Nuclear plants require radioactive material to operate. Certain metals that are
radioactive can be used to produce and sustain the nuclear reaction. This chapter
discusses the materials used in the various nuclear applications. The student
should refer to the Nuclear Physics and Reactor Theory Fundamentals Handbook
prior to continuing to better understand the material in this chapter.
LIST the four radioactive materials that fission by thermal neutrons
and are used as reactor fuels.
STATE the four considerations in selecting fuel material and the
desired effect on the nuclear properties of the selected fuel
Overview of Material Types
The reactor core is the heart of any nuclear reactor and consists of fuel elements made of a
suitable fissile material. There are presently four radioactive materials that are suitable for fission
by thermal neutrons. They are uranium-233 (233U), uranium-235 (235U), plutonium-239 (239Pu),
and plutonium-241 (241Pu). The isotopes uranium-238 (238U) and thorium-232 (232Th) are
fissionable by fast neutrons. The following text discusses plutonium, uranium, and thorium as
used for nuclear fuel.
Plutonium is an artificial element produced by the transmutation of 238U. It does exist in small
amounts (5 parts per trillion) in uranium ore, but this concentration is not high enough to be
Plutonium is produced by the conversion of 238U into 239Pu according to the following reaction.
U + 1o n 239
This reaction occurs in reactors designed specifically to produce fissionable fuel. These reactors
are frequently called breeder reactors because they produce more fissionable fuel than is used in
the reaction. Plutonium is also produced in thermal 235U reactors that contain 238U. Plutonium
can be obtained through the processing of spent fuel elements. To be useful as a fuel, plutonium
must be alloyed to be in a stable phase as a metal or a ceramic.