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FIGURE 13.19 The turbine deck of a nuclear generating
station. There is one large generator in line with four steam turbines FIGURE 13.20 Spent fuel rod assemblies are removed and
in this nonnuclear part of the plant. The large silver tanks are new ones are added to a reactor head during refueling. This shows
separators that remove water from the steam after it has left the an initial fuel load to a reactor, which has the upper part removed
high-pressure turbine and before it is recycled back into the low- and set aside for the loading.
pressure turbines.

amount of usable uranium and plutonium. For now, spent reac-
substance that slows neutrons so they are more readily absorbed tor fuel rods are mostly stored in cooling pools at the nuclear
by uranium-235 nuclei. Other reactor designs use heavy water plant sites. In the future, a decision will be made either to repro-
(dideuterium monoxide) or graphite as a moderator. cess the spent fuel, recovering the uranium and plutonium
Water from the closed primary loop is circulated through through chemical reprocessing, or to put the fuel in terminal
a heat exchanger called a steam generator (Figure 13.18). storage. Concerns about reprocessing are based on the fact that
The pressurized high-temperature water from the reactor plutonium-239 and uranium-235 are fissionable and could
moves through hundreds of small tubes inside the generator possibly be used by terrorist groups to construct nuclear explo-
as feedwater from the secondary loop flows over the tubes. sive devices. Six other countries do have reprocessing plants,
The water in the primary loop heats feedwater in the steam however, and the spent fuel rods represent an energy source
generator and then returns to the nuclear reactor to become equivalent to more than 25 billion barrels of petroleum. Some
heated again. The feedwater is heated to steam at about 235°C energy experts say that it would be inappropriate to dispose of
2
(455°F) with a pressure of about 68 atmospheres (1,000 lb/in ). such an energy source.
This steam is piped to the turbines, which turn an electric
generator (Figure 13.19).
After leaving the turbines, the spent steam is condensed
back to liquid water in a second heat exchanger receiving
water from the cooling towers. Again, the cooling water does
U-238 U-235
not mix with the closed secondary loop water. The cooling- 97% 3%
tower water enters the condensing heat exchanger at about
32°C (90°F) and leaves at about 50°C (about 120°F) before
returning to a cooling tower, where it is cooled by evapora-
tion. The feedwater is preheated, then recirculated to the
A
steam generator to start the cycle over again. The steam is
condensed back to liquid water because of the difficulty of Reaction
pumping and reheating steam. products 4.01%
After a period of time, the production of fission products in U-235 0.8%
the fuel rods begins to interfere with effective neutron transmis- U-238 Pu 0.89%
94.3%
sion, so the reactor is shut down annually for refueling. Dur-
ing refueling, about one-third of the fuel that had the longest
exposure in the reactor is removed as “spent” fuel. New fuel
rod assemblies are inserted to make up for the part removed
(Figure 13.20). However, only about 4 percent of the “spent” B
fuel is unusable waste, about 94 percent is uranium-238, FIGURE 13.21 The composition of the nuclear fuel in a
0.8 percent is uranium-235, and about 0.9 percent is plutonium fuel rod (A) before and (B) after use over a three-year period in a
(Figure 13.21). Thus, “spent” fuel rods contain an appreciable nuclear reactor.

13-19 CHAPTER 13 Nuclear Reactions 341