Corrosion Behavior of SUS316 Clad in Alkalis Molybdate Melt

Abstract

The study of a pyrochemical process by using alkalis molybdate melt has been carried out as a candidate reprocessing process for spent oxide fuels. In the previous studies, the UO2 pellet was dissolved into molten Na2MoO4-MoO3 mixtures and the dissolved uranium species in the melts were recovered by electrolysis. However, in a MOX dissolution test, we observed that iron species were dissolved into the melts from the stainless steel cladding tubes. The immersed corrosion tests of SUS316 tubes were thus carried out in molten Na2MoO4-MoO3 mixtures with MoO3 mole fractions ranging from 0.02 to 0.50 at 1023 K. Results showed that the concentration of dissolved iron and nickel species in the melt increased with an increase in MoO3 mole fraction, whereas the concentrations of dissolved chromium species remained low, independent of the MoO3 content. The cross-sectional morphology of the SUS316 specimen immersed for 2 h in the molten 0.5Na2MoO4-0.5MoO3 mixture at 1023 K clearly showed a layer of 22-29 nm thickness with a high content of chromium formed at the surface of the SUS316 specimen. It was thus found that the corrosion of SUS316 specimens in molten Na2MoO4-MoO3 mixtures at 1023 K proceeds with the iron and nickel preferably dissolved into the melt and with chromium-rich corrosion films formed at the surface.