There are many different kinds of materials used in the construction of a nuclear facility. Once constructed, these materials are subjected to environments and operating conditions that may lead to material problems. This chapter discusses considerations for selection and application of plant materials.

EO 1.1DEFINE the following terms:

a. Machinability c. Stability b. Formability d. Fabricability

EO 1.2IDENTIFY the importance of a material property and its application in a reactor plant.

Overview

During the selection and application of materials used for construction of a nuclear facility, many different material properties and factors must be considered depending upon the requirements for each specific application. Generally, these consist of both non-fuel reactor materials, used for structural and component construction, and fuel materials. This chapter discusses some of the considerations used in the selection process for plant materials including material properties, fuel, fuel cladding, reflector material, control materials, and shielding materials.

Material Properties

The following properties are considered when selecting materials that are to be used in the construction of nuclear facilities.

Machinabilitv

Components may be formed by removing metal "chips" by mechanical deformation. This process is referred to as machining. Machinability describes how a metal reacts to mechanical deformation by removing chips, with respect to the amount of metal effectively removed and the surface finish attainable. The mechanical properties of the metal will be the factors that influence the machinability of a metal.

Many components used in nuclear reactor construction use machined parts that require very close tolerances and very smooth surfaces. Thus, machinability becomes an important consideration when choosing materials for manufacturing these parts.

Formabilitv

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.

Ductilitv

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 Fracture.

Stability

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.

Availability

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.

Fabricability

Fabricability is a measure of the ease with which a material can be worked and made into desirable shapes and forms. Many components of a nuclear reactor have very complicated shapes and forms and require very close tolerances. Therefore, fabricability is an important consideration in the manufacturing of these components.

Heat Transfer

Good heat transfer properties are desirable from the fuel boundary to the coolant in order that the heat produced will be efficiently transferred.

For a constant amount of heat transfer, a degraded heat transfer characteristic requires higher fuel temperature, which is not desirable. Therefore, desirable heat transfer properties in the selection of reactor materials, especially those used as core cladding and heat exchanger tubes, are a major consideration.

Cost

Capital costs for building a typical nuclear facility can be millions of dollars. A major portion of the cost is for plant material; therefore, cost is an important factor in the selection of plant materials.