 Let's revisit what happens in a neutron induced fission reaction. Imagine a neutron hitting a uranium-235 nucleus. The neutron is neutral with no charge, so there is no coolant force preventing it from getting close to the uranium nucleus. Low-enaging neutrons have the highest probability of initiating a fission reaction. If you look at the masses of the various products of a fission reaction, there are hundreds of combinations that are energetically possible. Most of the fission pathways result in a heavier fragment with around 140 nucleons, a lighter fragment with around 95 nucleons, and the emission of 2-3 neutrons and the liberation of around 200 MeV of energy. This corresponds to an energy density for nuclear fuel that is remarkably high. If we completely burn 1 kilogram of coal, this will liberate between 25 and 35 megajoules of energy. Petrol is somewhat higher, with 1 kilogram releasing 46 megajoules. In comparison, the complete fission of 1 kilogram of uranium-235 releases huge amounts of energy. We can estimate this amount by working out the number of atoms in 1 kilogram of uranium and multiplying it by 200 MeV for each fission. This results in an energy release of 82 million megajoules. This means that the energy density of uranium-235 is around 2 million times higher than that for fossil fuels. The other key concept for understanding a nuclear reactor is the idea of a chain reaction. This is illustrated in the picture on the left. Remember that each fission reaction produces an average of 2.6 neutrons. Some of these neutrons can be lost. They might escape the fuel entirely, or they might be absorbed on something that is not uranium-235. However, some of them might be absorbed onto another uranium-235 nucleus in the target and initiate another fission reaction. This will generate more neutrons. Under the right conditions, each fission can generate sufficient neutrons that a continuous sequence of fission reactions occurs. This is known as a chain reaction. We can define the average number of neutrons from each fission event that goes on to create another fission. This factor is conventionally referred to by the letter K, and it's called the neutron reproduction factor. If K is less than 1, then the fission reactions will eventually die out. If K is greater than 1, then there will be an exponential increase in the number of fissions that occur. If K is much greater than 1, the time scale for neutron reproduction is short, and the fuel is physically contained from flying apart. The exponential increase can result in a large fraction of the fuel undergoing fission. Something designed to work in this way is the basis of a nuclear weapon. However, the key point is that it must be designed specifically to do this. It is impossible for a conventional fission reactor to undergo an explosion like a nuclear weapon. If K is equal to 1, there will be a sustained constant rate of fission reactions. This is the desired condition for power generation.