When people think of nuclear weapons they tend to think of bombs and missiles, but there have been (and possibly still are) some more unusual nuclear weapons.

Conventional Nuclear Weapons

Most nuclear weapons states (NWSs) use ballistic missiles as the primary tool in their nuclear arsenals. These can be battlefield (with a range less than 100 km), tactical (range up to 300 km), theatre (up to 3500 km), intermediate (up to 5500 km) or intercontinental (beyond 5500 km). These ballistic missiles can be land-based, either in underground silos or aboard mobile launchers, or submarine-launched; NWSs prefer SLBMs because they give a secure second strike capability (the UK’s only nuclear weapons are SLBMs).

A Trident II SLBM (as used by the USA and the UK) ignites its first stage rocket motor.

A Russian SS-25 Sickle ICBM aboard a transporter erector launcher vehicle.

Ballistic missile forces are frequently supplemented by cruise missiles, usually air-launched by fighter or bomber aircraft. Cruise missiles differ from ballistic missiles in that they do not follow ballistic paths but rather fly point-to-point, close to the ground (in a process known as terrain hugging). Ballistic missiles use rocket engines and can operate outside of Earth’s atmosphere, whereas cruise missiles are powered by air-breathing fanjet or ramjet engines (and are therefore unable to operate outside of the atmosphere), and generate lift and steer just as aircraft do.

A French Mirage 2000N fighter carrying an ASMP-A nuclear-capable cruise missile on its centre hardpoint.

Only the United States and China still use air-dropped nuclear bombs as part of their nuclear deterrent. The nuclear weapons programmes of Israel, India, Pakistan and North Korea are shrouded in secrecy, so it is possible – but unlikely – that they possess bombs as part of their arsenals.

A B83 bomb, the most powerful weapon in the US arsenal at 1.2 megatonnes TNT equivalent.

Unconventional Nuclear Weapons

Nuclear Land Mines (AKA Atomic Demolition Munitions)

The conventional nuclear weapons listed above are all designed to be delivered to their target remotely, whereas atomic demolition munitions (ADMs) are designed to be transported and emplaced by soldiers in the field. The US and the USSR/Russia developed a number of ADMs but no current weapons are known, though it is alleged that Israel have placed ADMs in the Golan Heights to secure the area.

Source: flickr/rocbolt

The most recent ADM, the US’s Special Atomic Demolition Munition (shown above) was decommissioned in 1989. It used the tiny W54 warhead, with a yield of 0.01-10 kilotons TNT equivalent, and had a mass of around seventy kilograms. It was designed to be carried in a backpack and emplaced by a team of paratrooper Special Forces.

Perhaps the strangest nuclear weapon of all falls into the ADM category. Blue Peacock was a 1950s British project to place ten kiloton nuclear landmines on the North German Plain to guard against a Soviet invasion from the east. The bomb’s designers were concerned that the cold weather would prevent the bomb’s electronics from operating correctly, and one suggestion was that live chickens be placed inside the bomb with a source of food and water and that the heat from the chicken’s bodies would be sufficient to keep the bomb operating correctly. The project was cancelled in 1958 before any bombs were placed.

So-called “suitcase nukes” also fall into the ADM category. A number of people have alleged that the US, USSR and Israel have produced suitcase nukes, but most nuclear scientists and engineers do not believe it to be possible to shrink a warhead – both in terms of size and mass – small enough to fit in a suitcase and be easily man-portable.

Nuclear Artillery

The W54 warhead used in the SADM was originally developed for the Davy Crockett recoilless rifle system. The Davy Crockett was fielded by US units between 1961-1971 and was designed to fire an M388 nuclear projectile containing a 0.01-0.02 kiloton W54 warhead up to four kilometres, and was envisaged as primarily an anti-personnel and area denial weapon due to the fallout it would produce.

An M388 nuclear projectile attached to the Davy Crockett system.

The US developed a number of other shell-firing nuclear artillery pieces: the M65 atomic cannon firing fifteen kiloton W9 and twenty kiloton W19 warheads; the M110 and M115 howitzers firing W33 warheads with a selectable yield up to forty kilotons; and the M109, M114 and M198 howitzers firing 0.072 kiloton W48 warheads. The USSR also produced a number of shell-firing nuclear artillery pieces, and the US, USSR, France and others operated nuclear rocket artillery pieces.

Nuclear Depth Charges

The UK, USA and USSR have all at one time fielded nuclear depth charges for use in anti-submarine warfare. The Mark 101 Lulu was an eleven kiloton nuclear depth charge fielded from 1958-1971, and the twenty kiloton B57 nuclear bomb (1968-1993) could also be used in this role.

A Mark 101 Lulu nuclear depth charge.

Air-to-Air and Surface-to-Air Rockets

Most nuclear rockets and missiles are air-to-surface or surface-to-surface, but some air-to-air and surface-to-air nuclear missiles have been created. Air-to-air nuclear missiles, such as the one-and-a-half kiloton AIR-2 Genie (1957-1985) and the 0.25 kiloton AIM-26 Falcon (1961-1972) were designed to guarantee a hit against incoming aircraft, and to destroy multiple aircraft with one device.

An F-106 Delta Dart fires an AIR-2 missile.

Surface-to-air nuclear missiles have also been used in anti-aircraft roles, but were, and are more commonly used in an anti-ballistic missile (ABM) role. The current Russian ABM system (A-135) uses ABM-3 Gazelle missiles with 10 kiloton nuclear warheads, and previously used much more powerful ABM-1 Galosh missiles with 2-3 megaton warheads. These ABM nuclear missiles are designed to destroy incoming ballistic missiles by damaging their electronic components via intense X-ray bombardment and neutron flux.

Source: wikimapia/ogima

The image above shows the A-135 ABM complex at Vnukovo (click to enlarge). Blue control buildings are on the right, and the sliding silo covers are visible to the left. There are a total of twelve silos at this site, with a further fifty-six silos spread between four other sites surrounding Moscow.

“The material is classified. Its composition is classified. Its use in the weapon is classified, and the process itself is classified.”

FOGBANK is the codename of a material used in modern nuclear warheads such as the W76 used in Trident submarine-launched ballistic missiles, the W78 used in silo-launched Minuteman III intercontinental ballistic missiles and the W80 used in air-launched cruise missiles and the Tomahawk cruise missile.

Exactly what FOGBANK is, and what it does, is unknown. It has been suggested that it is an aerogel-like substance that transfers energy from the fission stage of a thermonuclear (fission-fusion) device to the fusion stage; preventing the fission stage from destroying the fusion stage before it has time to react.

In 1996, during the refurbishment process for the aforementioned warheads, it was discovered that detailed records of the manufacturing process for FOGBANK did not exist, and the facility used to manufacture it had been mothballed.  Uncertain whether alternate materials would suffice, the National Nuclear Security Administration spent twenty-three million dollars on research and new facilities to recreate FOGBANK.

Unfortunately, the Mark II FOGBANK did not work correctly. Eventually it was discovered that this was due to the presence of an impurity, accidentally incorporated into original batches of FOGBANK, that was not included in the second manufacturing process. This impurity was included in the new formula as an additive and the refurbishment process was successful.

In nuclear weapon design a critical mass is the minimum amount of nuclear material required to sustain the chain reaction necessary for a nuclear explosion.

There are a number of factors that affect the critical mass.

• Material, enrichment and purity: Different nuclear materials have different properties (most importantly fission cross-section). To make a nuclear weapon far less plutonium-239 is required than uranium-235, because Pu-239 has a much larger fission cross-section.
• Shape: As neutrons escape through the mass’s surface, and collide throughout the mass’s volume, it is desirable to keep the surface area to volume ratio as low as possible: a sphere is therefore the appropriate choice.
• Density: If the density of a mass is increased then the nuclei are forced closer together and the critical mass required decreases. Modern nuclear weapons use explosive lenses to compress a sub-critical mass of plutonium into a super-critical state.
• Temperature: The likelihood of fission depends on the relative velocity of the fission neutrons and the nuclei of the nuclear fuel. If the temperature of the mass is decreased, fission becomes more likely.
• Reflectors and tampers: A neutron reflector placed around the critical mass prevents neutrons from escaping and reflects them back into the mass, decreasing the critical mass required. A tamper prevents the critical mass from expanding and keeps it concentrated in a small volume, also decreasing the critical mass. In many bomb designs the reflector and tamper are the same structure.

Changing any one of these factors could allow a material to go from sub-critical to critical. For example, a rod-shaped mass suddenly compressed into a sphere could become critical, as could a warm mass that is suddenly cooled.

The graph below shows the importance of choosing a nuclear material. Many isotopes require a smaller critical mass than uranium-235 or plutonium-239 but these are unsuitable for various reasons connected to the factors listed above. For example, isotopes of californium are too radioactive, neptunium-236 is too difficult to separate from its parent isotopes and plutonium-238 releases too much heat due to alpha-decay.

A fizzle occurs when a bomb creates a critical mass too slowly and nuclear predetonation occurs, destroying the bomb before the fission chain reaction can propagate throughout it. The first North Korean nuclear test in 2006 was a fizzle, with a yield of less than one kiloton, twenty times smaller than any other country’s initial test.

In the diagram above two sub-critical masses are brought together too slowly, causing a predetonation that throws the two masses away from each other, causing a fizzle and preventing a nuclear explosion.