How a Nuclear Power Plant Works

A nuclear power plant works to provide us with electricity by harnessing the energy released by the nuclear fission reaction of radioactive fuel, often Uranium 235, abbreviated to U-235.

The energy released in this reaction is either contained in the fast moving neutrons that continue the reaction or in excess energy released as heat. U-235 is a suitable fuel for fission reactors as it is readily split and releases large quantities of heat energy as it does so. U-235 is a very potent fuel and only comprises 5% of the material in the fuel rods used in nuclear reactors. The remaining component of the fuel rods is the non-fissile isotope U-238.

How a Nuclear Power Plant Works: Controlling the Reaction Rate

We are all aware of the effects of a chain reaction involving Uranium or other similarly behaving materials. In order to maintain a constant supply of electricity without allowing a chain reaction, nuclear reactors must control the amount of neutrons released in the reaction.

This is done by absorbing excess neutrons with what are called control rods. Control rods are made of materials that are very good at capturing free neutrons, such as Cadmium. If the amount of free neutrons is too high in the reactor core, the control rods are lowered into it to capture the excess. If the reaction is not moving quickly enough, the control rods are raised to keep more of the freed neutrons in the core, hence the reaction rate will speed up.

This is clearly a critical operation as failure of the machinery operating the control rods would lead to either the reaction coming to a stop or going too far and causing a melt down of the reactor core.

How a Nuclear Power Plant Works: The Role of Water

Another important component of the nuclear reactor core is water. Water serves two functions in the reactor core. Firstly, it serves to slow down the fast moving neutrons. This is necessary since these neutrons are moving at such a speed when initially released that they would pass straight through the nuclei of the U-235 atoms and would not cause the splitting, or fission, that is required.

The second function of the water is to act as a carrier for the heat energy so that steam can be generated to turn the turbine in the power plant, thus generating electricity.

The water in the reactor core is exposed to massive amounts of free subatomic particles. The Hydrogen atoms in the water gain neutrons to become either Deuterium or Tritium (heavy and super-heavy water respectively), both of which are unstable and radioactive and this water requires careful disposal.

How a Nuclear Power Plant Works: Reactor Types

There are two main types of Nuclear Fission reactors, the Pressurized Water Reactor and the Boiling Water Reactor. Each type also has two variants, one with normal water and one with heavy water.

Pressurized Water Reactor

The radioactive water in the reactor core is kept under extreme pressure so that it does not boil when it hits 100 degrees celsius but continues to absorb heat. This super hot water goes through a heat exchanger which transfers the heat to non-radioactive water. This water forms super-heated steam which is used to power the turbines of the power station.

In this reactor the water in the core is not pressurized and so it boils into steam in the core. This water is then piped out to the turbines where it is used to generate electricity. Upon cooling, the water is returned to the core. While this reactor type saves somewhat on the cost of pressurizing the core, it does mean that the radioactive water from the core is passed through the turbines which then also become contaminated with radiation. This reactor model carries a far greater clean up cost when it is dismantled as there are far more heavily radiated components.

Both the reactor types described above can also incorporate heavy water. Heavy water is water that is made of oxygen and an isotope of the element Hydrogen called Deuterium, which is the same as Hydrogen except it has an extra neutron in the nucleus and is radioactive. This is used because heavy water is better at slowing down the released neutrons than is regular water.

How a Nuclear Power Plant Works: Heat Loss and Water Return

The steam generates electricity by pushing the turbines as it passes through. This means that only some of the energy in the steam is converted into power. Once the steam has passed out of the turbine, it needs to be returned to water form to be passed back into the heat exchanger or the core, depending on the model of reactor in use. In order to cool the steam enough for it to return to liquid form it is passed through cooling towers. In these, some of the steam is sacrificed to remove sufficient energy to condense the rest. This is the source of the great white plumes coming from the classic curved towers we all associate with nuclear power plants.

This is done to minimize wasted heat. While at first it seems a little ridiculous to release steam to save heat, it is the only effective large scale way power stations can retain the maximum amount of hot water after it has turned into gaseous form.

Obviously, the steam coming out of the towers represents water loss, so the water in the reactor needs to be continually topped up from a local water source such as a lake or river.