Alloy selection and c-276 code design value extension for advanced molten salt technology test facilities experimentation

Molten halide salts are being considered as working fluids for nuclear and concentrated solar power applications. High temperature molten fluoride and chloride salts are known to preferentially attack and deplete Cr in alloys, which leads to the use of high-Ni low-Cr alloys in test facilities for advanced molten salt technology. Alloy C-276 is a commercially available Ni alloy that has adequate Cr contents and is qualified to the maximum temperature of 677°C (1,251°F) in the Boiler and Pressure Vessel Code. The alloy has good corrosion resistance to acids, is resistant to stress-corrosion cracking, and has long track records of use in the chemical industry. Therefore, it has been considered as a structural material for test facilities that require operations at 700°C (1,292°F) or greater to develop high-temperature molten salt technology. To meet the requirements, predictions of the Maximum Allowable Stress above the usage temperatures permitted by the Boiler and Pressure Vessel Code were developed with experimental data as an extension to the current code design values. Analysis showed that above current Codified maximum temperature, strength of the alloy is mainly controlled by creep rupture life under the average stress, although the Sc creep rate criterion is close to the Favg.Savg rupture criterion. This paper presents the intended test facilities and the design requirements, alloy selection considerations, literature review, data analysis, and proposed allowable stress extension based on some creep test data for C-276 at temperatures greater than 677°C (1,251°F). Further research activities are also briefly mentioned.