The boiling water reactor (BWR) is a type of nuclear reactor where heavy water boils in the reactor core to produce the steam that drives the turbine. This water is employed both as a coolant and moderator. Because of the axially changing void fraction, the axial flux becomes asymmetric. Having dried in moisture separator, the steam flows directly to the turbine. The loop is completed when the steam is condensed. BWRs are larger than power water reactors and they operate at a lower pressure than them, usually at 7-8 MPa.
Boiling water reactors burn uranium oxide fuel rods, which are grouped in a 6×6 or 8×8-rod square lattice. Thus, the full assembly is smaller than in power water reactor. Reactor control is carried out using control rods, which are inserted from the bottom of the core. Boron carbide is employed as an absorber material. In some of the older BWR designs, external pumps were used to make the water flow. In the latest designs, however, internal pumps are utilized. Using external pumps avoid the risk of loss of coolant in case there is an external line break.
Boiling water reactors are manufactured with a leak-tight containment that is conceived to withstand the load from a large break in the coolant or steam system. Safety systems were designed to inject water directly into the reactor vessel to cool the fuel. Containment pressure rise is moderated through condensation in water-filled areas. There is also an additional cooling system that sprays the chamber surrounding the reactor vessel.
BWR was first designed and developed by General Electric in the United States of America. The first commercial BWR, Dresden, sold to the Commonwealth Edison Company, was a 200 MW power plant commissioned in 1960. Over the years, ratings have been increased up to 1,300 MW plants, which are in operation today.
Below, diagram of a boiling water reactor
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