb to power a single-shot laser which is directed at a distant target.
Starfish Prime high-altitude nuclear test in 1962, an unexpected effect was produced which is called a
nuclear electromagnetic pulse. This is an intense flash of electromagnetic energy produced by a rain of high energy electrons which in turn are produced by a nuclear bomb's gamma rays. This flash of energy can permanently destroy or disrupt electronic equipment if insufficiently shielded. It has been proposed to use this effect to disable an enemy's military and civilian infrastructure as an adjunct to other nuclear or conventional military operations against that enemy. Because the effect is produced by high altitude nuclear detonations, it can produce damage to electronics over a wide, even continental, geographical area.
Research has been done into the possibility of
pure fusion bombs: nuclear weapons that consist of fusion reactions without requiring a fission bomb to initiate them. Such a device might provide a simpler path to thermonuclear weapons than one that required development of fission weapons first, and pure fusion weapons would create significantly less nuclear fallout than other thermonuclear weapons, because they would not disperse fission products. In 1998, the
United States Department of Energy divulged that the United States had, "...made a substantial investment" in the past to develop pure fusion weapons, but that, "The U.S. does not have and is not developing a pure fusion weapon", and that, "No credible design for a pure fusion weapon resulted from the DOE investment".
Antimatter, which consists of
particles resembling ordinary
matter particles in most of their properties but having opposite
electric charge, has been considered as a trigger mechanism for nuclear weapons.
 A major obstacle is the difficulty of producing antimatter in large enough quantities, and there is no evidence that it is feasible beyond the military domain.
 However, the U.S. Air Force funded studies of the physics of antimatter in the
Cold War, and began considering its possible use in weapons, not just as a trigger, but as the explosive itself.
 A fourth generation nuclear weapon design
 is related to, and relies upon, the same principle as
antimatter-catalyzed nuclear pulse propulsion.
Most variation in
nuclear weapon design is for the purpose of achieving
different yields for different situations, and in manipulating design elements to attempt to minimize weapon size.