The Nuclear Reactor

• Reactor core - The fission reaction takes place in the nuclear core. The reactor core contains the fuel rods, control rods and the water at high pressure. The reactor core has a thick steel wall to withstand the high temperatures and pressures inside
• Fuel rods - The fuel rods contain the uranium used for fission (mixture of U-235 and U-238)
• Moderator - The water acts as a moderator. This means that it slows down the fission neutrons. This is necessary because fast neutrons don't cause further fission in other uranium nuclei.

  The reactor is known as a thermal nuclear reactor because the neutrons are slowed down to kinetic energies comparable to the moderator molecules
• Control rods - The control rods absorb surplus neutrons, keeping the reaction under control

  The control rods may be lowered to absorb more neutrons and slow the reaction down. (Without the control rods the chain reactions would rapidly create too much energy too quickly and effectively be a nuclear bomb)
• Heat exchanger - Water also acts as a coolant and is pumped around the core and through the heat exchanger. This allows the heat to transfer to water that supplies steam to the turbine

  A pressurised water reactor (PWR) is just one type of reactor. Advanced Gas-cooled Reactors (AGR) use CO2 gas to cool the reactor
• Thick concrete walls - Thick concrete walls absorb ionising gamma radiation and neutrons (protecting those working in the power station)
• Nuclear fuel - The fuel rods contain enriched uranium which consists mostly of the non-fissionable and about 2-3% of the fissionable U-235. (Natural uranium consists of only about 1% U-235)

  For a chain reaction to occur, the mass of the fissile material (U-235) must be greater than some minimum mass, referred to as the critical mass