North Korea might soon have them, and President Trump seems to want more of the things. Seems like a good time to have a look at what a nuclear weapon really is.
Before turning to nuclear weapons, it’s important to look at regular explosives first. Regular explosives work through a chemical reaction. The molecules in the explosive react with some reagent in a so-called exothermic reaction. Exothermic means that the reaction releases energy into its surrounding, instead of taking energy from its surrounding. This release of energy, in the form of heat, kinetic energy and pressure waves, is what makes an explosion an explosion.
The energy in an explosion is not ‘produced’. Energy cannot increase or decrease. It can only change. In the case of an explosion, the atoms in the explosive combined with the atoms in the reagent result in a set of new molecules that require less energy to remain stable. The excess energy is released to the environment.
Fission and fusion
Nuclear explosives work differently. Energy is released not from the binding energy between atoms in a molecule, but from the binding energy inside the atom itself.
An atom consists of protons, neutrons, and electrons. Energy holds the three together. The number of protons in an atom determines what element it is on the periodic table. Hydrogen has a single proton, and uranium has 92.
Keeping one helium atom together (2 protons) requires less energy than keeping two 2 hydrogen atoms together (1 proton each). In other words, when putting two hydrogen atoms together into a helium atom , you will gain energy. This energy is actually the difference in mass between the two, as expressed in Einstein’s famous formula E=mv² which describes the relation between mass and energy. We call this reaction nuclear fusion. In other words, atoms fuse to create new atoms and release their excess binding energy.
Now, take a look at the diagram below, which shows the binding energy required to keep atoms together for each element.
Notice that the energy for holding things together starts to decrease beyond iron (Fe). What does this mean? Well, combining atoms with more protons than iron won’t gain you energy, it will cost you energy. This is because protons repulse each other, and if you put enough together, their repulsion starts to gain the upper hand over atomic binding.
The consequence is that breaking up (as opposed to fusing) atoms with more protons than iron will gain you energy. We call this nuclear fission.
If you look at the chart above, on the left you will see H, or Hydrogen. On the right is U, or Uranium. Yep, that’s the uranium used in nukes (alongside Plutonium, actually), and the hydrogen used in hydrogen bombs.
The nuclear fission bomb is the first one developed. The reason for this, is that a fusion reaction is even more difficult to start up than a fission reaction, even though it produces more energy. More on this later.
The theory that mass could be converted to energy was one thing, but developing an actual bomb took some doing. There were several practical problems to overcome.
For one, weapons-grade uranium isn’t easy to make. You have to use sophisticated techniques to separate the correct isotopes (I’ll leave the explanation of isotopes out of this article).
Once you have enough fissionable material, you need to detonate it in such a way that most of the material is actually fissioned, otherwise the amount of energy released isn’t large enough. You just get a tiny nuclear explosion that hurtles chunks of uranium every which way.
In the end, though, the Manhattan project managed to create a fission bomb and the US then proceeded to use it against Japan – for possibly depressing reasons.
The thing to realize about these weapons, is that the energy produced by this reaction per atom/molecule is far larger than with a chemical reaction. A single fission bomb releases the equivalent energy of one million tons of (the chemically-based explosive) TNT. In other words, converting mass to energy yields far more energy than reconfiguring atoms in molecules.
Hydrogen bombs are even more complicated. As you can see from the binding energy curve above, fusion will produce even more energy than fission does. However, getting the fusion going is not trivial. When you throw two Hydrogen atoms together, they won’t simply fuse: the electrons in orbit of the atom repulse each other, as do the protons in the atom core, meaning that the hydrogen atoms won’t want to come together.
One way to start a fusion reaction is to push the atoms together using gravity. That’s how nature does it. Over four billion years ago, hydrogen floated together into a cloud in space that collapsed under its own gravity until there was enough pressure that a fusion reaction started. Yep, the sun is a giant fusion bomb, with enough fuel to last for another four billion years.
So, pressure can start a fusion reaction. Another way is to pour enough energy into hydrogen atoms so that their energized state overcomes the repulsion forces. And that’s where the fission bomb comes in. By using the nuclear energy produced in a fission reaction, you can start a nuclear fusion reaction. And with that, we have the hydrogen bomb, the most explosive weapon ever produced by mankind. For now at least.
I hope the explanation above gives some insight into nuclear weapons work. The follow-up question, of course, is: what kind of damage do these weapons do. I’ll look at that one next week.