A nuclear reactor is a device where a fission chain reaction produces heat energy for generating electricity. The heat energy is used to produce steam, the mechanical energy of the steam drives the turbines, which generate electrical energy.
There is a large variety of nuclear reactors. They differ in fuel, power level and purpose. Most of them consist of the following components:
a) the core where fuel rods support a fission chain reaction;
b) the moderator which is used to slow down fast neutrons;
c) the coolant which is used to remove heat from the core;
d) control rods which are used to control the rate of the chain reaction;
As for the fuel, nuclear reactors use such fissile materials as U-233, U-235, and Pu-239.