Environmental Engineering Reference
reaction. The nuclear fuel, such
as U-235, is typically placed in
long rods. The rods are grouped
together in bundles called fuel
assemblies. There are also other
long rods, known as control rods,
made of a material that absorbs
neutrons. The control rods can
be inserted into the core and
removed as needed. If a chain
reaction starts to go too fast,
some of the control rods can be
inserted. They absorb some of
the neutrons l ying around in
the core, so that the neutrons
are not available to cause atoms
to undergo i ssion. If the reactor
power needs to be increased, the control rods can be pulled out.
This makes more neutrons available to cause i ssion.
Many reactor cores also contain a substance called a
moderator . This slows down the l ying neutrons to speeds low
enough that U-235 nuclei can capture them. In some reactors,
the moderator also serves as a coolant , keeping the core
from getting too hot and melting. Many reactors use water as
a moderator. Graphite is another substance that is sometimes
used as a moderator.
Uranium-235 is the most
widely used fuel for nuclear
power plants. Tiny amounts
of uranium occur nearly
everywhere on Earth. Some
countries have sizable
deposits of uranium ore.
About three-fi fths of the
uranium mined in the world
in 2007 came from Canada,
Australia, and Kazakhstan.
Obtaining Nuclear Fuel
U-235 is the only i ssionable form of uranium found in nature.
Natural uranium, however, has very little U-235. Instead,