PROPERTIES CONSIDERED WHEN SELECTING MATERIALS
Components may be formed by processes such as rolling or bending, which may cause
some parts of the metal to expand more than others. Formability of a material is its
ability to withstand peripheral expansion without failure or the capacity of the material
to be to manufactured into the final required shape. This becomes important in selecting
materials that have to be made into specific shapes by such means as rolling or bending
and still retain their required strength.
Ductility is the plastic response to tensile force. Plastic response, or plasticity, is
particularly important when a material is to be formed by causing the material to flow
during the manufacture of a component. It also becomes important in components that
are subject to tension and compression, at every temperature between the lowest service
temperature and the highest service temperature. Ductility is essential for steels used in
construction of reactor pressure vessels. Ductility is required because the vessel is
subjected to pressure and temperature stresses that must be carefully controlled to
preclude brittle fracture. Brittle fracture is discussed in more detail in Module 4, Brittle
Stability of a material refers to its mechanical and chemical inertness under the conditions to
which it will be subjected. Nuclear plants have a variety of environments to which materials
are subjected. Some of these environments, such as high temperatures, high acid, high
radiation, and high pressure, can be considered extreme and harsh; therefore, the stability of
the materials selected for service in these areas is a major consideration.
Corrosion mechanisms can become very damaging if not controlled. They are identified
in Module 2, Properties of Metals. High corrosion resistance is desirable in reactor
systems because low corrosion resistance leads to increased production of corrosion
products that may be transported through the core. These products become irradiated and
contaminate the entire system. This contamination contributes to high radiation levels after
shutdown. For these reasons, corrosion resistant materials are specially chosen for use in
the primary and secondary coolant systems.
The availability of a material used in the construction of nuclear plants refers to the ease
with which a material can be obtained and its cost.