By Richard Bleil
In principle, atomic weapons are easy to make. One has to start with an atomic fuel that undergoes fission, which is the process in which elements break down to form smaller elements. Uranium or plutonium (I believe the US uses plutonium) are the typical fuels.
Every time an atom breaks down in a process called fission, it releases three fast moving neutrons. If these neutrons hit the nucleus of another fuel atom, they will cause that element to undergo fission as well, which will create three more neutrons. Two other possibilities is to strike the nucleus of an impurity (such as the break-down products of the uranium or plutonium) or leave the fuel, either of which, of course, will stop the chain reaction.
If the odds of striking another fuel nucleus is greater than the neutrons leaving or colliding with impurities, you will have an uncontrolled fissions reaction, and an explosion. If, on the other hand, the odds are greater that the neutron will escape or hit an impurity, there will be no explosion. Simple, eh?
Historically, to create an atomic bomb, you needed to have an appropriate size of the fuel. The bigger the mass, the further the neutron has to travel, and the more likely it’ll hit another fuel atom. The mass where this probability tips is called the “critical mass”. The bombs dropped in Japan worked on this principle. There were two pieces of fuel, one donut shaped, the other like a plug. When detonated, a chemical bomb plunged the plug component into the donut, creating critical mass. Today, they surround the fuel with specially formulated and engineered bombs to compress the fuel. This increases density, pushing atoms closer together and changing the odds of the neutron striking a fuel atom. So today, they use “critical density” rather than critical mass. The explosives have to be carefully packed to compress the nuclear core rather than simply blow it apart.
In an atomic power plant, they use lead “control rods”. When the fuel is new, the rods are pushed deeper into it making the neutrons more likely to hit the control rods rather than a nucleus to propagate the reaction. As the fuel gets older and has more impurities from the breakdown of the fuel, the rods are not as deep so the neutrons are more likely to hit a uranium atom.
The hard part is to purify the fuel. It’s very challenging to purify something like uranium or plutonium because only one isotope will react. An “isotope” is the same element, but with different mass. Because it is the same element, these isotopes cannot be separated using a chemical technique. This is why they use a centrifuge, which separates the heavier from the lighter elements.
The more impurities, the less likely you will have an atomic explosion. This is why such high purity is required, but radioactive nucleotides will break down naturally, creating their own impurities. It’s kind of like alcohol. In the fermentation process, the microbes responsible for fermentation will eat sugar and excrete alcohol as their waste byproduct. Just like we would die in a septic tank because of the toxicity of our waste, the alcohol eventually reaches a high enough concentration to kill the microbes. As radioactive fuel breaks down, the concentration of the breakdown products eventually becomes too high for the atomic weapon to actually detonate. The US replaces the fuel core of atomic weapons every five to ten years (the exact time is, of course, classified). In ten to twenty years, the odds of an atomic fuel core detonating becomes inconsequentially small. In 1991, the Soviet Union fell apart. Some break-off republics ended up with nuclear weapons because there wasn’t time to remove them. Today, these weapons wouldn’t detonate (in fact, by 2000 the odds were very small) even if they did happen on the launch codes. Of course, the explosion and remaining radioactive fuel that would scatter would still be highly destructive and toxic, which is the concept behind a so-called “dirty bomb”. The radioactive waste would create a region that couldn’t be occupied for many years until the radioactivity decayed to safe levels.
The chemical bomb that provides the energy to cause the fusion bomb would wipe out, oh, a city block or two. The fusion bomb would be enough to destroy, I dunno, maybe downtown of a major city. This isn’t enough for modern military, so they use the energy of an atomic fusion bomb to set off a fission bomb, where the atoms of hydrogen are forced together in a fusion reaction (where they create larger, rather than smaller, elements). This would wipe out a downtown and surrounding neighborhoods, but the fission bomb itself produces mainly heat, which dissipates quickly. This is still not enough, so they wrap lower purity atomic fuel around the fission bomb, creating a fission-fusion-fission bomb. With enough energy, even less pure uranium or plutonium will set off an atomic explosion, Because of this, they can make this outer bomb much bigger than the original one that started the process, and the explosion will be larger still. A fission-fusion-fission bomb would wipe out the entire city, including all suburbs.
Hope you sleep well.