Nuclear Weapons and Radiation - Misc Facts

The US produced, and for many years deployed "Atomic Demolition Munitions." The Medium Atomic Demolition Munition (MADM) produced 1-15 kilotons of yield, and weighed 400 pounds. The Special Atomic Demolition Munition (SADM)  yielded .01-1 kilotons and weighed only 163 pounds.  

The smallest nuclear weapon the US produced was the "Davy Crockett"  - a recoilless rifle round. It weighed about 51 pounds, was 16 inches long and 11 inches in diameter. It produced a variable yield of up to 1 kiloton.

An excellent discussion of this issue is here.

The Soviets supposedly produced "suitcase nukes" and there is no reason to doubt this assertion. Former Soviet General Ledbed has asserted that a number of these are not accounted for. There are reasons, however, to doubt his assertions given his political position. Interestingly, Ledbed was recently killed in a helicopter crash.

The Soviets supposedly produced "suitcase nukes" and there is a US DOE estimate that only 4kg of Plutonium is necessary to make a fission weapon. Some believe that only 1kg is needed.

All known nuclear weapons require the fission of Uranium (235 or 233) or Plutonium 239. An "atomic bomb" (fission weapon) uses the fission energy directly, while a "hydrogen bomb" (thermonuclear weapon) uses fission to ignite a fusion reaction, achieving much higher energy release. In theory, there is no limit to the power of a fusion bomb. There has been speculation that it is possible to create useful fusion weapons without a uranium trigger, but no reliable unclassified information indicates that this is true, and there is a difficult physical principle to overcome.

Uranium consisting of unnaturally high amounts of isotopes 233 and 235 is called enriched uranium. Uranium is a common element on earth, but U-233 and U-235 constitute small percentages of it (<1%) and are always found mixed with the less useful U-238.

Fission is the process whereby an atom's nucleus splits, releasing a large amount of energy. In a simple fission weapon, fission occurs when a neutron is absorbed by a nucleus, causing it to be highly unstable, at which point the nucleus splits, releasing a large amount of energy and more neutrons. This only occurs easily in a few  isotopes (in Uranium, 235 or 233).

A chain reaction occurs because the fission of a nucleus releases additional neutrons, which can then cause fission in more nuclei.

Critical mass is the amount of fissile material needed for a chain reaction to become self-sustaining. This means that for each neutron released in the material, on average one more neutron will be released as a result. The critical mass is a factor not just of the type and amount of material, but also its instantaneous density and geometry. In other words, a mass of plutonium might not be critical until it is rapidly and highly compressed by high velocity  "implosion."

Fission weapons explode when the fissile material is suddenly placed in a configuration significantly greater than the critical mass - a state of supercriticality. If critical mass is reached too slowly, the weapon will explode with greatly reduced energy, possibly simply melting.

When a weapon has reached supercriticality, it still may not explode unless a neutron passes into the core. Since a high explosive implosion maintains criticality for only a few microseconds, a neutron flux generator may be required to guarantee this.

The easiest weapon to build uses a large amount (tens of kilograms) of enriched uranium. Because uranium releases neutrons at a very low rate, the weapon can use a relatively long "assembly time" to reach supercriticality. One design uses a sphere with a cylindrical hole in it, and a "gun" to fire a cylinder of uranium into that hole. Until the cylinder is inserted, both assemblies are well below critical mass, but when the cylinder is inserted, the mass rapidly rises to supercriticality. A neutron randomly released by the material during this process triggers the chain reaction. This weapon is so simple that the US used one against Nagasaki without ever testing the design. These weapons tend to be fairly large and inefficient, although the design was used in a US nuclear artillery shell.

A plutonium based weapon cannot use the "gun" approach, because plutonium releases too many neutrons, which would cause the chain reaction to start long before the mass was supercritical enough to cause a large explosion. Hence plutonium weapons require assembly by compressing a sphere or shell of plutonium very rapidly, using high velocity explosives. This neccessitates very high quality explosives, a very precise machining of all parts, and an electrical detonating system which can deliver very high energy pulses to a number of detonators with great timing precision. Hence plutonium based weapons are significantly harder to build. The US tested one at Trinity Site before deploying another against Nagasaki.

A uranium weapon can also use the implosion approach, to achieve greater efficiency. However, in this case it requires a neutron flux generator to assure that enough neutrons flood the core during the maximal period of compression that the chain reaction will start and the weapon will be efficient.

Uranium weapons require the production of enriched uranium (although it does not have to be as highly enriched as is used in some power reactors). This is a complex process because it requires the separation of isotopes of the same element (which means they have the same chemical behavior) and the isotopes have almost the same weight. Thus multiple stages of gas diffusion, centrifuges or electromagnetic (calutron) separation are required. This is inevitably a major industrial project, and is likely to be detectable by intelligence agencies. However, the centrifuge method can be  distributed, making it hard to spot. It is believed that Saddam Hussein intended to use this approach once he was rid of UN inspectors. Laser separation has also been used. In addition, there may be new technologies that make enrichment much easier to do or at least easier to conceal. Only a ton or so of natural uranium is required to produce enough entriched uranium for a weapon.

Plutonium weapons require the production of highly pure plutonium. Because plutonium is a not found in nature, it must be made in a nuclear reactor. Once made, however, it is relatively easy to extract because it is chemically different from the other elements in the mix, although the extraction process must take place in an environment made extremely radioactive by other elements mixed with the plutonium.

Fusion weapons can be  much more powerful than fission weapons, but require a subtle and difficult design. However, at least two different teams (Teller-Ulam and Sakarov) independently discovered the same approach. Since that time knowledge of fusion designs has spread through espionage and possibly technology trades. The details of fusion weapons will not be discussed here - see the Nuclear Weapons FAQ for vastly more information.