The boiling water reactor is similar in design to the pressurized water reactor. Light water is used in the boiling water reactor, too, as coolant and moderator, but unlike the pressurized water reactor the water flows in a single circulation. At pressures up to approximately 75 bar the water already boils in the nuclear reactor. The weakly radioactive steam is fed directly to the turbine system. In the boiling water reactor the main coolant pumps are almost always located inside the reactor vessel (as opposed to the external arrangement

in the pressurized water reactor). They suck the water out of the annular chamber between the reactor core and the pressure vessel wall, and force it into the reactor core. Hence the name forced circulation pump. In the boiling water reactor all pumps – including their seals – involved indirectly or directly with the steam circulation are contaminated to a greater or lesser degree (radioactive). Mechanical seals are employed in all pumps in a boiling water reactor.