Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission or a combination of fission and nuclear fusion reactions, producing a nuclear explosion. Both bomb types release large quantities of energy from relatively small amounts of matter.
Nuclear weapons have had yields between 10 tons and 50 megatons for the Tsar Bomba. Yields in the low kilotons can devastate cities. A thermonuclear weapon weighing as little as can release energy equal to more than 1.2 megatons of TNT. Apart from the blast, effects of nuclear weapons include extreme heat and ionizing radiation, firestorms, radioactive nuclear fallout, an electromagnetic pulse, and a radar blackout.
The first nuclear weapons were developed by the United States in collaboration with the United Kingdom and Canada during World War II in the Manhattan Project. Production requires a large scientific and industrial complex, primarily for the production of fissile material, either from nuclear reactors with reprocessing plants or from uranium enrichment facilities. Nuclear weapons have been used twice in war, in the 1945 atomic bombings of Hiroshima and Nagasaki that killed between 150,000 and 246,000 people. Nuclear deterrence, including mutually assured destruction, aims to prevent nuclear warfare via the threat of unacceptable damage and the danger of escalation to nuclear holocaust. A nuclear arms race for weapons and their delivery systems was a defining component of the Cold War.
Strategic nuclear weapons are targeted against civilian, industrial, and military infrastructure, while tactical nuclear weapons are intended for battlefield use. Strategic weapons led to the development of dedicated intercontinental ballistic missiles, submarine-launched ballistic missile, and nuclear strategic bombers, collectively known as the nuclear triad. Tactical weapons options have included shorter-range ground-, air-, and sea-launched missiles, nuclear artillery, atomic demolition munitions, nuclear torpedoes, and nuclear depth charges, but they have become less salient since the end of the Cold War.
, there are nine countries on the list of states with nuclear weapons, and six more agree to nuclear sharing. Roughly 40 countries have defense policies linked to nuclear armament, through NATO membership or agreements with other nuclear powers. Nuclear weapons are weapons of mass destruction, and their control is a focus of international security through measures to prevent nuclear proliferation, arms control, or nuclear disarmament. The total from all stockpiles peaked at over 64,000 weapons in 1986, and is around 9,600 today. Key international agreements and organizations include the Treaty on the Non-Proliferation of Nuclear Weapons, the Comprehensive Nuclear-Test-Ban Treaty and Comprehensive Nuclear-Test-Ban Treaty Organization, the International Atomic Energy Agency, the Treaty on the Prohibition of Nuclear Weapons, and nuclear-weapon-free zones.
Testing and deployment
Nuclear weapons have only twice been used in warfare, both times by the United States against Japan at the end of World War II. On August 6, 1945, the United States Army Air Forces detonated a uranium gun-type fission bomb nicknamed "Little Boy" over the Japanese city of Hiroshima; three days later, on August 9, the USAAF detonated a plutonium implosion-type fission bomb nicknamed "Fat Man" over the Japanese city of Nagasaki. These bombings caused injuries that resulted in the deaths of approximately 200,000 civilians and military personnel. The ethics of these bombings and their role in Japan's surrender are to this day, still subjects of debate.Since the atomic bombings of Hiroshima and Nagasaki, nuclear weapons have been detonated over 2,000 times for testing and demonstration. Only a few nations possess such weapons or are suspected of seeking them. The only countries known to have detonated nuclear weapons—and acknowledge possessing them—are the United States, the Soviet Union, the United Kingdom, France, China, India, Pakistan, and North Korea. Israel is believed to possess nuclear weapons, though, in a policy of deliberate ambiguity, it does not acknowledge having them. Germany, Italy, Turkey, Belgium, the Netherlands, and Belarus are nuclear weapons sharing states. South Africa is the only country to have independently developed and then renounced and dismantled its nuclear weapons.
On the October 30, 2025, US President Donald Trump called for renewed nuclear weapons testing in order to keep pace with other nuclear armed states such as Russia and China however it was not specified whether he referred to nuclear explosive testing or testing of delivery systems for nuclear warheads.
Types
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.Fission weapons
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. 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 is forced into supercriticality—allowing an exponential growth of nuclear chain reactions—either by shooting one piece of sub-critical material into another or by compression of a sub-critical sphere or cylinder of fissile material using chemically fueled explosive lenses. The latter approach, the "implosion" method, is more sophisticated and more efficient than the former.
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 of TNT.
All fission reactions generate fission products, the remains of the split atomic nuclei. Many fission products are either highly radioactive or moderately radioactive, and as such, they are a serious form of radioactive contamination. Fission products are the principal radioactive component of nuclear fallout. Another source of radioactivity is the burst of free neutrons produced by the weapon. When they collide with other nuclei in the surrounding material, the neutrons transmute those nuclei into other isotopes, altering their stability and making them radioactive.
The most commonly used fissile materials for nuclear weapons applications have been uranium-235 and plutonium-239. Less commonly used has been uranium-233. 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.
Fusion weapons
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, as they rely on fusion reactions between isotopes of hydrogen. 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—the United States, Russia, the 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 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 results in an explosion hundreds of times stronger than that of a fission bomb of similar weight.
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 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 this "two-stage" design, 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. This is in contrast to fission bombs, which are limited in their explosive power due to criticality danger. The largest nuclear weapon ever detonated, the Tsar Bomba of the USSR, which released an energy equivalent of over, was a three-stage weapon. Most thermonuclear weapons are considerably smaller than this, due to practical constraints from missile warhead space and weight requirements. In the early 1950s the Livermore Laboratory in the United States had plans for the testing of two massive bombs, Gnomon and Sundial, 1 gigaton of TNT and 10 gigatons of TNT respectively.
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. Furthermore, high yield thermonuclear explosions have the force to lift radioactive debris upwards past the tropopause into the stratosphere, where the calm non-turbulent winds permit the debris to travel great distances from the burst, eventually settling and unpredictably contaminating areas far removed from the target of the explosion.