The very high temperature gas reactor (VHTR) has been chosen by the Generation IV International Forum as one of the next-generation nuclear reactors. Due to the high operating temperatures of VHTR, Inconel alloy 617 is being considered as a primary candidate material for the intermediate heat exchanger (IHX) of the VHTR. In this study, the microstructures of creep specimens under various creep loads in a He environment were investigated. As the creep time increased, the thickness of Cr-oxide on the outer layer of the specimens clearly increased, and delaminated after a long creep time. Depths of the decarburized zones in the specimens increased slowly with creep time. However, precipitates at grain boundaries near the surface disappeared before the bulk diffusion of Cr in the matrix. It is considered that decarburization caused by minor gas impurities in He caused the reduction in creep rupture time.

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