How Reactors Work

How do nuclear reactors work?

Power nuclear reactors used to generate electricity work in a similar manner to regular coal or gas power plants, with one important difference - instead of burning coal or gas to heat the water that turns the turbines, fission is used.

What is Fission?

Sometimes when the nucleus of an atom is very heavy (contains many protons and neutrons), and a neutron is fired at it, it will split into two smaller nuclei. This is called fission. When the nucleus splits, it releases energy, which creates the heat we need in the reactor to boil the water. The most commonly used nucleus in nuclear power reactors is Uranium-235 or U-235 for short. The diagram below is a summary of what happens when a neutron collides with a U-235 nucleus and causes it to fission. The red balls are neutrons, the blue are U-235, and the green are the smaller nuclei the U-235 splits into (called fission fragments).

A single fission event (one U-235 nucleus splitting into two smaller nuclei) doesn't release a lot of energy by everyday standards; only about 200 MeV. This is about enough energy to raise the temperature of 1 gram of water by 7.65x10-12 oC or about a hundredth of a billionth of a degree!

So why is fission interesting? It's because in a nuclear reactor, billions of fission events occur every second, so as a whole, a reactor can give us a lot of heat energy which we can then put to useful work!

What happens to the other Neutron?
You may have noticed that the fission event in the diagram above produced more than one neutron. This is a common event; in commercial power reactors, a fission event will produce on average 2.43 neutrons (taken for a typical light-water reactor).

For the moment let's assume that only 2 neutrons are produced per fission. If we fire a neutron into a reactor so that it causes a nucleus in the fuel to fission, will produce 2 more neutrons. If both these neutrons go on to cause fission in two more nuclei, we will have four neutrons, after the next generation of fissions we will have 8, then 16, 32, 64, 128, and so on. If all neutrons always caused fission; after 30 generations of fissions there will be over a billion neutrons flying around in the reactor, looking for a nucleus to fission with. Since each fission event releases some energy, we can see that this chain reaction will release ever-increasing amounts of energy, which can be very dangerous.

Fortunately, it is not the case that every neutron in a reactor is destined to fission. There are other possible ends to the life of a neutron in the reactor. Neutrons may be absorbed by nuclei present in the reactor (without fission occurring), they may escape the core where the fissile fuel is held (often referred to as 'leaking' from the core), or the control rods may absorb them.

All reactor cores are designed so that for each fission event only one of the neutrons produced will go on to induce another fission. This condition is called 'criticality'. It is actually quite difficult (though with clever desigining not impossible) to achieve criticality with ordinary Uranium ore dug up from the ground, as the natural level of U-235 is too small (another type of Uranium, U-238 - which is not fissile - is much more abundant). Usually expensive enrichment of fuel is required to get just the right percentage of U-235.

Enrichment isn't the only variable in getting the reactor to function properly; the moderator, coolant, and control mechanisms must be carefully chosen to balance safety, stability, cost effectiveness, longevity, and other important parameters.