Application of Mossbauer spectroscopy and positron annihilation spectroscopy for testing of neutron-irradiated reactor steels

The study is focused on the application of Mossbauer spectroscopy (MS) and positron annihilation spectroscopy (PAS) in the evaluation of the microstructure parameters of materials used in nuclear industry. The practical applications of these methods are documented on the evaluation of degradation processes currently in practise in nuclear power plant (NPP) reactor pressure vessel (RPV) steels. The samples originating from the Russian 15Kh2MFA and Sv10KhMFT steels, commercially used at WWER-440 reactors, were irradiated near the core at NPP Bohunice (Slovakia) with neutron fluences ranging from 7.8 x 10(23) m(-2) to 2.5x 10(24) m(-2). The systematic changes in the MS and PAS spectra were observed mainly during the first period (I-year stay in irradiation containers under operating conditions by "speed factor" of about 10). A possible explanation could be due to the changes caused by precipitation of elements like Cu or Cr mainly in carbides to the surface. These MS results confirm that the close environment of Fe atoms in b.c.c. lattice of RPV steels remains almost stable after initial changes and perhaps could be correlated with the ductile-brittle transition temperature curve from mechanical tests.