Uranium, like all other atomic elements, occurs in several different forms, known as isotopes. The most common isotope of uranium is U-238, which makes up 99.28% of all uranium atoms. The second-most common isotope, U-235 (0.71%), is the one used to generate electricity, because it can easily undergo fission.
Because nuclear fission is a very efficient source of energy, nuclear reactors require very little fuel. A single 20-gram uranium fuel pellet can produce the same amount of energy as 400 kilograms of coal, 410 litres of oil, or 350 cubic metres of natural gas.
Uranium (both U-235 and U-238) is relatively stable before entering the reactor: it emits such a small amount of radiation that unused fuel pellets are safe to be near. However, when a U-235 atom is bombarded with a neutron, it often splits (or “fissions”) into several pieces, ejecting two or three extra neutrons. This process releases heat, which can be converted into electricity.
In what is often called a chain reaction, these neutrons interact with other nearby U-235 atoms and cause the effect to continue – much the way billiard balls can continue striking other ones after the cue ball has stopped moving. By controlling the concentration of nuclear fuel and slowing or absorbing neutrons, nuclear reactors stabilize this chain reaction at the desired rate.
The basic parts of a nuclear reactor are the core, a moderator, control rods, a coolant, and shielding.
The core of a reactor contains the uranium fuel. CANDU heavy water reactors use natural uranium, of which 0.7% is U-235, while light water reactors use uranium that has been enriched so that U-235 makes up about 3 – 5% of the total.
The moderator is a light material, such as water, that allows the neutrons to slow down without being captured. By slowing down the fast neutrons created during fission, it can increase their efficiency of causing further fission.
Control rods are made of materials that absorb neutrons, such as boron, silver, indium, cadmium, or hafnium. They are introduced into the reactor to reduce the number of neutrons and thus stop the fission process when required. They are also used to control the level and distribution of power in the reactor.
A coolant is a fluid circulating through the reactor core that is used to absorb and transfer the heat produced by nuclear fission. At the same time, it maintains the temperature of the fuel within acceptable limits.
Shielding is a structure around the reactor and its steam generators, designed to protect it from intrusion and to protect those outside from the effects of radiation in the event of any serious malfunction inside. It is typically a metre-thick concrete and steel structure.