Schematic layout of a boiling water reactor (click to enlarge) (illustration: GRS)

Boiling water reactors have a closed water-steam cycle. In the cooling water system, the coolant water flows through the reactor core, where the heat in the fuel elements, produced through nuclear fission, heats the water to around 286 degree Celsius so that it boils in the reactor pressure vessel. There is relatively low pressure in the reactor pressure vessel (about 70 bar). Boiling and evaporation of the coolant water in the reactor pressure vessel is the main feature distinguishing the BWR from a pressurised water reactor (PWR).

The steam is then routed through the cyclones and steam dryer located in the upper plenum (i.e. the top) of the reactor pressure vessel, reducing its water content. The “dried” steam is then routed to the turbine, which is connected to a generator that converts its rotational energy into electricity. Behind the turbine, the steam is cooled and turned back into water in large condensers. It is then routed back to the reactor pressure vessel as feedwater.