Tag Archives: nuclear


“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.

The Trestle

The Trestle (or more formally the Air Force Weapons Lab Transmission-Line Aircraft Simulator) is a unique structure built by the US government in the Albuquerque desert and which was used to test aircraft’s resilience against the electromagnetic pulses created by nuclear weapons.


The Trestle is three hundred metres long and nearly two hundred metres tall and made entirely from wood and glue. The presence of any metal would distort readings from EMP testing and therefore The Trestle does not even use metal nails or braces. It was built from more than fifteen thousand cubic metres of Douglas Fir and Southern Yellow Pine and was strong enough to support the weight of a fully loaded two hundred tonne B-52 Stratofortress strategic bomber.



The Trestle was equipped with a two hundred gigawatt, ten megavolt Marx generator and was used to test bomber, fighter and transport aircraft and even long-range missiles. The Trestle programme was shut down in 1991 when computer simulations became good enough to simulate the effects of EMPs and the dried-out, creosote-soaked wood now poses a serious fire hazard.

Efforts are being made to have the Trestle site declared a National Historic Landmark, but these efforts are being hampered by the fact that The Trestle is located on Kirtland Air Force Base. Kirtland houses a number of Top Secret units such as the US Air Force Nuclear Weapons Centre, the 498th Nuclear Systems Wing and the Air Force Research Laboratory and therefore access to the site is highly restricted.

The composition of nuclear electromagnetic pulses

When a nuclear weapon is detonated at high altitude the effects are very different to those created by a low-altitude detonation. Aside from creating a much larger, much faster-expanding fireball, the nuclear detonation also creates an electromagnetic pulse (EMP) that can damage electromagnetic equipment on the ground.

A nuclear EMP differs from other EMPs such as those generated by lightning strikes or by conventional EMP weapons (such as flux compression generators) in that it is much more powerful* and is composed of three different pulses called E1, E2 and E3.


The E1 pulse is the most destructive, occurring far too quickly for protective equipment to activate. It is the component that destroys computers and communications lines by causing the insulating components of these devices to become conducting and allowing electrical current to flow between regions that are not supposed to be connected, effectively short-circuiting all circuits simultaneously.

The E1 pulse is created when the intense gamma radiation from the nuclear detonation ionises atoms in the upper atmosphere (Compton scattering); releasing electrons which travel downward at relativistic speeds, about 95% of the speed of light. Any charged particle in a magnetic field will experience a force (the motor effect) and the Earth’s magnetic field causes the electrons liberated by the gamma radiation to follow a spiral path around the magnetic field lines. As the electron oscillates back and forth it creates an electromagnetic field and as there are about 1025 electrons doing this simultaneously, this creates a very powerful (about 50000 volts per metre, 6.6 megawatts per square metre) but very short-lived electromagnetic pulse. The E1 pulse typically reaches its peak value in about five billionths of a second (five nanoseconds) and ends after about one millionth of a second (one microsecond) as the scattered electrons are stopped by collisions with air molecules.

The shape of the region affected by the E1 pulse depends on latitude, due to the changing orientation of the Earth’s magnetic field. Away from the equator the region is U-shaped, and towards the equator it is more symmetrical.


The E2 component is caused, like E1, by Compton scattering when scattered gamma rays, gamma rays produced by interaction of fission neutrons with atoms in the air and gamma rays produced by radioactive decay of fission fragments ionise air particles.

The E2 component lasts from about one microsecond after detonation to one second after detonation and is very similar to the pulses created by lightning strikes. The E2 component is easy to protect against using conventional lightning protection equipment, but this protective equipment is likely to have been damaged by the E1 component and will therefore not function correctly, allowing the E2 component to cause further widespread damage.


The E3 component is very different to the E1 and E2 components. It lasts tens to hundreds of seconds and is caused when the Earth’s magnetic field “snaps” back into place after being pushed out of the way by the ionised plasma created by the expanding nuclear fireball. This induces electrical currents in conductors on the ground such as pipelines, power lines and transformers.

The E3 component is similar to the pulse created by a geomagnetic storm, when a severe (X-class) solar flare pushes on the Earth’s magnetosphere and is sometimes referred to as “Solar EMP”. In 1989 a solar EMP-type event caused the collapse of the Hydro-Québec power grid when a huge coronal mass ejection followed a X15-class solar flare.

* During a Soviet nuclear EMP test called K-3 the E3 component of the pulse fused the entire length of a 570-kilometre overhead telephone line (with currents reaching up to 3400 amperes) and the E1 component caused all of the attached overvoltage protectors to fire. The test also started a fire that caused the Karaganda power station to burn down and penetrated nearly a metre into the Earth to burn out 1000 km of buried power cables.

Critical mass and fizzles

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.

Nuclear mines in the Bay of Naples

During the Cold War the USSR considered the Bay of Naples, off the south-west coast of Italy, to be an important strategic location. They considered it so important that K-8, a November-class submarine, was tasked with mining the area with nuclear torpedo mines.

The Bay of Naples


Project 627A November-class submarine

A torpedo mine is laid anchored to the sea floor and waits patiently to be activated. Once activated, the mine uses passive sonar to listen for an approaching submarine. Once an enemy submarine is detected and within range, the mine’s torpedo is automatically launched. It then tracks and destroys its target as a normal torpedo would.

K-8, the submarine said to have carried out the operation on 10th January 1970, originally carried a complement of twenty-four mines. Three months later, during a large-scale naval exercise two fires occurred simultaneously in two different compartments aboard K-8, causing all hands to abandon the boat. As it was being towed for repairs through the Bay of Biscay it sank in rough seas – allegedly with only four mines still aboard – killing fifty-two members of the crew.

It is alleged that the nuclear mines are still in place, but after more than forty years they are very unlikely to still be functioning. Still, they may pose a serious contamination risk as they rust and degrade.

Disclaimer: It’s worth noting that this information comes from Mario Scaramella, who has been the subject of some controversy. The IAEA lists the event as “not confirmed” in a report entitled Inventory of Accidents and Losses at Sea Involving Radioactive Material.