The Pebble Bed Modular Reactor is a nuclear power plant that derives its name from the type of fuel that it consumes and the incremental, modular fashion in which it can be built.
Pebble bed reactor technology uses spheres or ‘pebbles’ of coated uranium dioxide encased in graphite. When fully loaded the reactor holds about 452,000 fuel spheres each of which is 60cm in diameter.
The reactor itself is a vertical, cylindrical, steel pressure vessel approximately 27m high and 6.2m in inner diameter. The nuclear reaction that takes place in the reactor is cooled by helium gas, which enters the top of the vessel at approximately 500°C, flows down between the spheres and leaves the bottom of the vessel having been heated to 900°C. This heated gas then passes through a gas turbine that drives an electricity generator.