Today’s post is about nuclear bombs. Big boom. Mushroom clouds. Yep, those bombs. In our last post, we discussed the basic mechanism behind the uranium fission chain reaction. We also briefly talked about the difficulties involved in making it a continuous, feasible reaction. In this post, I’ll talk about the basic principles behind the design of a nuclear fission bomb. We’ll see two classic designs – the designs of the Hiroshima and the Nagasaki bombs, known as Little Boy and Fat Man. I assume that you have read the previous posts, or are familiar with basic scientific terminology related to nuclear reactions.
The explosive power of a nuclear fission weapon is derived from the rapid fission of a large number of atoms at the core of the warhead. Nuclear fission releases far more energy than a conventional reaction. A nuclear chain reaction is one in which one reaction event produces products that initiate more reaction events. If each reaction produces more reactions in its wake, the entire process grows exponentially, with each passing second resulting in a larger and larger number of reactions. The primary difficulty in designing a nuclear weapon is keeping the fission reaction from ending too quickly. When a lump of fissile matter becomes critical, it starts a self-sustaining nuclear reaction. Usually, this causes the sample to melt or vaporize and expand. This process tends to cause the sample to start radiating energy, but not quickly enough to cause a massive explosion. This phenomenon is known as a criticality excursion. The trick to a nuclear weapon is in holding a sample of fissile material close together for long enough for it to actually cause an exponentially growing cascade of fissions. By the time the sample disintegrates and separates, massive quantities of energy should already have been produced. More powerful bombs not only use more fissile material but also use more complicated designs to hold them together for longer and under tighter conditions so that the maximum amount of energy can be extracted before the entire contraption explodes. It is this delicate requirement that complicates the construction of a nuclear weapon.
One early design to keep fissile weapons took the approach of slamming two pieces of U-238 together. A critical piece of uranium rapidly generates heat, melts and separates. But if you take that same critical sample and slice it into two sub-critical samples, you can then slam the two of them together. The force from slamming them together will hold them together for just long enough to cause enough fissions to cause a nuclear explosion. Of course, when I say slam, what I actually mean is put one piece in a gun and shoot it at the other. The solution to the problem of building a nuclear bomb turned out to be “more dakka“. This weapon design is, unsurprisingly, known as a gun-type assembly. Two sub-critical fragments of uranium are mounted on either ends of a long tube/barrel. A conventional explosive is mounted at one end. When the explosive is set off, one of the pieces is sent hurtling down the barrel, accelerating to extremely high speeds. By the time it gets to the other side of the barrel, even though the configuration is critical, the force from the explosive forces the fragments together, setting of a nuclear chain reaction. The timescale involved in exceedingly small – microseconds or shorter. But that is what it takes. Within microseconds, the entire device explodes. The actual efficiency of this device is quite low. A lot of uranium ends up getting scattered in the process without undergoing any fission at all. Nevertheless, the design is relatively simple and its efficacy with demonstrated over Hiroshima, Japan at the end of the second world war.
Curiously Shaped Explosives
As you may have realized, gun-style nuclear bombs require the fragments to be travelling really rapidly to come together quickly enough to cause a nuclear explosion. If the fragments do not get together fast enough, there is a risk that a nuclear reaction will start too early, causing the fragments to explode without having released much of its nuclear power in a nuclear fizzle, leaving a lot of radioactive waste behind. Designing explosives to accelerate fissile fragments without exploding the barrel of the bomb or adversely affecting the fragment is a tough challenge. It is also dangerous because an accident that triggers the conventional explosive could cause a nuclear explosion, or criticality excursion harming a lot of people and leaving behind a hard to clean mess. A new design was invented that involved only one piece of fissile material instead of two. A single spherical plutonium core, if compressed quickly and really hard, would form a very dense smaller sphere which would be super-critical. This dense sphere would then undergo lots of nuclear fissions to release the desired energy. This design was much safer because it is extremely hard to accidentally compress a plutonium core (without destroying and scattering it) to the point where it is critical. In fact, it was extremely hard to do so even deliberately. Any asymmetric explosion would simply fragment the core. A complicated set-up involving a cocoon of explosives that burned at various speeds was used to create an explosive lens. This lens created a spherically symmetric explosive wave and would compress the core from all sides. All the explosives involved would need to be detonated in a perfectly synchronized manner. If any one of them exploded too early or too late, the wave would not be spherical and the core would simply be ripped to shreds. If all went well, the core would instead be compressed into a tiny critical sphere and be held there for the microseconds it took to fission a large number of its atoms. This type of device is known as an implosion-type assembly. The result is a massive explosion, such as the one over Nagasaki, Japan at the end of the WW2.
This is the basic principle behind designing nuclear explosives. Get fissile material that is currently non-critical to become critical and keep it critical for long enough for a lot of fission reactions to take place. As always, easier said than done.