High temperature reaction tests between high-Cr ODS ferritic steels and U–Zr metallic fuel

Highlights: •Aluminum addition to ODS steel improves compatibility between U-Zr fuel and the ODS steels. •It increases the threshold temperature for reaction layer formation by 50 K. •The compatibility improvement mechanism was discussed using general thermodynamic data. -- Abstract: Out-of-pile high-temperature reaction tests are carried out on the diffusion couples between U–Zr fuel and ODS ferritic steels, in which the concentrations of Cr and Al were systematically changed for improvement of corrosion resistance. The results show that the Al addition to ODS ferritic steel considerably improves the compatibility between U–Zr fuel and the ODS steel. The threshold temperature for reaction layer formation is roughly 50 K higher in the Al-containing ODS ferritic steels than in the steels without Al addition for the testing time up to 900 min. The compatibility improvement mechanism by Al addition is discussed from the viewpoint of activity change. The activity calculation results obtained using general thermodynamic data indicate the possibility that stabilization of the intact α-Zr layer by Al addition is the main mechanism and occupation of steel lattice sites by Al that does not easily diffuse into γ-U–Zr also partly contributes to suppression of the inter-diffusion