Reactor Core Components

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, see neutron capture cross section
Cladding - contains fuel and prevents release of radioactive fission products
Coolant - gas or liquid circulated through core of reactor for heat extraction
Control rods - usually B or Cd with large σ_c (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 electricty for the National Grid in the UK are known as Magnox, AGR (Advanced Gas Cooled Reactors), PWR (Pressurised Water Reactors) and Fast Breeder Reactors.