The hot functional test (HFT) represents the first exposure of primary circuit materials to near operational coolant conditions and play a key role in subsequent corrosion product release and radiation field development. This paper evaluates primary circuit passivation during a HFT at the nuclear power plant (NPP) Mochovce Unit 4 (a water-water energetic reactor (VVER)-type pressurized water reactor (PWR)). The passivation stage lasted 15 days, during which thermohydraulic conditions and coolant chemistry were kept stable and well controlled. The resulting passivation layer was evaluated using two complementary approaches. First, surface replicas collected from selected primary circuit components were used to assess the condition and uniformity of passivated surfaces. Second, corrosion coupons exposed in the primary circuit were characterized separately using scanning electron microscopy (SEM)/energy dispersive X-ray spectroscopy (EDS) and the complementary techniques, including focused ion beam scanning electron microscopy (FIB‑SEM), transmission electron microscopy (TEM)/selected area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), and Raman microscopy and atomic force microscopy (AFM) to determine oxide‑layer thickness, structure, and phase composition. A continuous and adherent two-layer oxide film was formed, consisting of a compact iron-chromium-nickel (Fe-Cr-Ni) spinel inner layer and an outer magnetite-rich layer. The total oxide thickness was typically 2–3 µm. Both the HFT course and the characteristics of the formed passive layer were compared with results from NPP Mochovce Unit 3. The results confirm effective and reproducible primary circuit passivation under controlled HFT chemistry at Mochovce nuclear power plant.
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