The Chemistry of War: Nuclear Bombs

By Helen Sakharova, Mika Thomas, and Ko Cheng Chan

In modern warfare, explosives are used all the time. They destroy bridges and barricades, stop armored vehicles in their tracks, and bury enemies in their own tunnels. But the most terrifying weapon ever invented by humans, the nuclear bomb, is on a different level entirely. Here is a weapon that can level a city with a single strike. Enough of them could even destroy the world. Yet a nuclear bomb, from a blinding explosion to a billowing mushroom cloud, is just a particularly deadly manifestation of chemistry. Lets start with the basics.

Fission

The first nuclear bombs, including the ones detonated over Hiroshima and Nagasaki, were fission bombs. They utilized uranium or plutonium, large, heavy elements that can undergo chain fission reactions. Fission is when a large nucleus falls apart into smaller nuclei, releasing neutrons and gamma rays in the process. In a chain reaction, the newly released neutrons strike the nuclei of other atoms, which then undergo fission as well, releasing even more neutrons. This cascade of fission reactions results in the huge release of energy accompanied by the explosion of an atomic bomb.

The kinetics of a chain fission reaction are particularly interesting to observe. Chain fission reactions don’t really have a “rate”, since the size of the reaction increases exponentially over time, assuming the reaction is sustained. It is necessary to note that not all of the neutrons necessarily cause further fission reactions: they may simply escape the system as a whole. They can also be captured without causing another fission reaction. If not enough neutrons cause secondary fission reactions, the number of reacting atoms decreases over time and the chain reaction sputters out. Thus, the number of fission reactions in progress, i.e. the size of the reaction, at a certain point in time can be determined by the formulae^{(k-1)t/\Lambda}. Here, Λ is the mean generation time, or the average time it takes for a newly released neuton to be “captured” by another nucleus and cause a fission reaction. As you can probably see, whether or not the chain reaction will be able to sustain itself or not depends on k, the effective neutron multiplication factor. If k is less than one, the system is subcritical, and any spontaneous or intentional fission reaction that occurs will quickly grow weaker and stop. If k is one, the system is critical, and any fission reactions will continue at a constant rate. Nuclear power plants operate at the critical point, since the constant fission reactions provide a steady source of energy. However, if k is greater than one, the system becomes supercritical. In this case, the reaction will get faster, and faster, and faster, accelerating exponentially until it spirals out of control and becomes a nuclear explosion.

This is the fission reaction of Uranium 235. Note how the chain reaction leads to exponentially more reactions taking place.

Fusion

The most powerful bomb ever detonated was the Tsar Bomba, a thermonuclear bomb that generated over 210 petajoules of energy, the equivalent of 50 million tons of TNT. Like all thermonuclear weapons, the Tsar Bomba utilized fusion. Nuclear fusion is the opposite of fission.  Nuclear fusion is the process where two light elements (elements with low atomic numbers) are fused together, forming a new nucleus and releasing a lot of energy.

At first glance, this seems illogical. How can forcing two atoms together possibly release energy? Wouldn’t the positive charges of the nuclei repel each other at close range? Indeed they do. However, at extremely close ranges, we encounter a force seldom mentioned in chemistry: the nuclear force. The nuclear force is the attractive force between two nucleons (neutrons or protons). By forcing two nuclei close enough together, this attractive force overcomes the repulsion between the protons and the nuclei join together. At this point, some of the mass is directly converted into photons, or energy. The exact amount of energy created strictly relates to the change in mass according to the equation E=Δmc2. Even though only tiny amounts of mass are lost, relatively large amounts of energy are produced.

Here is a video from CrashCourse explaining nuclear fusion.

The Hydrogen Bomb

Hydrogen bombs, or thermonuclear weapons, utilize both fission and fusion to cause the most powerful explosions ever created by humans. Inside the casing, a thermonuclear warhead of the typical Teller-Ulam design basically carries two bombs encased in polystyrene foam. The first is a fission bomb which releases large amounts of X-rays and gamma rays that are reflected off the inside of the casing back into the polystyrene foam. The irradiated polystyrene foam then becomes a plasma. This plasma compresses the second bomb, which consists of fusion fuel encased in uranium with a fissile spark plug inside. The compression from the plasma sets off a fission reaction in the spark plug, which pushes against the fusion fuel. Compressed from both sides and heated to extreme temperatures, the fuel, lithium-6 deuteride, produces tritium, which sets off a fusion reaction between the tritium and deuterium.

Step-by-step explosion of a hydrogen bomb. The arrows symbolize x-rays and gamma rays.

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