New alloy developments for advanced spent fuel reprocessing using nitric acid solutions

The development of three types of corrosion resistant alloys have been carried out for application to the advanced Purex reprocessing equipments used in several oxidizing grade of nitric acid environments. The highest corrosion resistance is required to the heat conducting surface with high oxidizing potential near 1.8 V caused from the thermodynamic decomposition of boiling nitric acid. Type 304ULC steel treated by EB-SAR process with immunity to grain boundary attacks, and high Cr-W-Si nickel base alloy so-called the RW designed for inhibiting the trans-passive corrosion by stable oxide film formation were tested in Cr6+ added nitric acid solutions under heat flux control for evaluating the applicability to heat conducting components of evaporators operated at boiling points lower than 70 deg C. The corrosion rate of the EB-SAR treated type 304ULC steel and RW alloy decreased in 30% and less than 10% respectively, compared with it of type 304ULC commercial heats. From viewpoint of the practical applicability; the cladding process between these alloy and type 304L steel used at the steam heater side was examined. The appropriate diffusion bonding process was selected. As alternative refractory materials of type 702 grade metallic zirconium which has high susceptibility to SCC and the accelerated corrosion by the low amount of fluorine in nitric acid solutions. W added Nb base binary alloy was selected for optimizing both the mechanical strength and the corrosion resistance, by considering the chemical stability of M2O5 protective oxide film. (author)