Introduction to Basic Manufacturing Processes and ... - always yours

Non-Ferrous Materials 99
vessel materials. Uranium, thorium, plutonium, zirconium, beryllium, niobium and their
alloys are primarily used for nuclear engineering applications.
5.16.1 Uranium
Most important metal found in nature and used for nuclear energy is uranium. It is used as
a nuclear fuel and is radioactive, easily oxidized mat and exists in three allotropic forms. It
has a poor resistance to corrosion and needs to be protected for use as fuel elements by roll
cladding a thin aluminium or zirconium jacket. Therefore it is strengthened by a thin film
of aluminium or zirconium through a rolling process known as cladding. This metal in the
pure condition is weak and is susceptible to severe irradiation damage and growth in the
reactor environments. Addition of some alloying elements such as chromium, molybdenum,
plutonium and zirconium are added to make great use of this material for generating nuclear
power. Uranium compounds have been found to give better service. Uranium oxide is re
highly refractory, shows no phase change in an inert atmosphere, possesses a good strength
and a high corrosion resistance. But it has a low thermal shock resistance, poor thermal
conductivity and a high coefficient of in expansion. The melting point of uranium is 1850°C.
5.16.2 Thorium
Thorium is another possible fuel and is also a radioactive metal like uranium. It is a nuclear
fuel and is free from phase changes below 1480°C. It can be converted into uranium by beta
decay. It can be converted into uranium by beta decay. In its pure form it is weak and soft,
but its mechanical properties are improved by adding small amount of other elements e.g.,
only 0.2% of carbon addition raises its tensile strength from. Its mechanical properties change
drastically by small addition of impurities. Small additions of titanium, zirconium, and niobium
decrease the strength and hardness of the metal. Uranium addition increases the strength
of thorium. Thorium like uranium is an emitter of alpha rays and releases considerable
quantity of the radioactive products during processing, but this being a cubic metal is less
susceptible to irradiation damage. The melting point of thorium is 1845°C.
5.16.3 Plutonium
Plutonium is not a natural element but synthetic. It is produced through the absorption of
neutrons by uranium 238 and subsequent beta decays. It is a concentrated type of fuel used
in a fast reactor without a moderator. It is identified as highly toxic and chemically very
reactive material. It has a poor resistance to corrosion. This material can be easily oxidized.
It is frequently used for production of atomic weapons and it possesses tremendous applications
in breeder reactors.
5.16.4 Zirconium
The natural form of zirconium contains 0.5 to 2% halfnium which is a strong absorber of
neutrons and must therefore be removed. It possesses excellent corrosion resistance against
most of the acids and having good strength, good weldability and fatigue resistance. It has a poor
resistance to CO 2
at elevated temperatures and that can be further improved by adding 0.5%
Cu and 0.5% Mo. It is specifically used in gas cooled reactors and cladding of fuel elements.
5.16.5. Beryllium
Beryllium is specially used as a moderator, reflector and neutron source in nuclear reactors.
It is a coarse grained light and brittle metal which possesses highly developed texture. During