112 Electrical Power Systems Technology
tor as the number of neutrons is increased. If a reactor is left uncontrolled,
it may destroy itself. Moderating a reactor, therefore, means controlling the
multiplication of neutrons in the reactor core. There are several methods
used for moderating nuclear reactors.
Boiling-Water Reactor (BWR)—Water is a popular coolant for reactors.
In this type of reactor, shown in Figure 4-18, water is pumped into the re-
actor enclosure. The water is then converted into steam, which is deliv-
ered to a steam turbine. The water also serves as the moderator material
of the reactor.
Pressurized-Water Reactor (PWR)—The pressurized-water reactor,
shown in Figure 4-19 is similar to a boiling-water reactor, except that the
coolant water is pumped through the reactor under high pressure. Steam
is produced in an adjacent area from a separate stream of water, which is
pumped through the steam-production system. Just as in the BWR, the
water within the reactor serves as the moderator.
High-temperature Gas-cooled Reactor (HTGR)—The high-temperature
gas-cooled reactor, shown in Figure 4-20 uses pressurized helium gas to
transfer heat from the reactor to a steam-production system. The advan-
Figure 4-18. Simplified diagram of a boiling water reactor (BWR)