Splitting Up

Fission is the splitting of an atom. Not all atoms will go through fission; as a matter of fact, very few do under normal circumstances. A small percentage of Uranium atoms have an atomic mass of 235 amu (atomic mass units). Only U-235 undergoes fission, so these atoms must be separated from the far more numerous U-238 atoms. The difficulty and cost of completing this separation is what has prevented most countries from having nuclear weapons (thank goodness). 

In a nuclear reaction, scientists shoot a whole bunch of neutrons at uranium-235 atoms. When one neutron hits the nucleus, the uranium becomes U-236. When it becomes 236, the uranium atom wants to split apart. After it splits, it gives off three neutrons and a lot of energy. Those neutrons hit three other U atoms in the area and cause them to become U-236. Each cycle, the reaction gets three times bigger. A reaction that, once started, continues by itself, is called a chain reaction. A chain reaction that keeps getting bigger is called an uncontrolled chain reaction. Left alone, and with sufficient U-235 (which you do not have in a reactor), the energy would grow large enough to cause an explosion – A BIG one! The worst that can happen with a nuclear reactor that gets out of control would be a melt down; which is plenty bad, but not as bad as an explosion. 

Fizz vs. Fuse

When we were kids we always got fusion and fission confused. The confusion wasn't because the processes were similar; the words were just similar. You need to remember that one process is a breaking down process and the other is a process of building up. When things fuse (fusion), you start with smaller objects (tritium, deuterium) and build larger objects (helium). When things "fiss" or break down, you start with a larger object (uranium) and finish with smaller objects (strontium, calcium, barium, etc). 

Isotope Action

Why do we say that atoms are radioactive? There is another isotope of uranium with the number 238. When one of those free neutrons hits a 238, it will bump it up to 239 (that just makes sense). But that 239 is radioactive and releases a beta particle when it decays. It's not over. That U-239 breaks down into neptunium-239. The neptunium is also radioactive. It will release another beta particle when it breaks down into plutonium-239. The plutonium will eventually give off an alpha particle (not as strong as beta and less dangerous). That's a lot of particles being given off. 

Einstein's Legacy

The fission process, however radioactive it is, is the main reaction that happens in many nuclear devices. It all started with Einstein's equationE=mc^2. As soon as scientists thought there were huge amounts of energy available in each atom of the universe, the military began to develop weapons that had enormous destructive abilities. Usually uranium or plutonium is in the bomb and a smaller explosion of material that surrounds the uranium and plutonium sets off the fission reaction. They have even developed fusion bombs that are set off by a fission reaction.