If uranium fission is to be harnessed in a power station, the nuclear chain reaction must first be tamed. The chain reaction is explosive and dangerous. It must be curbed by both slowing the neutrons released by each fission reaction, by carrying away the energy and by controlling the neutron numbers.
As seen above, the chain reaction takes place when each fission reaction causes more than one further identical reaction. If the fission of a single uranium-235 atom causes only one identical reaction to take place, the reaction will carry on indefinitely - or at least until the supply of uranium-235 has been used up - without accelerating. But if each fission reaction leads to an average of less than one further reaction, the process will eventually die away naturally.
The operation of a nuclear reactor is based on the above idea that a nuclear chain reaction can be controlled so that the process continues indefinitely, but is never allowed to run away and become a chain reaction. A reactor in which each nuclear reaction produces one further nuclear reaction is described as critical. Once the product of each nuclear reaction is more than one further reaction, the reactor is described as supercritical.
A nuclear reactor contains uranium which has generally been enriched so that it contains more uranium-235 than it would in nature. Enrichment to about 3% is common. Using enriched uranium makes it easier to start a sustained nuclear fission reaction.
In addition to the uranium, the reactor also contains rods made of boron. Boron will absorb the neutrons generated during the nuclear reaction of uranium-235, removing them and stopping the chain reaction from proceeding. By moving the rods in and out of the reactor core, the nuclear process can be controlled.
One further crucial component is needed to make the reactor work, something to slow the fast neutrons down. The neutrons from each uranium-235 fission move too fast to stimulate a further reaction but they can be slowed by adding a material called a moderator. Water makes a good moderator and is used in most operating reactors. Graphite also functions well as a moderator and has been used in some reactor designs.
When a uranium fission reaction takes place the energy it releases emerges as kinetic energy. The products of the fission process carry the energy away as energy of motion, and they move extremely fast. Much of this energy is carried away by the fast neutrons. These neutrons will dissipate their energy in collision with atoms and molecules within the reactor core. In many reactors this energy is absorbed by the moderator, water. So while the neutrons are slowed, the water within the core becomes hotter. By cycling the water through the reactor core this heat can be extracted and used to generate electricity. This helps maintain the reactor in a stable condition.
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