Materials for Nuclear Engineering
Nuclear engineering is the branch of engineering concerned with the application of the nuclear fission as well as the nuclear fusion and the application of other sub-atomic physics, based on the principles of nuclear physics. In general, nuclear engineering deals with the application of nuclear energy in a variety of branches, including nuclear power plants, naval propulsion systems, food production or medical diagnostic equipment such as MRI machines.
Our goal here will be to introduce material engineering of nuclear reactors. The knowledge of thermophysical and nuclear properties of materials is essential for designing nuclear power plants.
In general, there are two basic types of materials, that are used in nuclear power plant.
- Materials having specific nuclear properties. These materials must fulfill very specific requirements that originates especially in interactions of atomic nuclei. This class corresponds to the nuclear fuels, neutron absorbing materials or alloys of low neutron capture cross-sections. For these materials proper nuclear properties are a priority and the best chemical state (atomic properties) could be selected (e.g., boron as a neutron absorber can be used in water solution as boric acid or as boron carbide – B4C in control rods).
- Standard engineering materials. These materials does not differ from material used in other engineering branches. It must be added nuclear engineering puts higher requirements on quality and reliability of all materials than in other engineering branches. This class corresponds, for example, to alloys, such as structural steels, stainless steels, aluminum alloys, etc. Few specific alloys have been developed for particular applications, such as Zr alloys in water reactors, which belong also to the materials having specific nuclear properties.
Materials essential for designing nuclear power plants can be divided into the following groups:
- Nuclear Fuels. Nuclear fuel is generally any material that can be ‘burned’ by nuclear fission to derive nuclear energy. Common nuclear reactors use an enriched uranium and plutonium as a fuel. Most of PWRs use the uranium fuel, which is in the form of uranium dioxide, but other fuels and matrices are developed.
- Neutron Moderators. The moderator, which is of importance in thermal reactors, is used to moderate, that is, to slow down, neutrons from fission to thermal energies. Commonly used moderators include regular (light) water (roughly 75% of the world’s reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). Beryllium and beryllium oxide (BeO) have been used occasionally, but they are very costly.
- Neutron Absorbers. The materials that absorb neutrons are used in reactor core in the following three cases:
- In control rods, that are an important safety system of nuclear reactors.
- As burnable absorbers, which can be dispersed uniformly in fuel or placed in certain sections.
- As additives to a moderator for compensation of an excess reactivity.
- Coolants. In nuclear power plant, water and steam are a common fluids used for heat exchange in the primary circuit (from surface of fuel rods to the coolant flow) and in the secondary circuit. But many other materials can be used for this purpose. In power reactors, carbon dioxide, heavy water, helium of liquid metals can be used.
- Structural Materials. Many various materials are used in the designs of nuclear reactors. Some materials must have lower neutron capture cross-section, especially inside a reactor core, where fission chain reaction takes place. On the other hand there are many materials, that must have higher capture cross-sections. In this section, the basic physical properties of some structural materials (especially steels, alloys and concretes) under normal conditions will be discussed.
- Radiation Shielding. Radiation shielding usually consist of barriers of lead, concrete or water. There are many many materials, which can be used for radiation shielding, but there are many many situations in radiation protection. It highly depends on the type of radiation to be shielded, its energy and many other parametres. For example, even depleted uranium can be used as a good protection from gamma radiation, but on the other hand uranium is absolutely inappropriate shielding of neutron radiation.