The figure shows the principle of a boiling-water reactor plant. Another figure shows a cross section of a Swedish boiling-water reactor plant.
The reactor tank contains the reactor's fuel, uranium, in the form of fuel rods collected into fuel elements. The fuel is cooled by water pumped up by the main circulation pump through the reactor core containing the fuel.
The water reaches boiling point in the reactor core and the water-steam mixture separates into water and steam in the steam separator and steam driers located above the reactor core. The dry steam is led to the turbine plant which drives the electricity generator.
The steam is condensed in the condenser which is cooled by sea water, river water, or water from a cooling tower. The condensate is led through the condensate preheaters, in which it is heated and finally pumped into the reactor tank again by means of the feed-water pumps.
The reactor's output is controlled by means of control rods inserted through the bottom of the reactor tank. The control rods absorb neutrons and thus slow down the chain reaction.
During ordinary operation output is regulated by the main circulation pumps using the self-stabilizing properties of the reactor core.
Boiling-water reactors in the west are provided with a leak-tight reactor containment and with safety systems that prevent the release of radioactive materials in an accident.
In recent years several proposals have been made for safer, so-called advanced reactors with passive safety systems. Two advanced boiling-water reactors are being built in Japan (1994).