More than
500 nuclear power plants operate around the world. Close to 300 operate
pressurized water reactors (PWRs), more than 100 are built with boiling-water
reactors (BWRs), about 50 use gas-cooled reactors, and the rest are heavy-water
reactors.
The steam separators
are above the core, and the steam dryers are at the top of the reactor. The
reactor is enclosed by a concrete dome.
In addition
a few fast breeder reactors are in operation. These reactors are built for
better utilization of uranium fuel. The modern nuclear plant size varies from
100 to 1200 MW.
Pressurized
Water Reactor
The general
arrangement of a power plant with a PWR is shown in Fig. 59.8(A). The
Reactor heats
the water from about 550 to about 650° F. High pressure, at about 2235 psi,
prevents boiling.
Pressure is
maintained by a pressurizer, and the water is circulated by a pump through a
heat exchanger. The heat exchanger evaporates the feedwater and generates
steam, which supplies a system similar to a conventional power plant.
The
advantage of this two-loop system is the separation of the potentially
radioactive reactor cooling fluid from the water-steam system. The reactor core
consists of fuel and control rods. Grids hold both the control and fuel rods.
The fuel
rods are inserted in the grid following a predetermined pattern. The fuel
elements are Zircaloy-clad rods filled with UO 2 pellets. The control rods are
made of silver (80%), cadmium (5%), and indium (15%) alloy protected by
stainless steel.
The reactor
operation is controlled by the position of the rods. In addition, control rods
are used to shut down the reactor. The rods are released and fall in the core
when emergency shutdown is required.
Cooling
water enters the reactor from the bottom, flows through the core, and is heated
by nuclear fission.
Boiling-Water
Reactor
In the BWR
shown in Fig. 59.8(B), the pressure is low, about 1000 psi. The nuclear
reaction heats the water directly to evaporate it and produce wet steam at
about 545°F.
The
remaining water is recirculated and mixed with feedwater. The steam drives a
turbine that typically rotates at 1800 rpm. The rest of the plant is similar to
a conventional power plant.
A typical
reactor arrangement is shown in Fig. 59.9. The figure shows all the major
components of a reactor. The fuel and control rod assembly is located in the
lower part.
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