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“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.

A bhang­meter is a radiometer, a device that meas­ures the power of elec­tro­mag­netic radi­ation. Most people are already familiar with one type of radiometer, the Crooke’s radiometer, which detects infrared radiation.

A Crooke’s radiometer. The higher the flux of infrared radi­ation the faster it spins.

Bhangmeters are placed on recon­nais­sance satel­lites* in order to detect nuclear weapon det­on­a­tions and to measure their yield. Bhangmeters are designed to look for the char­ac­ter­istic “double flash” created when nuclear weapons det­onate: the first initial bright flash being caused by the actual det­on­a­tion of the weapon and the second being caused when the ionised gas shock wave cools enough to allow light from the fire­ball to escape.

The name “bhang­meter” was created by Frederick Reines (who later won the Nobel Prize for Physics for his work on detecting neut­rinos), who sug­gested that one would have to be on bhang (an Indian drink made from marijuana) to believe that the detector would work.

* The US Department of Defense’s GPS satel­lites also carry bhangmeters.