What makes up a Reactor?

There are several components common to most types of reactors:

Fuel: This is where the nuclear fission takes place and is the energy source for nuclear power. The fuel within the reactor is usually in the form of pellets of uranium oxide (UO2) that are arranged in tubes to form fuel rods. The fuel rods may be individually loaded into the reactor or can be organized into a grid pattern containing many rods, which is known as a fuel assembly, and placed in the reactor core.

Moderator: Its job is to slow down the neutrons released from fission so that they can react with other material in the core and cause more fission. Depending on the type of reactor, the material used as a moderator can be water, heavy water, carbon or graphite. This moderator is placed around the fissionable material (usually uranium-235) in the core of the reactor.

Control rods: The control rods are an important component of the reactor in order to control the rate of reaction in the core. These rods are made with neutron-absorbing material such as cadmium or boron that capture neutrons in the reactor core to slow down the fission process. They can be inserted or withdrawn from the core at any time to slow down or speed up the rate of reaction, or to halt the reaction completely.

Reactor vessel: This consists of a robust steel vessel containing the reactor core and moderator (see PWR), or a series of tubes holding the fuel and conveying the coolant through the moderator (see BWR).

Coolant: This is usually a liquid or gas that is circulated through the core in order to transfer the heat from the core to the steam generator.

Steam generator: This section is housed inside the containment structure where water from the cooling system is turned into steam from the heat generated in the reactor core. This steam is then used to drive the turbine that generates electricity.

Containment Structure: This is the structure around the reactor core designed to protect it from outside intrusion and to protect those outside from the effects of radiation or any malfunction inside. It is typically a metre-thick concrete and steel structure.