A Mössbauer Study of Austenitic Steel Originating from the Steam Generator of a VVER-440 Type Nuclear Reactor, Paks, Hungary

Abstract

Real specimens after more than ten years of operation in the steam generator of a VVER-440 type nuclear reactor, Paks, Hungary have been studied. Conversion electron Mössbauer spectroscopy was used for phase analysis of the primary side surfaces of austenitic stainless steel pipes. From the measurements, we concluded that the surface layer of the pipes consisted of a thin (<100 nm) and highly iron depleted oxidic film of nickel and chromium rich compounds. The ratio of the metallic constituents was found to be substantially different from that of the bulk steel to a depth of about 1–1.5 microns as revealed by independent SEM-EDAX studies. These findings may most likely be attributed to the irradiation enhanced diffusion and selective dissolution of iron.

The bulk steel was investigated by transmission Mössbauer spectroscopy and X-ray diffraction. The samples originated from different sections of the collector head of the steam generator representing different contact times with steam and liquid water. Besides the paramagnetic peak of austenite, the Mössbauer spectra contained a magnetic sextet of ferrite in various amounts. This difference in phase composition may be attributed to the combination of irradiation effect and different thermal stress.