The main categories of nuclear reactor are:
Pressurised Water Reactor (PWR) - the most dominant form of nuclear power generation system. It does not boil the water so uses a steam generator.
Boiling Water Reactor (BWR) - this system uses light water as a coolant and enriched uranium as a fuel
Heavy water moderated reactors - this uses heavy water as the coolant instead of light water. This system does not need to use enriched fuel so has a more effective use of fuel
Gas-Cooled Reactors - This is an older type of reactor and no new gas-cooled power reactors have been built for over 20 years.
Fast Reactors - they do not have a moderator and have a high utilisation of mined uranium
Thorium Reactors - Future reactors could be fuelled with thorium which is an abundant nuclear fuel which could provide even longer-term security of energy supply.