Yes, foam! That’s not some acronym with lots of fancy science words either. Your normal Household Polystyrene Foam is one of the key components in a thermonuclear warhead. I’m sure that you’re just as surprised as I was to discover this fact, so let me fill you in on the details.

When a Thermonuclear (fusion) bomb detonates, the first thing to happen is a small size fission bomb is triggered to provide the initial energy for the reaction. The energy from this fission bomb wants to expand away from the bomb. This is usually exactly what you would want from a bomb, however in the case of a fusion bomb the process is a little more complicated. All the energy needs to be retained inside the bomb for as long as possible. If you’ve ever done research into the topic of Nuclear fusion as a power source, you’ll be well aware of the claim that it is an extremely safe method of power production. The moment the reaction isn’t contained, the energy dissipates and quickly stops. This same fact is true for fusion bombs. The initial energy from the fission explosion needs to be held inside the bomb for as long as possible so that Nuclear fusion can take place.

Now it’s time for the foam!

The energy created by this initial explosion not only needs to be contained within the bomb itself but also transferred to the secondary stage of the weapon where the fusion will take place. The X-Rays produced by the fission stage super heat the Polystyrene Foam, turning it into hot plasma. This plasma then compresses the casing of the second stage, while also heating it to extremely high temperatures and bombarding it with neutrons. This last part is a bit of a problem however. The second stage contains two main parts. The fuel for the fusion, which comes in the form of lithium deuteride and then a ‘Spark-plug’ which will kick-start the fusion at the appropriate time. The spark plug consists of Fissile material (U-235 or PU-239) and if too many of these bombarding neutrons reach it then a fission reaction could occur too soon and no fusion would take place. Which is why the outer shell of the second stage has ‘Uranium Tamper’. This is Urainium-238, which absorbs the stray neutrons and prevents a premature reaction.

At this point in the detonation the fusion fuel is very hot and under extreme pressure. The spark plug has been compressed too from a long cylinder into a smaller size where it is dense enough to become a critical mass and react. This energy breaks the lithium fuel into tritium and deuterium (isotopes of hydrogen) which will fuse and release massive amounts of energy. In a turn of events the final part of this explosion, the source of half the energy released in a Thermonuclear blast, is the very same thing that was preventing an explosion earlier in the process. With all this energy from the fusion taking place, the uranium tamper itself starts to fission!

The uranium tamper, while providing most of the energy created in the blast, is also responsible for the vast majority of the radioactive fallout that occurs from this otherwise clean explosion (as clean as a nuclear bomb can be of course). So to reduce the fallout the tamper can be reduced or even replaced with beryllium shielding  but at the cost of blast yield.

FOGBANK

Everything I’ve laid out before you thus far is correct… in theory. The problem is that there are very few people who are aware of what goes into a nuclear weapon and the US government along with other nuclear states will do anything in their power to keep this a secret. Most of the nuclear weapons held by the US today were created in the 80s with some dating back further, which means that they are nowhere close to being declassified. This secrecy even poses a problem for the US itself.

There is a substance known as Fogbank that the US created to put in a nuclear weapon. If that sentence sounds vague, then that’s because it is all the information the general population knows about Fogbank. It is such a big secret that when a project was begun by the US to refurbish their arsenal in 2000, no one knew how to make it. Everyone who worked on Fogbank had moved on or forgotten the production process completely, with no documents recording how to make it. This led to a decade long, hundred million dollar initiative to reverse engineer Fogbank.

The general consensus is that the inter stage medium used to transfer energy from the primary to the secondary stage is definitely a foam of some description. Many sources will say polystyrene foam is used, and others will say Fogbank.

So that’s it! That is how foam is a vital component in thermonuclear warheads. I recommend reading up on the disaster that is Fogbank and its re-creation as it is a long and disaster filled story that I don’t have time for in this post. Thank you for Reading!