Nuclear weapon

Nuclear weapons are explosive devices that derive their destructive energy from nuclear reactions, either through nuclear fission, nuclear fusion, or a combination of both in thermonuclear designs. Their immense energy output from small quantities of matter has shaped modern military strategy, international security, and global arms control efforts.
The destructive effects of nuclear detonations arise not only from the blast itself but also from thermal radiation, ionising radiation, radioactive fallout, electromagnetic pulses, and large-scale fires. The destructive potential is measured in terms of the TNT equivalent yield, which ranges from tens of tonnes to tens of megatonnes.

Development and early use

The first nuclear weapons were created during the Second World War under the Manhattan Project, a vast scientific and industrial programme involving the United States, the United Kingdom, and Canada. Its purpose was to produce fissile materials—uranium-235 and plutonium-239—and design reliable explosive mechanisms.
In August 1945 the United States used nuclear weapons in warfare for the first and only time to date. The Little Boy uranium gun-type bomb was detonated over Hiroshima, followed by the Fat Man plutonium implosion bomb over Nagasaki three days later. These attacks caused the deaths of an estimated 200,000 civilians and military personnel and contributed to Japan’s surrender. Their ethical and strategic implications remain subjects of debate.
Following the war, nuclear technology advanced rapidly. The earliest designs relied entirely on fission, but by 1952 the first successful thermonuclear weapon—a device achieving fusion—was detonated, demonstrating yields far beyond those of fission bombs alone. The development of these weapons led to an accelerating nuclear arms race during the Cold War.

Types of nuclear weapons

Nuclear weapons can be broadly divided into fission bombs and thermonuclear bombs, although all thermonuclear weapons contain fission components.

• Fission weaponsThese devices derive most of their energy from the splitting of heavy nuclei. A mass of fissile material is rapidly assembled into a supercritical configuration using one of two methods:• gun-type assembly, in which two subcritical masses are driven together;• implosion assembly, in which high explosives compress a subcritical core.Implosion is far more efficient and is used in the majority of modern designs. Yields may range from under one kiloton to several hundred kilotons. All fission events produce radioactive fission products and free neutrons, both of which contribute to fallout and long-term environmental contamination.
• Thermonuclear weaponsAlso known as hydrogen bombs, these use a fission primary to generate the intense temperatures and pressures required to initiate fusion between isotopes of hydrogen, typically deuterium and tritium. Fusion contributes much of the total energy released, and can also drive secondary fission reactions in surrounding materials. Modern designs employ staged configurations, such as the Teller–Ulam arrangement, enabling yields that can exceed several tens of megatonnes. Only a small number of states have successfully tested such devices.

Effects and delivery systems

Nuclear explosions produce a characteristic set of destructive effects. The blast wave devastates structures; thermal radiation ignites fires; prompt ionising radiation can cause acute radiation sickness; and fallout results in long-term contamination. A detonation at high altitude can generate an electromagnetic pulse, disrupting electrical infrastructure, while the ionisation of the upper atmosphere can cause radar blackouts.
Delivery systems evolved to increase range, survivability, and accuracy. By the late Cold War, major nuclear powers developed the nuclear triad, consisting of:• land-based intercontinental ballistic missiles,• submarine-launched ballistic missiles,• strategic bombers capable of delivering gravity bombs and cruise missiles.
Tactical nuclear weapons were designed for battlefield roles and included short-range missiles, artillery shells, torpedoes, and demolition charges. Their relevance diminished after the Cold War, although some remain in national arsenals.

Proliferation and international control

Since 1945 more than 2,000 nuclear tests have been conducted in the atmosphere, underwater, underground, and in space, contributing to weapon refinement and effects research. A number of states have sought to limit or eliminate testing. Countries that no longer conduct tests typically maintain their arsenals using science-based stockpile stewardship, combining simulation, high-energy laboratories, and subcritical experiments.
Nine states are widely recognised as possessing nuclear weapons or having tested them: the United States, Russia, the United Kingdom, France, China, India, Pakistan, North Korea, and Israel (the latter maintaining a policy of deliberate ambiguity). Several NATO allies—Germany, Belgium, the Netherlands, Italy, Turkey, and Belarus—participate in nuclear sharing arrangements.
South Africa remains the only state to have developed and then voluntarily dismantled its nuclear arsenal.
The global nuclear order is supported by a number of legal and institutional frameworks:• the Treaty on the Non-Proliferation of Nuclear Weapons (NPT), which aims to prevent the spread of nuclear weapons and promote disarmament;• the Comprehensive Nuclear-Test-Ban Treaty (CTBT) and its monitoring organisation;• the International Atomic Energy Agency (IAEA), responsible for safeguarding civilian nuclear programmes;• nuclear-weapon-free zone treaties and the Treaty on the Prohibition of Nuclear Weapons.Debate continues over the effectiveness of these instruments, particularly amid concerns about regional tensions and modernisation programmes.

Strategic doctrine and deterrence

Nuclear strategy has historically centred on deterrence, the notion that the threat of unacceptable retaliation can prevent an adversary from initiating nuclear war. During the Cold War this often took the form of mutually assured destruction, in which both superpowers maintained second-strike capabilities.
Strategic planning also distinguishes between counterforce targets—military infrastructure and missile sites—and countervalue targets such as cities and industrial centres. Over time, improvements in accuracy, command-and-control systems, and early-warning technologies have altered calculations about escalation and stability.

Modern issues and contemporary relevance

Advances in missile technology, including hypersonic glide vehicles and improved ballistic missile defences, continue to influence nuclear planning. Simultaneously, debates over disarmament, arms control renewal, and the humanitarian consequences of nuclear use remain central to international diplomacy.