Reactor Pressure Vessel Steel Degradation Studied by Mössbauer and Positron Annihilation Spectroscopy

The work applies Mössbauer spectroscopy (MS) and positron annihilation spectroscopy (PAS) to evaluate the microstructure parameters of materials. These methods are used for collecting additional characteristics for understanding the degradation processes in reactor pressure vessel steels. Samples from the Russian 15Kh2MFA and Sv10KhMFT steels, commercially used at WWER-440 nuclear reactors, were irradiated near the core at NPP Bohunice (Slovakia) to neutron fluences in the range 7.8x10^23 m^-2 to 2.5x10^24 m^-2. Systematic changes in the MS and PAS spectra were observed mainly during the early period of irradiation. These could be due to changes caused by precipitation of elements like Cu, P or Cr, mainly in carbides, on the surface. The MS results confirm that the close environment of Fe atoms in the b.c.c. lattice of RPV steels remains almost stable after initial changes and can be correlated with the ductile-brittle transition temperature curve from mechanical tests. Positron annihilation lifetime measurements using the Pulsed Low Energy Positron System (PLEPS) were applied for investigation of defects of irradiated and thermally treated reactor pressure vessel (RPV) steels. PLEPS results showed that the changes in the microstructure of the RPV-steel properties caused by neutron irradiation and post-irradiation thermal treatment can be detected in the surface as well as the bulk region.