High energy neutron exposure results in accumulation of many defects like point defects (vacancies and interstitials) and dislocations, and causes redistribution in the chemistry (phase change or radiation-induced segre­gation (RIS)) in the materials. These modifications lead to deterioration in mechanical and corrosion properties of the exposed materials. The micro­scopic defects produced in materials due to irradiation are referred to as radiation damage. The crystal defects thus produced modify the macro­scopic properties (physical, thermal and mechanical) of materials which are referred to as radiation effects.

The high energy neutron knocks out a stationary atom from its equilib­rium position and transfers some kinetic energy (KE) to it which in turn displaces more atoms to cause a cascade effect, resulting in a number of interstitials leading to Frenkel defects.5 This process continues until the energy of all primary and secondary knock-on atoms is insufficient (<25 eV) to displace those from the lattice sites. The atomic displacements per atom (dpa), defined as the number of times each atom is displaced from the lat­tice site, is estimated using various models, among which the Kinchin-Pease model is often used:

ЛЕ 4 A

dpa = o-el n Ф, with Л = , [1.1]

el 4Ed (1 + A)2’ L J

where Ф = 0t is the fluence, ф is neutron flux (/m2s), t is irradiation time, En is the mean neutron energy, Ed is the atomic displacement energy, A is atomic mass (in amu) and oe is the elastic neutron scattering cross section. This dpa is a better damage evaluation unit than the commonly used fluence (Ф) since it takes into account the spectral distribution of neutron energy in a given reactor.5 This process continues until the KE falls below that needed to cause further displacements. The interstitials thus formed segregate as small disc shaped clusters.6 They can either dissolve by vacancy emission or coalesce to large nano-voids. Clearly, the creation of excess point defects not only changes the physical dimensions due to a reduction in density but also enhances the diffusion kinetics and the phenomena controlled by atomic diffusion such as creep, while the production of line, areal and volumetric defects result in radi­ation hardening and embrittlement. In RPV steels, the excess vacancies pro­duced by irradiation favour long range diffusion of copper atoms to form a copper-rich-phase, which has a BCC structure and is coherent with the steel matrix. This phase gets enriched in iron, nickel and manganese, and increases in volume.7 These phenomena pertaining to RPV steels are further discussed in detail in a later chapter of the book. Before discussing the radiation effects a brief description of various materials properties and phenomena is presented. It should be noted that materials issues specific to various reactor compo­nents as well as detailed descriptions on various phenomena are included in specific chapters later in the book: Part I covers various fundamental materi­als phenomena and Part II covers reactors and components.