Estimation of the liquefaction temperature of metallic fast reactor fuel. Reaction between U-Pu-Zr alloys and Fe at 923 K

The peak cladding temperature of the metallic fast reactor fuel during normal reactor operation should be limited to the level at which a liquid phase is not formed in the reaction zone at the fuel alloy-cladding interface. In order to characterize this reaction zone, ex-reactor tests with the diffusion couples consisting of U-Pu-Zr alloys and Fe have been conducted at 923 K. A liquid phase has been observed at the interface between U-22at%Pu-22at%Zr and Fe, while no liquid phase has been found for U-13at%Pu-22at%Zr. It indicates the liquefaction temperature depends upon Pu content in the fuel alloy. Compositions of the low-Pu (<5at%Pu) phases in the reaction zone have been found to be independent of Pu content in the U-Pu-22at%Zr alloys. In the U-22at%Pu-22at%Zr/Fe couple, the high Pu content in the (U, Pu, Zr)6Fe phase, exceeding the Pu solubility limit in that phase, has caused the liquid phase. These results will serve development of a method to predict the phases in the reaction zones for various compositions of the U-Pu-Zr alloys. (author)