Solidification-induced nonuniformity in U-Zr-RE metallic nuclear fuel rods.

Metallic fuel is being developed for the next generation of sodium-cooled fast reactors due to its safety and fuel cycle economics through spent fuel reprocessing technology. After spent fuel reprocessing, rare earth elements that are immiscible with Uranium and Zirconium are included in the raw material for metallic fuel. Achieving a uniform composition and microstructure of RE inclusions is challenging because phase separation and temperature inhomogeneities during solidification lead to non-uniform composition and microstructure. This study explores the solidification and inclusion formation of U-Zr-RE metallic fuel rods. We show how RE inclusions are formed in the U-Zr matrix and how temperature inhomogeneities during solidification control the inclusion size distribution and edge migration of RE inclusions along the U-Zr-RE rods. Understanding the inhomogeneity of inclusions due to solidification can provide hints for making homogeneous nuclear fuel rods.