The Foundations of Chemistry

Light Water Reactors

Most commercial nuclear power plants in the United States are “light water” reactors,
moderated and cooled by ordinary water. Figure 26-12 is a schematic diagram of a light
water reactor plant. The reactor core at the left replaces the furnace in which coal, oil,
or natural gas is burned in a fossil fuel plant. Such a fission reactor consists of five main
components: (1) fuel, (2) moderator, (3) control rods, (4) cooling system, and (5) shielding.

Fuel

Rods of U 3 O 8 enriched in uranium-235 serve as the fuel. Unfortunately, uranium ores
contain only about 0.7%^235092 U. Most of the rest is nonfissionable^238092 U. The enrichment
is done in processing and reprocessing plants by separating gaseous^235 UF 6 from^238 UF 6 ,
prepared from the ore. Separation by diffusion is based on the slower rates of diffusion
of heavier gas molecules (Section 12-14). Another separation procedure uses the ultra-
centrifuge.
A potentially more efficient method of enrichment would involve the use of sophisti-
cated tunable lasers to ionize^235092 U selectively and not^238092 U. The ionized^235092 U could then
be made to react with negative ions to form another compound, easily separated from the
mixture. For this method to work, we must construct lasers capable of producing radia-
tion monochromatic enough to excite one isotope and not the other—a difficult challenge.

Moderator

The fast neutrons ejected during fission are too energetic to be absorbed efficiently by
other nuclei. Thus, they must be slowed by collisions with atoms of comparable mass that
do not absorb them, called moderators.The most commonly used moderator is ordinary
water, although graphite is sometimes used. The most efficient moderator is helium, which

26-15 Nuclear Fission Reactors 1027

Molten sodium

or

Liquid water

under high

pressure

(carries heat

to steam generator)

Nuclear

reactor

Control rod

Uranium fuel

Pump

Steam

generator

Steam Steam turbine

(generates electricity)

80 ° F 100 ° F River

Pump

Condenser

(steam from turbine

is condensed

by river water)

P

P

P

+

Pump

Figure 26-12 A schematic diagram

of a light water reactor plant. This

design includes two closed loops of

water. The water that carries heat

from the reactor to the steam

generator is in a closed loop and is

not released to the environment.

Uranium is deposited on the

negative electrode in the

electrorefining phase of fuel

reprocessing. The crystalline mass

is about 97% LiCl and KCl. The

remaining 3% uranium chloride is

responsible for the amethyst color.