Microstructure of the secondary circuit piping of a WWER-440 nuclear power plant

Basic metallographic data characterizing the morphology and microstructure of the inner tube surface were obtained for a damaged component of the secondary circuit of a WWER-440 reactor where wall thinning and rupture had occurred in 50 000 hours of operation. The techniques of optical microscopy, transmission and scanning electron microscopy (TEM, SEM), and energy dispersive spectroscopy (EDS) were employed. The thinning of the examined site near the pipe perforation was 80% of the initial thickness. The appearance of the inner surface, coated by a thin oxide layer, is a result of a long-time erosion-corrosion effect of the water-steam mixture. While the macroscopic relief (grooving) of the inner surface is not associated with pearlite, the tiny surface defects (pits, cavities) seem to form at pearlite nodule sites. The occurrence of surface pits, cavities and cracks indicates the involvement of local corrosion in the corrosion-erosion processes. The material has a ferritic-pearlitic structure. No appreciable differences were found between the microstructure of the piping examined and a reference specimen which had not been engaged in the reactor operation; the minor differences (interlamellar distance, dislocation density) are due to the different manufacturing procedures rather than to the reactor operation. The microstructure of the material of the operated piping near the inner surface as observed by TEM (dislocation substructure and morphology of pearlite) is consistent with the condition after the manufacture of the pipes and does not exhibit any changes that could be attributed specifically to the reactor operation. Although the chromium concentration is at the EDS detection limit, Cr distribution heterogeneity was observed, presumably due to a preferential damage of pearlite. A detailed assessment of the degradation mechanism and its relation to the microstructure would require determination of chromium in the cementite lamellae and evaluation of the surface oxide layer, for which appropriate experimental techniques have to be developed. (P.A.). 16 figs., 6 refs