Reactor Core

A nuclear reactor core constitutes the following components 

Fuel - Originally uranium metal but now many variations, most common variant is ceramic UO2
Moderators - carbon, H2O, D2O, used to slow down the fission neutrons to increase the rate of fission in 235U
Cladding - contains fuel and prevents release of radioactive fission products
Coolant - gas or liquid circulated through the core of the reactor for heat extraction
Control rods - usually B or Cd with large σc (neutron capture cross section)
Shield - usually steel and concrete used for radiation protection and pressure vessel

Each different type of reactor will typically have a different combination of these components. The four types of reactors that have been used to produce electricity for the National Grid in the UK are known as Magnox, AGR (Advanced Gas Cooled Reactors), PWR (Pressurised Water Reactors) and Fast Breeder Reactors.