Mossbauer and positron annihilation spectroscopy applied to WWER-1000 RPV steels

Mossbauer spectroscopy (MS) and Positron annihilation spectroscopy (PAS) were applied in the evaluation of the microstructure parameters and degradation processes of nuclear reactor pressure vessel (RPV) steel surveillance specimens. Study was oriented to the material investigation of Russian WWER-1000 steels (15Kh2MNFAA and 12Kh2N2MAA) with higher Ni content (1.26 wt.% in base metal and 1.7 wt.% in weld). For comparison, the WWER440 weldmetal (Sv10KhMFT) without Ni was measured too. Specimens were studied in as received form, after irradiation in LVR-15 experimental reactor to the neutron fluence F-(E > 0.5 Mev) = 1.47 x 10(14) m(-2) s(-1) and after annealing process in vacuum at 475 degrees C/2h. Changes due to different chemical composition and due to irradiation were registered using MS. Post-irradiation thermal treatment and annealing of defects was well detected by different PAS techniques. Results confirm the hypothesis that Ni affects size (decrease) and distribution (more homogeneous) of the Cu- and P-rich clusters and MxCx carbides.