Positron annihilation study of neutron-irradiated nuclear reactor pressure vessel steels and model radioactive waste glasses

First we are concerned with the irradiation-induced changes in the microstructures of the nuclear reactor pressure vessel (RPV) steels: such as the formation of the solute nano-clusters (SNCs) and matrix damages (MDs) due to irradiation-induced point defect clusters and dislocation loops. The irradiation-induced embrittlement of the RPV is currently a vital issue for ensuring the safe operation of nuclear power plants and is caused by the irradiation-induced changes. Thus in this area, positron annihilation (PA) methods are expected to provide us with unique and detailed information on the SNCs and the MDs. We have integrated the PA results of the SNCs and MDs with those by three dimensional atom probe (3DAP) to clarify the mechanisms of the irradiation-induced hardening and embrittlement. We employed two kinds of RPV steels (A533B) of the first and second generation: the first one has high impurity contents of harmful elements of Cu, P and S, while the second one has been purified to reduce these impurities. Furthermore we briefly present new methods of positron annihilation to obtain number densities and sizes of embedded Cu nano-precipitates (ECNPs) in thermally aged model Fe-Cu alloys of RPV steels based on positron affinity trapping of the ECNPs: Age Momentum Correlation (AMOC) and Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) momentum smearing. Second we present positronium studies of intrinsic structural subnanovoids in model silica- based radioactive waste glasses. Dependence of the average sizes of the structural subnanovoids estimated from the pick-off annihilation rates of ortho-positronium (o-Ps) on the chemical composition is investigated. The average size decreases rapidly with increasing the mole fraction of the network modifier oxides, such as Na2O and K2O, because the cations in the oxides occupy selectively larger subnanovoids. This finding gives us useful information about the structural subnanovoids and suggests a possible tool for the study of the stability of the nuclear radioactive waste glasses during their long-term storage. (author)