A Basic Introduction to Nuclear Weapons Design
Dropping Some Nuclear Know-How
With Russia in increasingly dire straits in Ukraine, many commentators are once more discussing the likelihood of nuclear use by Moscow. This is an enormous and important issue, of course, and is perhaps something I’ll address more explicitly in the coming weeks. But it is striking to me just how little people understand about the nuts and bolts of nuclear weapons, the nuclear fuel cycle, and nuclear reactors. Thus, over the coming weeks, I’ve decided to put out a few basic explainers that focus on the technical side of nuclear systems. This week’s post will address the basic structure of single-stage nuclear fission bombs, with particular focus on the two warhead types employed by the United States at the end of World War Two. Next week will continue the discussion of warhead design by explaining how multi-stage thermonuclear (fusion) warheads work, which is important given that these are the warheads that now comprise most nuclear powers’ arsenals. If these posts do well with viewership, I’ll probably write two more posts on the nuclear fuel cycle and nuclear reactor design, so if you’re interested in seeing more share this piece widely!
With the logistics of the next few weeks cleared up, it’s time to talk about nukes! At its core, a nuclear explosion is simply an uncontrolled nuclear reaction. For a nuclear warhead to be effective, in other words, it needs to release as much nuclear energy as possible in the shortest feasible time. Importantly, though, it must only do this when desired! Nuclear warheads balance these requirements by changing the density of their pits - the part of the warhead that contains the fissile material (either U-235 or Pu-239). This works because nuclear explosions result from chain reactions created by neutron emissions. As the fissile material decays, it produces neutrons which collide with other uranium or plutonium atoms, leading them to fission and release even more neutrons. As increasing numbers of neutrons get released, a critical point is eventually reached that triggers a chain reaction. Nuclear warheads regulate these neutron collisions by altering the density of the core. Density is crucial because higher density equate to greater odds that a neutron will collide with, and fission, an atom in the pit.
The first nuclear bomb ever used in combat was a gun-type device. This style of warhead features a uranium core with two sub-critical masses that are fired into each other. In many ways, this design is not dissimilar to a musket in that it fires a uranium projectile down a smooth bore barrel using a type of explosive powder (in the case of Little Boy, the first nuclear device dropped on Japan, it was cordite). The collision of the two masses creates a sufficiently dense mass of fissile material to support a chain reaction, ultimately producing a nuclear detonation.
Of course, the design is a bit more complex than this. Immediately after the impact of the two subcritical masses, a polonium-beryllium neutron generator is triggered, flooding the core with additional neutrons to help jump-start the reaction. Remember, the key is to maximize the number of neutron collisions, which can be achieved by increasing density or by increasing the number of neutrons within the core. The uranium pellets are also surrounded by a tungsten-carbide tamper which, after firing, completely encases the core. The point of the tamper is simple: hold the exploding core together for as long as possible, because each additional fraction of a second the core remains critical equates to significant gains in yield. The tamper also serves as a neutron reflector, meaning that it prevents many of the neutrons generated by the reaction from escaping, trapping them in the exploding core and thus further increasing yield.
These gun-type device are, at least relatively speaking, quite simple. In fact, American engineers working on the Manhattan project were so confident their gun-type design would work that they did not even bother testing it prior to its use against Hiroshima. But for all its simplicity, the gun-type device has major limitations. For one, it is unable to utilize plutonium in its core because Pu-239 emits far more neutrons than U-235. This means that, were it to be used in a gun-type device, it would explode prematurely, leading to a low-yield fizzle due to the detonation occurring before optimal density was achieved. In addition, the uranium slugs utilized in the gun design must fit within the barrel, preventing pit levitation, which is a technique that further increases yield and reduces weight.
Consequently, the U.S. concomitantly developed a separate warhead design for Fat Man, the bomb dropped on Nagasaki. This design, known as an implosion device, relied on a complex lattice of explosive lenses to crush a sphere of plutonium into a sufficiently dense state to trigger an uncontrolled chain reaction. Much as with the gun-type design, the reaction was abetted by a neutron generator within the pit (called an urchin), which helped jump-start the fission chain reaction. The advantage of using plutonium is that it requires a lower mass to achieve criticality. Whereas a uranium bomb utilizes 10-12 kilograms of HEU, plutonium warheads need only 5 kilograms of Pu-239. Implosion-type devices also enjoy several additional advantages beyond weight savings, although these were mostly incorporated into post-WWII designs. One obvious benefit of implosion warheads is that they allow for a levitated pit, which simply means that there is a gap between the tamper/pusher and the fissile pit within the core. This increases the power of the shockwave that hits the fissile material, leading to greater compression and thus higher density.
Implosion-type devices have another advantage, which is that they can utilize hollow pits. These pits feature a cavity in their center into which the plutonium can collapse. This means the outer layer of the pit can serve as a type of tamper/pusher in its own right, helping to compress the inner plutonium layers and reducing the size and weight requirements by decreasing the amount of uranium needed to form the main tamper. Hollow pits can also be filled with deuterium and tritium gas, which undergoes fusion during the explosion, boosting the explosive yield without increasing radioactive fallout. Boosting can also be used to reduce the amount of plutonium needed within the core, further reducing weight and raising efficiency. Beyond their weight-saving benefits, boosted cores are also especially useful in variable-yield warheads, as pilots can program the amount of deuterium-tritium mix to inject into the pit just before release, allowing them to tweak the size of the explosion depending on battlefield requirements.
As is hopefully clear by now, nuclear warhead designs are fairly intuitive. Of course, the process of engineering them is fiendishly complicated, but the basic principles are easy to grasp. The goal is simply to compress fissile material to a critical mass and flood it with neutrons. Thus, every warhead design is just a combination of tamper/pusher, pit, and neutron generator. There are often other features, such as beryllium neutron reflectors or tritium gas, but the basic design really just involves those three aforementioned components.
The problem with single-stage fission devices like those discussed in this post, however, is that their yield is limited, typically topping out in the hundreds of kilotons. Thus, thermonuclear weapons, which can achieve yields of several megatons, increasingly came to replace fission-only devices by the late 1950s and early 1960s. Designing these higher yield devices is no straightforward task, however, and it took very clever engineering to realize their full potential. That will be next week’s post, so if you enjoyed learning the basics of fission bombs, stay tuned because next Sunday the explosive potential only gets bigger!