Last week’s post detailed the basic workings of single stage nuclear fission warheads. This type of warhead, however, is fairly uncommon to see in nuclear states’ arsenals today because it was largely supplanted by the hydrogen bomb in the 1950s and 1960s. Unlike first generation nuclear weapons, hydrogen bombs (thermonuclear warheads) rely at least in part on nuclear fusion to generate their explosive yield. But, as anyone who follows developments in nuclear energy knows, fusion is a far trickier process to master than fission. Indeed, over seven years elapsed between the Trinity Test and the detonation of the first thermonuclear device, codenamed “Mike,” largely due to the complexity inherent in the design. This post aims to elucidate the basic principles of thermonuclear devices so that you understand not just the designs behind the bombs dropped on Hiroshima and Nagasaki but also those behind the warheads found in modern nuclear arsenals.
Before getting into the technical workings of thermonuclear warheads, it’s useful to briefly consider the reasoning behind countries’ decisions to build them. Basically, it can be summarized by one word: yield. Hydrogen bombs contain far greater explosive potential than first generation atomic weapons, enjoying lethal radii around 5-10 times greater. This explosive potential was particularly valuable during the early Cold War for two reasons. The first is simply that larger yields enabled countries to inflict far greater damage on opponents’ cities and military installations, rendering the threat posed by nuclear weapons, and thus their deterrent value, even greater. The second reason is related, but a bit more nuanced. As my first post on nuclear targeting detailed, the total kill probability of a nuclear strike is largely determined by two factors, accuracy and yield. Early nuclear missiles were, unsurprisingly, quite inaccurate due to the limitations of the electronics and missile guidance systems of the time. Increasing yield was a way to compensate for this inaccuracy because, by drastically increasing the explosive power of a warhead, planners could increase the odds of destroying their targets, even if the missiles landed fairly wide of the mark.
So it’s clear why countries sought out hydrogen bombs, but how do these thermonuclear devices work? Unlike fission warheads, thermonuclear warheads have multiple stages. The first stage is simply an implosion-type fission device. This is because fusion reactions require immense heat and pressure to begin, and thus thermonuclear warheads actually employ a small nuclear detonation as a trigger for the second stage. The second stage is where things get trickier, as nuclear engineers had to invent a way to channel the immense pressure generated by the first stage (typically called the primary) in such a way that it compresses the second stage and triggers a fusion reaction. Ultimately, the problem was solved by the Hungarian-American physicist Edward Teller and Polish-American physicist Stanislav Ulam. They invented a way to employ the principle of radiation implosion devised by John Von Neumann and Klaus Fuchs to trigger the second stage (also known as the secondary). Specifically, it is believed that thermonuclear weapons exploit radiation pressure, with the X-rays generated by the detonation of the primary compressing and triggering the secondary.
The exact manner in which X-ray compression works in thermonuclear devices is still contested in the unclassified literature, but what is known is that American warheads include a so-called “interstage” component filled with a material known as “fogbank.” The composition and exact purpose of this material is highly classified, and in fact so few people are privy to its production methods that the U.S. temporarily forgot how to produce it in the early 2000s. The commonly accepted theory, however, is that it is some kind of aerogel that is converted to plasma by the X-rays produced by the primary. This plasma then compresses the tamper/pusher surrounding the secondary, initiating the fusion process.
But this raises an important question: what comprises the secondary? The secondary has three major components. The outer of these is a tamper/pusher, much like that found in a single-stage fission device. It is generally thought that the tamper/pusher around the secondary is comprised either of U-235, U-238, or Pb-82, although U-235 would likely be superior in most cases because it could itself undergo fission, releasing more neutrons and thus increasing the yield. Underneath the tamper/pusher is lithium deuteride. This compound is used because it reacts with neutrons to produce tritium. Tritium, an isotope of hydrogen, can undergo fusion with deuterium which, usurpingly, happens to be present in lithium deuteride. Finally, within the secondary is a device called the “sparkplug.” This is a subcritical piece of plutonium that is compressed to the point of criticality by the tamper/pusher after the detonation of the primary, generating an additional fission reaction but also, more importantly, generating neutrons that react with the lithium deuteride to breed additional tritium.
In theory, there is no limit to the number of stages engineers could place within the warhead. For example, a tertiary could be placed underneath the secondary, being triggered by X-ray compression generated by the fusion reaction. In practice, however, it is very uncommon for thermonuclear warheads to contain more than two stages. Thermonuclear weapons can also be designed for special effects missions. One notable example is the enhanced radiation warhead, otherwise known as a neutron bomb. These devices are encased in material either transparent to neutrons or that actively breeds them. This design choice reduces the blast damage of the weapon by allowing neutrons to escape the reaction and by forgoing the potential yield gains created by encasing the warhead in fissionable material, but it massively increases the number of neutrons and, therefore, the radiation zone. The neutron radiation these warheads produce eviscerates organic matter but, because neutrons decay rapidly, generates relatively little fallout. Consequently, the radiation effects are short-lived, allowing friendly troops to move into the radiation zone in relative safety.
There are many more fascinating areas to explore within the topic of nuclear warhead design (if you want to learn about a particularly nasty type, look up “salted bomb”), but I think this post represents the last one I’ll be writing on the matter, at least for a good while. I still plan on writing an explainer on the nuclear fuel cycle and nuclear reactor design, but I’ve now decided to break up the nuclear posts a bit in case nuclear weapons aren’t everyone’s cup of tea. More nuclear content will be coming soon, though, so do stay tuned!