Carbon Transfer Behavior of Materials for LMFBR's Steam-Generator in Flowing Sodium

Abstract

Using an AISI 304 stainless steel loop, the effect of exposure to flowing heated sodium was examined on materials considered suitable for LMFBR steam generators. The test specimens were exposed for 1,000 hr at temperatures ranging of 550"-425"C, descending in the direction of sodium flow. The oxygen concentration in the sodium was about 8 ppm. The materials tested were ferritic 2)i'Cr-1Mo, 2)i'Cr-1Mo-Nb, high Cr-1Mo and austenitic AISI 316 stainless steel. The loss of weight through exposure to liquid sodium was found to depend upon the heat-treatment history in the case of the 2)i'Cr-1Mo steels. The principal cause of this loss of weight was confirmed to be decarburization, and differences in the heat-treatment history may possibly influence the decomposition behavior of carbides. Increasing the chromium content in Cr-1Mo steel specimens tended to modify the behavior of the specimen in the direction of increasing carbon concentration near the specimen surface during exposure to liquid sodium, altering the effect of exposure from decarburization to carburization at a chromium content somewhere between 3 and 5%. Thermal aging of annealed 2)i'Cr-1Mo steel was found to reduce its tensile strength at 500"C to a marked degree.

Sodium effects on mechanical performance and consideration in high temperature structural design for advanced reactors

“…In a mono-metallic sodium system, austenitic stainless steels tend to decarburize in the reactor core region and carburize in the IHX region [2]; low-Cr ferritic steels, such as 2.25Cr-1Mo, are susceptible to decarburization in sodium due to its inherently high C activity [3][4][5]; high-Cr materials such as 9Cr and 316 steels have a relatively lower C activity and are more resistant to carbon loss in a sodium environment [6][7][8]. In bimetallic sodium loops constructed of austenitic and ferritic steels, the ferritic steel located in the low temperature region tends to decarburize and the austenitic stainless steel located in the high temperature region tends to carburize [9][10][11].…”