Nuclear Materials and Irradiation Effects

Abstract

Two types of materials are selected by the nuclear industry to be used in nuclear reactors: either materials having specific nuclear properties, or standard engineering alloys corresponding to the thermomechanical loadings and environment. The first class corresponds to the fuels neutron absorbing isotopes or alloys of low neutron capture cross sections. These atomic properties do not preclude the chemical state of the species, and the best chemical state could be selected (e.g., B, a neutron absorber, can be used in water solution as boric acid or as refractory carbide: B4C). The second class corresponds to alloys, such as structural steels, stainless steels (SS), aluminum alloys, etc. Few specific alloys have been developed for particular applications, such as Zr alloys in water reactors or vanadium alloys in fusion devices. All these materials have to support the environment of a nuclear reactor. In addition to standard engineering constrains (mechanical loadings, corrosion, high temperatures, etc.), the irradiation itself induces major changes in structure, properties, and behavior of all the materials. Irradiation damage includes chemical changes induced by irradiation, with the specificity of in situ He formation, by (n, α) reactions, promoting swelling. However, the major mechanism of irradiation damage is mostly due to elastic interaction of neutrons with the atoms, leading to displacement cascades and generation of point defects (PD). The migration and clustering of these PD induce major changes in microstructure with corresponding changes in behavior. Irradiation hardening, reduction of ductility, irradiation creep and growth, and swelling are described in detail in their physical mechanisms and their specific characteristics for the alloys and ceramics for current use and of future interest. Other irradiation effects such as radiolysis on water, or changes in electrical properties of insulating ceramics are also described. After a generic description of the physics of the transformations induced in the microstructures by irradiation, the phenomena of major concern are presented for the different components of various reactors. The corresponding conditions are analyzed for various types of experimental reactors, power reactors (thermal and breeder), and fusion devices.