One of the main goals of the nuclear industry is to increase the nuclear safety and reliability of nuclear power plants (NPPs). As the steam generator (SG) is the most corrosion sensitive component of NPPs, it is important to analyze the corrosion process and optimize its construction materials to avoid damages like corrosion cracking. For this purpose two different kinds of SGs and its feed water distributing systems from the NPP Jaslovske Bohunice were studied by nondestructive M？ssbauer spectroscopy. The samples were scraped from the surface and analyzed in transmission geometry. Magnetite and hematite were found to be the main components in the corrosion layers of both SGs. Dependant of the material the SG consisted of, and the location in the system where the samples were taken, the ratios between magnetite and hematite and the paramagnetic components were different. The obtained results can be used to improve corrosion safety of the VVER-440 secondary circuit as well as to optimize its water chemistry regime. 1. Steam Generator In the Russian design of VVER-440, NPPs horizontal SGs are used. A pipeline, passing the SG wall via a nozzle, supplies the secondary side of the SG with water. The secondary side is also connected to a feed water distributing system located inside the SG. The feed water distributing system consists of a T-junction and of the left and right parts of a horizontal pipe collector with a number of short cylindrical water outlet nozzles. The horizontal pipe collector is inserted into an SG tube bundle, and it consists of 22K Russian carbon steel (GOST 22K, STN 12022; see Table 1) [1–10]. Table 1: Chemical composition of the investigated base material from the steam generators SG35 and SG46 from Jaslovske Bohunice [ 1– 3, 11]. As serious damages were observed in the region of the T-junction of the pipe collector and the outlet nozzles after 10 years of operation (the strong erosion caused holes as is visible in Figure 1), the former feed water distributing system was replaced by an advanced system, developed at NPP Jaslovske Bohunice [12, 13]. Figure 1: VVER-440 (Bohunice) steam generator cross-section. Bohunice innovation [ 14– 16]. The original (old) steam generator with technical mark RGV-4E is one body SG [14, 15]. The heat-exchange area is incorporated inside as surface of primary pipelines bundle with U-shape. The ends of these pipelines are fixed to the walls of the primary collector. Inside of SG body several separators and system of the steam water distribution are placed. The PGV-4E steam generator is foreseen for dry
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