Thermodynamics of cobalt-containing alloys proposed as materials for high-temperature gas-cooled reactor systems

Advanced high-temperature gas-cooled reactors demand appropriate high-temperature alloys as materials for the components situated outside the reactor core made of graphite. Some important candidate materials are the cobalt-containing alloys NIMONIC alloy PE 13, INCONEL alloy 617 and alloy IN-643. A contamination of the primary cooling cycle can occur if alloying constituents such as 59Co, 50Cr and 58Fe get into the reactor core, where radioactive nuclides are generated by neutron capture. In order to predict the increase in radioactivity by deposition of the nuclides formed, the partial pressures of cobalt, chromium and iron over the alloys are of interest. Therefore, and also to get more data on the thermodynamics of the alloys, their vaporization has been investigated by high-temperature mass spectrometry. Chemical activities and activity coefficients of chromium, nickel, iron and cobalt in NIMONIC alloy PE 13 and IN-643 as well as of chromium, nickel and cobalt in INCONEL alloy 617 were determined. Polished ground sections were made of the measured samples; these indicated the presence of chromium carbides in the alloys. The chemical activities of chromium in Cr3C2, Cr7C3 and Cr23C6 were computed from the free energies of formation of these carbides given in the literature. In order to check these results the activity of chromium in Cr7C3 was determined by high-temperature mass spectrometry. The chemical activities and activity coefficients determined for the alloys are discussed with respect to the microstructure of the alloys and the chromium activities in Cr3C2, Cr7C3 and Cr23C6 that were obtained. (author)