Nuclear 101


Module 4:

Nuclear Weapons

What are nuclear weapons?

  • A nuclear weapon is a weapon that derives its incredible destructive force from the sudden release of the energy created by a self-sustaining nuclear fission and/or fusion reaction.
  • Fission-based weapons derive their energy from the splitting of atoms, which includes all first generation U.S. nuclear weapons, including the bombs dropped on the Japanese cities of Hiroshima and Nagasaki. Modern nuclear weapons are far more destructive, and derive some or most of their explosive yield from another process called fusion.
  • Fusion is conceptually the inverse of fission, fusing rather than splitting atoms. When two or more light atoms (e.g., hydrogen isotopes deuterium and tritium), fuse, a large amount of energy is released.

How Fusion Works

Deuterium Tritium FusionImage Source: WikiMedia Commons

  • Fusion can only occur under intense heat and pressure similar to the temperature of the sun’s core. In fact, the sun generates its energy purely through the fusion process. Weapons designers produce the conditions for fusion through the use of a “primary” fission explosion to trigger a fusion-based “secondary.” This is why to-date it has been prohibitively difficult to harness fusion for peaceful applications; facilities such as the U.S. National Ignition Facility conduct fusion experiments in the hopes of eventually being able to do so.

How are nuclear weapons different from conventional weapons?

  • The humanitarian, economic, and environmental consequences of nuclear war are unimaginable. While the likelihood of a full-scale nuclear exchange has decreased significantly since the U.S.-Soviet Cold War, the continued existence of around 15,700 nuclear weapons poses ongoing risks of intentional, accidental or unauthorized nuclear weapons use. [1]

visualization of U.S. Nuclear Firepower: 1945 vs. 2012

  • Nuclear reactions, whether fission or fusion, allow nuclear weapons to harness far greater energy than conventional explosives, which rely upon chemical reactions. Fissioning one kilogram of a fissile material, such as uranium-235 or plutonium-239, can generate about 15 million times more energy than one kilogram of the conventional explosive TNT.
  • Nuclear explosions are typically measured using a standard unit known as tons of TNT equivalent. For example, the W87, a modern U.S. nuclear weapon, has an explosive yield of 300 kilotons, which is equivalent to 300,000 tons of TNT.
  • The difference is not just about the size of a nuclear explosion, but also the unique effects it generates. The enormous destructive power of a nuclear weapon comes from the blast (which causes shock waves); thermal radiation (which generates enormous heat); nuclear radiation (which has both short and long-term effects), and the Electromagnetic Pulse (which is a short burst of electromagnetic energy that disrupts and damages electronics and other infrastructure).

Slideshow: Effects of a Nuclear Explosion

XX34 BADGER atmospheric nuclear test

1. Fireball

When a nuclear bomb explodes, an enormous fireball of hot gases begins to form in less than a millionth of a second. The formation of the fireball—seen as a mushroom cloud—triggers the destructive effects of a nuclear explosion.

Photo: XX-34 BADGER atmospheric nuclear test performed by the United States in April 1953. | Photo Credit: NNSA

2. Blast

The rapidly expanding fireball heats and compresses the surrounding air, causing a blast wave to form and move outward from the fireball. The blast wave generates a strong wind far more powerful than any hurricane, and immense pressure. The blast wave causes many of the most visible effects of a nuclear explosion, destroying or damaging buildings and infrastructure. About 50 percent of the energy released from a nuclear explosion comes from the blast effects.

Photo: U.S. Trinity test fireball at .025 seconds | Photo Credit: CTBTO

Thermal Radiation fireball

3. Thermal Radiation

The fireball emits intense heat and light (known as thermal radiation), accounting for roughly 35 percent of the energy released in a nuclear explosion. Even an observer 50km away from the fireball will suffer permanent retina damage. Temperatures at ground zero briefly exceed 3,000°C, causing massive firestorms (fires so intense they create hurricane-force winds). The fires produce carbon dioxide, which is heavier than air and settles along the ground, asphyxiating all living things in proximity to the fires.

Photo: From 11 kilometers away, U.S. military personnel observe the Dog Test on November 1, 1951 at the Nevada Test Site | Photo Credit: U.S. Government


4. Nuclear Radiation

The explosion generates two types of nuclear radiation, prompt radiation and delayed radiation which together account for about 15 percent of the energy released.

Prompt Radiation

Prompt radiation is released within the first minute of the explosion, and can cause radiation sickness resulting in death, or increase survivors’ risks for cancers later in life. It affects people directly beneath the fireball.

Photo: Radiation | Photo Credit:

HazMat Radiation Sweep

4. Nuclear Radiation Cont.

Delayed Radiation

Delayed radiation results from the radioactive fission fragments and large amounts of pulverized debris, known as fallout, which is drawn up into the mushroom cloud following the explosion. Climbing high into the atmosphere, radioactive fallout eventually drops, and dependent on weather patterns, can spread over very large geographic areas. The fallout damages organisms’ tissues and contaminates food, soil, and water supplies.

Photo: Sweeping for ‘Radiation’ during a HazMat Exercise | Photo Credit: 1st Lt. Michael Thompson, 78th Homeland Response Force, Georgia Army National Guard

Operation Hardtack - Teak shot

5. Electromagnetic Pulse (EMP)

A nuclear weapon will also generate an electromagnetic pulse (EMP) effect lasting only a few seconds after detonation. An EMP is not believed to cause damage to the human body, but can damage or disrupt electronic equipment. For example, studies have suggested that a single nuclear detonation at high-altitude over the United States could seriously affect the country’s communications infrastructure, and potentially shut down the entire power grid. [2]

Photo: Operation Hardtack, Teak shot | Photo Credit: U.S. Government via WikiMedia Commons

What are the different kinds of nuclear weapons?

Slideshow: Nuclear Weapon Types

Diagram of a Gun Type Weapon

1. Gun-Type Nuclear Weapon

Gun-Type Bomb

A gun-type bomb is the simplest of all nuclear weapon designs. The “Little Boy” bomb the U.S. detonated over Hiroshima, Japan on August 9, 1945 was a gun-type bomb.

Conventional explosives are used to shoot one piece of uranium-235 into another to form a critical mass. The impact generates a burst of neutrons, causing an uncontrolled, and explosive, fission chain reaction.

Gun-type bombs can only use highly enriched uranium. Plutonium’s physical properties are such that a gun-type device cannot combine two pieces of plutonium fast enough to form a critical mass.

Photo: Mock-up of the Little Boy bomb dropped on Hiroshima. | Photo Credit: U.S. Government

Little Boy Bomb

1. Gun-Type Nuclear Weapon Cont.

Gun-type bombs require larger amounts of enriched uranium than the more efficient implosion design, making them a relatively heavier and bulkier design. For example, Little Boy contained 64 kg of enriched uranium, and the bomb was over ten feet long and weighed about four tons.

In the 21st century, gun-type designs are unlikely to be attractive to countries that currently possess or are seeking nuclear weapons, since they cannot be placed atop long-range missiles.

Photo: Mock-up of the Little Boy bomb dropped on Hiroshima. | Photo Credit: USG

Atom Bomb Little Boy

1. Gun-Type Nuclear Weapon Cont.

The gun-type bomb is the most likely design to be pursued by non-state actors, who could likely build one if they were able to acquire enough highly enriched uranium. Much of the design information is in the open literature, and the design is so simple that it does not need to be tested prior to use. The United States never tested the Little Boy design before using the bomb during World War II. Non-state actors also might not mind a bulkier design, which could be delivered to a target using a truck or shipping container.

Photo: Atom Bomb, Little Boy | Photo Credit: U.S. Government

Fat Man Bomb

2. Implosion Nuclear Weapon

The first-ever nuclear explosion involved an implosion device, when the United States carried out the Trinity Test in Alamagordo, New Mexico on July 16, 1945. A few weeks later, the United States detonated an implosion bomb, codenamed “Fat Man,” over Nagasaki, Japan on August 9, 1945.

Other nuclear weapon states have reportedly started their programs with implosion rather than gun-type designs, except for South Africa. In the 1980s, the pro-apartheid government in South Africa built six gun-type weapons and had started constructing a seventh before dismantling them in the early 1990s.

Photo: Replica of the original “Fat Man” bomb | Photo Credit: U.S. Government

Implosion Trinity Test

2. Implosion Nuclear Weapon Cont.

Unlike gun-type designs, implosion designs can use plutonium. Implosion designs can also use HEU, and because of their efficiency require much less of it to achieve the same yield as a gun-type weapon. China’s first nuclear test in October 1964 used an HEU-based implosion device.

Photo: Diagram of an Implosion Nuclear Weapon | Photo Credit: U.S. Government

Diagram of an Implosion Nuclear Weapon

2. Implosion Nuclear Weapon Cont.

An implosion bomb detonates conventional explosives around a spherical shell of plutonium or HEU, known as the pit, which compresses the material into a tight sphere.

An implosion bomb can fizzle—or fail to achieve criticality—if the conventional explosives around the plutonium pit do not detonate simultaneously and uniformly. As such, engineering an implosion bomb is much more challenging than constructing a gun-type device. It requires significant expertise, precision manufacturing, and extensive testing.

Photo: Fireball from the 1945 Trinity Test that used an implosion device dubbed “the gadget.” | Photo Credit: U.S. Government

Operation Greenhouse George

3. Boosted-Fission Nuclear Weapon

Early nuclear weapons were inefficient, and would blow up before a substantial portion of their fissile material had fissioned, resulting in smaller yield explosions than would otherwise be possible. To fission more material, weapons designers added a few grams of Deuterium-Tritium gas inside the core of an implosion device. This technique, known as boosting, improved overall efficiency, enabling weapons designers to decrease the size and weight of bombs without decreasing their yields. When fission begins and the temperature increases, nuclei of the Deuterium-Tritium gas fuse (a process known as fusion), flooding the core with a burst of high-energy neutrons that fission the surrounding material more completely.

Photo: Operation Greenhouse, George Event | Photo Credit: U.S. Government via WikiMedia Commons

Greenhouse Dog

3. Boosted-Fission Nuclear Weapon Cont.

Although a small amount of fusion energy is released, a boosted bomb is not considered a fusion weapon since the purpose of the boost gas is to accelerate the fission chain reaction. Very little of the explosive energy produced is derived from fusion.

Photo: Operation Greenhouse, Dog Shot | Photo Credit: U.S. Government via WikiMedia Commons

Teller Ulam Device

4. Thermonuclear Weapons

Thermonuclear weapons, or hydrogen bombs (H-bombs), produce explosions with yields in the megatons as opposed to kilotons.

Most thermonuclear weapons are two-stage devices, containing a distinct fission bomb, known as the primary, which triggers a fusion bomb, known as the secondary.

The United States tested the first thermonuclear bomb in November 1952 on Enewetak. The test, codenamed Ivy Mike, yielded 10.4 megatons.

Photo: Teller Ulam Device | Photo Credit: Fastfission via WikiMedia Commons

Tsar Bomba

4. Thermonuclear Weapons Cont.

The largest thermonuclear explosion, the Soviet “Tsar Bomba” test in 1961, measured 55 megatons, nearly 4000 times more destructive than the Hiroshima bomb.

The development of thermonuclear weapons vastly improved the yield-to-weight ratio of nuclear weapons, making it possible to deploy them on ballistic missiles. Modern thermonuclear weapons weigh less than a couple hundred kilograms but are considerably more destructive than the first nuclear bombs.

Fatman vs Modern Warhead Graphic Source US Government

Photo: Tsar Bomba Mushroom Cloud | Photo Credit: via WikiMedia Commons

How much fissile material is required to build a nuclear weapon?

Fissile Material Amounts Source US Government

  • The amount of fissile material required to build a nuclear weapon, often referred to as a critical mass, varies based on whether plutonium or HEU is used; the quality of the plutonium and/or enrichment level of the HEU; and many other design considerations, such as whether the weapon’s design employs a neutron reflector (which reduces the amount of material needed).
  • The first implosion nuclear weapon, “Fat Man,” used 6 kg of plutonium. If 90% HEU had been used in a similarly primitive design, it would have required about 20 kg of HEU. [3] The United States has declassified the fact that 4 kg of plutonium is sufficient to build a modern nuclear weapon, and some experts suggest a modern HEU-based weapon might contain only 12 kg of HEU. [4]
  • To help guide the development of nuclear safeguards, the International Atomic Energy Agency (IAEA) developed the concept of a significant quantity (SQ), defined as “the approximate amount of nuclear material for which the possibility of manufacturing a nuclear explosive device cannot be excluded.” The current IAEA safeguards standard sets a significant quantity at 25 kg of uranium-235 in HEU and 8 kg of plutonium. These figures are different from critical mass numbers listed in this tutorial, because they are political rather than technical definitions. The significant quantity is a standard agreed upon by the international community as the basis for applying IAEA safeguards, and is widely acknowledged to be higher than the minimum amount of nuclear material required to build a weapon in an efficient design.

How difficult is it for a country to develop nuclear weapons?

Developing nuclear weapons is a complex endeavor, requiring substantial financial, technical, and human resources; however, these barriers are not insurmountable, and many countries could develop nuclear weapons if they chose to do so.

Step 1: Acquiring Fissile Material

Acquiring fissile material is the most significant hurdle to a nuclear weapons capability, requiring a country to enrich uranium, produce plutonium, or illicitly procure such materials through theft or purchase.


Step 2: Weapons Fabrication

  • Individuals with expertise in chemistry, physics, metallurgy, electronics, and explosives would be required to design and fabricate a nuclear weapon. Many non-nuclear components and high-end manufacturing techniques are also required.
  • However, the basic design concepts are in the public domain, and many of the components and manufacturing techniques could be procured legitimately because of their dual-use nature. Modern proliferators would benefit from advances in high-performance computing (supercomputers), which were unavailable to the first nuclear weapons programs.

Step 3: Testing

  • Countries use nuclear testing to validate whether a particular nuclear weapon design works. For an unsophisticated design, such as a gun-type weapon, nuclear testing is not necessary to provide high confidence a bomb will detonate.
  • Weapons designers would need to test the non-nuclear components in an implosion-based weapon, but might not need to conduct a full-scale nuclear text. Testing at full nuclear yield would be required for a country seeking very low-weight weapons, or thermonuclear weapons.
  • All five nuclear weapon states recognized by the NPT and at least three of the four additional nuclear weapons possessing states have conducted nuclear tests. All but one of these countries (North Korea) currently observe testing moratoria. Because of the Comprehensive Nuclear Test Ban Treaty Organization’s (CTBTO) International Monitoring System, conducting clandestine nuclear tests is extremely difficult.

Step 4: Delivery Systems

  • In addition to the nuclear weapon’s design, the sophistication of a country’s delivery systems—such as missiles, combat aircraft and drones—determine how, when and against whom a country can use nuclear weapons. These systems also enable states to deploy their nuclear forces for deterrence and signaling purposes.
  • States prefer aerial methods of delivering nuclear weapons (and especially ballistic missiles) because they are fast, cover large distances, carry large payloads and can penetrate an adversary’s defenses.
  • Long-range systems place tight constraints on the size and weight of nuclear weapons. Miniaturization to fit a nuclear weapon atop a missile, for example, can pose significant challenges to a proliferating country, and would require nuclear testing.
  • Non-state actors such as terrorists may resort to crude delivery methods to carry out attacks. Were such groups to build a crude nuclear device, a human carrier, truck or possibly a civilian aircraft might be sufficient to deliver it to a target.

How many countries have developed nuclear weapons?

Rotate your device horizontally to improve chart display.

Nuclear Stockpiles

nuke holdings per country

Nuclear Holdings

  • Nine countries possess nuclear weapons: China, France, India, Israel, North Korea, Pakistan, Russia, the United Kingdom, and the United States. South Africa also developed nuclear weapons, but voluntarily gave them up in the early 1990s.
  • Many other countries are believed to have the technological capacity to develop nuclear weapons. To learn more about why the vast majority of countries have not acquired nuclear weapons, see the Nonproliferation Regime Tutorial.
[1] “Status of World Nuclear Forces,” Federation of American Scientists, 2015,
[2] “Nuclear Detonation: General Information,” U.S. Department of Health and Human Services,; “Electromagnetic pulse,” Glossary of the Comprehensive Test Ban Treaty Organization,
[3] Global Fissile Material Report 2013: Increasing Transparency of Nuclear Warhead and Fissile Material Stocks as Steps toward Disarmament, International Panel of Fissile Materials, October 2013, p. 92.
[4] Global Fissile Material Report 2013: Increasing Transparency of Nuclear Warhead and Fissile Material Stocks as Steps toward Disarmament, International Panel of Fissile Materials, October 2013, p. 92.