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Nuclear Corner

Nuclear Corner

Providing resources to assist arms control treaty implementers with keeping up-to-date on nuclear treaty negotiations and the status of compliance verification activities, weapon system reductions, and securing nuclear materials.

Weapons & Warheads

Quick reference information on different types of nuclear weapons, their delivery systems, and warheads.

Strategic Offensive Arms under New START
Nuclear Warheads
Fission Weapons
Fusion Weapons
Significant Nuclear Explosions

Strategic Offensive Arms under New START

U.S. Department of State; Under Secretary for Arms Control and International Security; Bureau of Arms Control, Verification and Compliance (AVC); New START Treaty.

Nuclear Warheads

¹Nuclear Nonproliferation Treaty (NPT)

Arms Control Association’s sources included the Carnegie Endowment for International Peace, Central Intelligence Agency, Congressional Research Service, U.S. Department of Defense, Institute for Science and International Security, International Atomic Energy Agency, and Natural Resources Defense Council.

Fission Weapons

These are weapons that only use fission reactions as a source of energy. Nuclear fission is a process in which a neutron collides with an atom’s nucleus, splitting the atom into two roughly equal-mass fragments and releasing a significant amount of energy. Every collision also releases more neutrons, which in a critical mass of fissile material will sustain a chain reaction of fission. By manipulating the size and speed of the chain reaction, nuclear fission can be exploited for power generation or alternatively, for weapons of mass destruction.

Fission Weapons, also called atomic bombs or fission bombs, operate by rapidly assembling a subcritical configuration of fissile material (plutonium or enriched uranium) into one that is highly supercritical.

These are the easiest nuclear weapons to design and manufacture, and the capability to do so is a prerequisite for developing any of the other weapon types.

There are practical limits to the size of pure fission bombs. Larger bombs require more fissionable material, which:

  • becomes increasingly difficult to maintain as a subcritical mass before detonation and
  • makes it harder to assemble into a high efficiency supercritical mass before stray neutrons cause predetonation.

The principal materials used for fission weapons are U-235 and Pu-239.

Uranium Gun-Type Device (HEU)

This is the “simplest” type of nuclear explosive because it does not require sophisticated explosive or electronic components. The design uses highly enriched uranium (HEU) as fissile material, which is obtained by concentrating atoms of the rare U-235 isotope. When uranium is extracted from the ground, less than 1 percent of the ore is U-235. Ninety-nine point three percent is the heavier U-238 isotope, which cannot sustain a chain reaction of nuclear fission.

A gun-type weapon uses chemical explosives to "shoot" one subcritical mass of HEU into another at high speed, much as a bullet is shot from a gun. The impact generates more neutrons, ensuring a fission chain reaction. The gun-type nuclear explosion is the most inefficient in terms of burning up the fissile material; only about 1.4 percent of the HEU in the Hiroshima bomb actually fissioned. The exact amount of HEU depends on the level of enrichment of the uranium used in the weapon, the explosive yield desired, and the technical sophistication of the bomb design.

Yet a large amount of fissile material is required to ensure that a nuclear chain reaction will take place. Therefore, gun-type weapons will necessarily be heavier and bulkier than other types of nuclear weapons. While this suggests that States seeking strategic nuclear weapons would look to more advanced designs, the simplicity of a gun-type device may be attractive to terrorists. A weapon of this type is too large to be mounted on a long-range missile, but it could be dropped from a plane or delivered in a truck or a shipping container.

It is impossible to achieve a large nuclear explosion by using plutonium in a gun-type device. Nonetheless, a plutonium gun-type bomb could release as much energy as a few tons of TNT, which could conceivably cause many casualties. Moreover, this kind of bomb would release large amounts of plutonium and other radioactive materials, thereby making it a potent radiation dispersal device, or "dirty bomb."

In terms of testing, no nuclear components need to be verified with the gun-type design; only the conventional components must be tested.

Implosion Design (Plutonium or HEU)

An implosion-type fission weapon is more sophisticated than a gun-type design. An implosion weapon uses a complex arrangement of explosives to rapidly compress one or more spheres of fissile material into a critical mass. They are more difficult to design and build than gun-type weapons, because they often require advanced explosive components and sophisticated fusing systems.

Implosion weapons can use either plutonium or HEU to create nuclear explosions with yields in excess of 10 kt. Also, they typically require much less fissile material than gun-type weapons, because they use the fissile material available in the core more efficiently. According to the International Atomic Energy Agency (IAEA), with 25 kg of HEU or 8 kg of plutonium, "the possibility of manufacturing a nuclear explosive device cannot be excluded," but sophisticated, advanced designs could require less fissile material. Some sources estimate lower thresholds of 12-15 kg of HEU and 4 kg of plutonium.

A smaller amount of plutonium than HEU is required to achieve a self-sustaining chain reaction of nuclear fission, however, plutonium’s physical properties are such that a gun-type device cannot combine two separate masses fast enough to achieve this critical mass. A nuclear explosion using plutonium actually begins as an implosion that relies on a sophisticated arrangement of high explosive lenses that must fire inwards simultaneously from all directions towards a plutonium pit.

For an instant, the plutonium is compressed to a high density, making the mass critical. However, simply compressing the plutonium to critical mass does not ensure that a nuclear chain reaction will begin. For this to happen, high energy neutrons need to be available at the moment of compression. Since relying solely on natural decay of plutonium is too risky, the certainty provided by a neutron initiator is needed.

The implosion design can also be used with HEU, allowing a smaller device to achieve the same yield as a gun-type device.

In order to test an implosion device, the compression of the pit must be uniform and needs to be tested quickly enough to avoid a premature nuclear explosion, a so-called fizzle. Before or after this particular moment, the conditions are not right for sustaining the chain reaction until most of the fissile material has been consumed. This is a far more challenging engineering problem than building a gun-type device, but experts at the Nuclear Control Institute warn that it could still be accomplished by a small group of people with the right training and experience if they have access to plutonium.

Fusion Weapons

Fusion reactions power the sun. The fusion of deuterium and tritium, both heavy isotopes of hydrogen, releases energy as well as a neutron with seven times more energy than a fission neutron. Fusion’s energy output per kilogram of source material is much higher than that of fission. Fusion can be used inside a fission explosion to improve the efficiency of the weapon (boosting), or a large amount of fusion fuel can be triggered separately (thermonuclear weapon). The fusion of deuterium and tritium is initiated by the extremely high temperatures and radiation that result from fission.

(Fusion) Boosted Fission Weapons

When fusion takes place in a fission weapon, the high-energy neutrons are more likely to collide with fissile atoms. Those higher-energy collisions release even more neutrons than simple fission, speeding up the chain reaction. This enables more material to fission before the device blows itself apart.

Boosted weapons are typically implosion devices with deuterium and tritium gas introduced into the hollow pit in the centre of the fissile pit. As fission begins, the high temperature causes fusion, and the high-energy neutrons released by fusion accelerate the fission chain reaction. Boosting has led to a hundred-fold increase in the efficiency of fission weapons since 1945, and it plays a role in nearly every nuclear weapon deployed today.

Thermonuclear Weapons (Hydrogen Bombs)

Thermonuclear bombs yield explosions in the megaton range, orders of magnitude more powerful than fission weapons alone.

The destructive energy of a thermonuclear weapon is the result of three separate but nearly simultaneous explosions. The first is the detonation of chemical explosives that surrounds a sphere (or “pit”) of plutonium metal. The force from this blast is directed inward, compressing the pit and bringing its atoms closer together. Neutrons (atomic particles with no electric charge) that have been introduced into this dense core collide with the plutonium nuclei, sometimes causing them to split, or fission. Together, this chemical and fission explosion is known as the nuclear “primary.”

The fission primary triggers a powerful fusion secondary. The energy released from the primary explosion compress and ignite the secondary device, more commonly known as a hydrogen (or H-) bomb. This term derives from the process by which fusion combines two hydrogen atoms to form helium. The massive release of high-energy neutrons from the fusion reactions in the secondary causes even the U-238 tamper (not fissile) to fission, allowing for massive explosive yields.

The main threat posed by thermonuclear weapons is their ability to pack huge amounts of explosive power into small, light-weight packages that can be delivered by missiles.

Mastering thermonuclear weapons technology makes it possible for countries to miniaturize their weapons, allowing for flexibility in delivery and yield. U.S. warheads such as the B83 can be adjusted on the battlefield for either a low sub-kiloton yield (primary only) or higher yields up to 1.2 megatons. Many of the missile-delivered warheads currently deployed by the nuclear weapon States are light-weight thermonuclear weapons that fall in the 100-300 kiloton range.

Significant Nuclear Explosions

For a listing of significant nuclear explosions compiled by the Comprehensive Nuclear Test-Ban Treaty Organization, please see Significant Nuclear Explosions.

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