4.05.4.4.1 Introduction

The purpose of this subsection is to establish the level of understanding regarding the irradiation-induced formation of CECs, and how this understanding sup­ports the mechanistic framework outlined above. Insight has been developed by characterizing popula­tions of irradiation-induced CECs in Cu-containing steels and, in particular, the dependence of CEC structure, composition, size, and number density on material and irradiation parameters (e. g., steel com­position, fluence, flux, etc.). Measurements have also been made on the level of Cu in the matrix (Cumatrix) which is not associated with any CEC or precipitate. This is an important measure as, clearly, Cumatrix will decrease during irradiation as CECs are formed, and at SOL it may be lower than at the bulk level if Cu is precipitated during the final heat treatment.

Several techniques have been successfully em­ployed to characterize the CECs formed during neu­tron irradiation of Cu-containing pressure vessel
steels (Table 6). TEM was first used to observe CECs, but the cluster sizes were close to the resolu­tion limit for most TEMs, so atom probe (3DAP) and small angle neutron scattering (SANS) became the most commonly used methods to characterize CECs. However, these are not the only techniques; for example, an important development has been the advent of experiments employing a positron annihi­lation technique, coincidence Doppler broadening (CDB) spectra of positrons annihilating in aged or irradiated Cu-containing alloys or steels. CDB pro­vides a means of identifying the elements around the annihilation site.52 It has become the standard practice to characterize the same as-irradiated specimens with a number of techniques.53,54 Part of the logic behind this is that all the techniques have limitations, either in terms of volume analyzed or complexity of interpret­ing experimental data, and that combining techniques provides better information. These microstructural techniques have been discussed in greater detail in English and Hyde.55,56