Components Inside a Nuclear Reactor

There are several components common to most types of reactors:

Fuel - Usually pellets of uranium oxide (UO₂) arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core. The core of the reactor contains all of the nuclear fuel and generates all of the heat. It contains low-enriched uranium (<5% U-235), control systems, and structural materials. The core can contain hundreds of thousands of individual fuel pins.

Moderator - This is material in the core which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite.

Control rods - These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it. In some reactors, special control rods are used to enable the core to sustain a low level of power efficiently. 

Coolant - The coolant is the material that passes through the core, transferring the heat from the fuel to a turbine. A liquid or gas circulating through the core so as to transfer the heat from it. It could be water, heavy-water, liquid sodium, helium, or something else. In light water reactors the water moderator functions also as primary coolant. Except in BWRs, there is secondary coolant circuit where the steam is made.  (see also later section on primary coolant characteristics)

Pressure vessel or pressure tubes - Usually a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the moderator.

Steam generator - Part of the cooling system where the primary coolant bringing heat from the reactor is used to make steam for the turbine. (not in BWR)

Containment - The structure around the reactor core which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any malfunction inside. It is typically a metre-thick concrete and steel structure.

The turbine transfers the heat from the coolant to electricity, just like in a fossil-fuel plant.

The containment is the structure that separates the reactor from the environment. These are usually dome-shaped, made of high-density, steel-reinforced concrete. Chernobyl did not have a containment to speak of.

Cooling towers are needed by some plants to dump the excess heat that cannot be converted to energy due to the laws of thermodynamics. These are the hyperbolic icons of nuclear energy. They emit only clean water vapor.

                     Example of Pressurized Water Reactor (PWR)

                           Example of Boiled Water Reactor (BWR)

(pics taken from NuclearRC USA )