Phase Characterization of (Mn, S) Inclusions and Mo Precipitates in Reactor Pressure Vessel Steel from Greifswald Nuclear Power Plant

This study presents a comprehensive analysis of the microstructural characteristics and chemical composition of base and weld materials from reactor pressure vessels in the first (units 1 and 2) and second (unit 8) generations of Russian VVER 440 reactors at the Greifswald nuclear power plant. We measured the specific activities of $^{60}$Co and $^{14}$C in activated samples from units 1 and 2. $^{60}$Co, with its shorter half-life (t$_{1/2}$ = 5.27 a), is a key dose-contributing radionuclide during decommissioning, while $^{14}$C (t$_{1/2}$ = 5700 a) plays an important role in a geological repository for low- and intermediate-level radioactive waste. Our findings reveal differences in the proportions of trace elements between the base and weld materials as well as between the two reactor generations. Microstructural analysis identified Mo-rich precipitates and (Mn, S)-rich inclusions containing secondary micro-inclusions in the unit 1 and 2 samples. Raman spectroscopy confirmed iron oxides (γ-Fe$_2$O$_3$, Fe$_3$O$_4$), silicates (Mn-SiO$_3$), and Cr$_2$O$_3$/NiCr$_2$O$_4$ in the base metal as well as MnFe$_2$O$_3$ in the weld metal. ... mehrX-ray photoelectron spectroscopy identified Mn inclusions as MnS, MnS$_2$, or mixed Mn, Fe sulfides, and the Mo precipitates as MoSi$_2$. These findings offer valuable insights into the speciation of elements and the potential release of radionuclides through corrosion processes under repository conditions.