Positron Annihilation Spectroscopy Study On Irradiation Damage Of Chinese Domestic Reactor Pressure Vessel Steels

The irradiation-induced embrittlement of a reactor pressure vessel (RPV) steel is one of the vital issues for ensuring safe operation of the plants. The lifetime of the RPV correlates with that of the plants because the RPV is irreplaceable. Hence, it is necessary to predict the future embrittlement of the RPV more precisely. In addition, The safety operation of the nuclear power plants is strongly required after the Great East Japan Earthquake. Therefore, understanding of the mechanism of irradiation-induced embrittlement is particularly important.Embrittlement of the RPV steels is considered to be mainly caused by irradiation-induced changes in the microstructure as the formation of matrix defect(MD)(due to irradiation-induced point defect clusters), solute cluster (SC) and grain boundary segregation. These microstructural changes result in hardening of the steels through obstructing the motion of dislocations, thereby causing degradation of the fracture properties, such as the ductile-to-brittle transition temperature shift. Hence, understanding of the correlation between these microstructural changes and the mechanical property changes, such as hardening, is very necessary,In the present study, both proton and iron ions irradiation were carried out at room temperature as a primary knocked atom to induce sufficient matrix defects and solute cluster, the different ions irradiation-induced microstructural evolutions of the RPV A508-3steels and Fe-Cu alloys were investigated using a combination of Positron Annihilation Spectroscopy (PAS), Three-dimensional Atom Probe (3D-AP) and Transmission Electron Microscopy (TEM)?The main contents and results as follows:In the results of110keV and240keV proton-irradiated RPV steels, the S parameter increases evidently with dose after irradiation, means the number of bigger vacancy-type defects matrix damage increased rapidly with increasing dose. The LINAC (Linear Accelerator)-based slow positron beams annihilation lifetime confirmed that number of vacancies increase from V6to V10or more than V10with higher dose. That means the proton bombardment caused the formation of larger defect agglomerations, both the density and the average size increased with dose. In contrast, MD showed a trend of saturation rapidly which caused by a high indensity of V10at0.05dpa dose for3MeV iron ion irradiated steels. It is suggested that proton irradiation induces the migration of vacancies and agglomeration to vacancy clusters while larger size vacancy clusters with a high density can be easily induced at low dose by iron irradiation. However, there is no saturation even through the value of Sd parameter of proton irradiation is bigger than Fe13+ion irradiation at high dose.Furthermore, neither proton irradiation nor Fe13+ion irradiation induced evident precipitates, clusters or other forms of solute segregation observed by3D-AP and TEM. Combined the results to PAS, it clearly indicates that the defects induced by ions irradiation under room temperature mostly are matrix damages, rather than solute-clusters.In addition, Microhardness increases with dose level for the proton irradiated steels, which corresponds to irradiation hardening. That indicates matrix damages are the main reason which cause the embrittment of A508-3steels under ions irradiation.Pure Fe and Fe-Cu alloys were also investigated by PAS with proton irradiation at room temperature. Compared to the results of A508-3steels, the Sav parameter of alloys is bigger than A508-3steels, indicates that solute atom in A508-3steels will restrain the formation of matrix damages, especially the Cu-rich Solute Cluster (CRSC). Positrons are trapped at the vacancies decorated with Cu atoms (V-Cu complexs) in Cu-riched alloys under irradiation, which can induce the S parameter decrease.RPV A508-3steels and Fe-1.0%Cu alloys were thermally aged at375?and550?for several hours, respectively. And then studied the evolution of Cu precipitates by the coincidence Doppler broadening (CDB). The CDB results show that the CRSC will increase at first hours aging, while it decreases at a longer aging time, the reason is beacuse the the Cu precipitates which are too large to trap positrons.