Visualizing Environmental Science

Nuclear Energy 431

PROCESS DIAGRAM

There are two different nu-
clear reactions that release en-
ergy: fission and fusion. In nuclear
fission, the process nuclear power
plants use, energy is released when
a single neutron crashes into the
nucleus of a large atom, usually
uranium or plutonium. The contact causes the large atom
to break into smaller particles, releasing energy in the pro-
cess (ˆ}ÕÀiʣǰ£Î). In fusion, the process that powers
the sun and other stars, two small atoms are combined,
forming one larger atom of a different element.

Conventional Nuclear Fission

Uranium ore, the mineral fuel used in conventional nu-
clear power plants, is a nonrenewable resource present

in limited amounts in sedimen-

tary rock in Earth’s crust. Approx-

imately 11 percent of the world’s

uranium deposits are located in

the United States.

Uranium ore contains three

isotopes: U-238 (which makes

up 99.28 percent of uranium), U-235 (0.71 percent),

and U-234 (less than 0.01 percent). Because U-235, the

isotope used in conventional fission reactions, is such a

minor part of uranium ore, uranium ore must be refined

after mining to increase the concentration of U-235

to about 3 percent. This refining process, called

enrichment, requires a great deal of energy.

After enrichment, uranium is processed into small

pellets of uranium dioxide; each pellet contains the

enrichment The

process by which

uranium ore is

refined after mining

to increase the

concentration of

fissionable U-235.

fission The splitting

of an atomic

nucleus into two

smaller fragments,

accompanied by the

release of a large

amount of energy.

(^1) Uranium-235
(U-235) nucleus
undergoes
nuclear
bombardment.
(^2) U-235 nucleus splits into
two smaller radioactive
fragments and several
free neutrons.
(^3) Free neutrons bombard
nearby U-235 nuclei,
causing them to split.
(^4) More free
neutrons are
released in a
chain reaction.
Neutron
Uranium-235
Uranium-235
Atomic
fragment
Atomic
fragment
Atomic
fragments
Atomic
fragment Atomic
fragment
Free
neutron
Free
neutrons
Free
neutrons
Free
neutron
Energy
Energy
Energy
ENERGY
Nuclear fissionÊUʈ}ÕÀiʣǰ£Î
✓✓THE PLANNER
Starting at the left side of the figure, neutron bombardment^1 of a uranium-235 (U-235) nucleus causes it to
split into two smaller radioactive fragments^2 and several free neutrons^3. The free neutrons bombard nearby
U-235 nuclei, causing them to split and release still more free neutrons^4 in a chain reaction. Many different
pairs of radioactive atomic fragments are produced during the fission of U-235.