BOILING WATER REACTORS
In a typical boiling water reactor the reactor core creates heat and a single loop both
delivers steam to the turbine and returns water to the reactor core to cool it. The cooling
water is force-circulated by electrically powered pumps. Emergency cooling water is supplied
by other pumps, which can be powered by onsite diesel generators. Other safety systems, such
as the containment building air coolers, also need electric power. Electricity is needed to
operate the reactor and its support structures; and the source of their electricity, like
your own, is generated off-site, the power grid.
In the figure above, water is circulated through the Reactor Core picking up heat as the
water moves past the fuel assemblies. The water eventually is heated enough to convert to
steam. Steam separators in the upper part of the reactor remove water from the steam.
The steam then passes through the Main Steam Lines to the Turbine-Generators. The steam
typically goes first to a smaller High Pressure (HP) Turbine, then passes to Moisture
Separators (not shown), then to the 2 or 3 larger Low Pressure (LP) Turbines. The Generator
produces the electricity. This electrical power is then distributed to a Generator
Transformer. Then the power is distributed to a switchyard or substation where the power is
then sent offsite.
The steam, after passing through the turbines, then condenses in the Condenser, which is at a
vacuum and is cooled by ocean water. The condensed steam then is pumped to Low Pressure
Feedwater Heaters (shown but not identified). The water then passes to the Feedwater Pumps
which in turn, pump the water to the reactor and start the cycle all over again.
The Control Rods, used to shutdown the reactor and maintain a uniform power distribution
across the reactor, are inserted from the bottom by a high pressure hydraulically operated
system. The BWR also has a Torus (shown above) or a Suppression Pool. The torus or
suppression pool is used to remove heat released if an event occurs in which large quantities
of steam are released from the reactor or the Reactor Recirculation System, used to circulate
water through the reactor. In addition, Pilgrim installed a Torus Vent to release radioactive
high pressure steam generated in a severe accident by allowing the unfiltered release
directly to the atmosphere through the 300 foot vent stack.
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