Mitigation of segregation of rare-earth elements in U–Zr metallic fuel via Zr-induced melt density control

This study investigates how melt density affects the incorporation of REEs in U–Zr metallic fuels fabricated by injection casting. Melt density was controlled by varying the Zr content of the matrix while maintaining a constant RE charge representative of Nd, Ce, Pr, and La. Metallic fuel slugs were fabricated and characterized for density, microstructure, phase constitution, and chemical composition. Cross-sections of the associated melt residues were examined to assess microstructural evidence of segregation. Increasing the Zr fraction lowered the melt density and markedly reduced buoyancy-driven segregation, leading to near-target incorporation of REEs. Melt residues with higher Zr content showed no continuous REE-rich surface layer and only fine, uniformly dispersed inclusions, indicating reduced flotation of REEs during casting and improved retention in the fuel slug during solidification. These results indicate that melt-density control is a practical approach for fabricating metallic fuel slugs with high REE content, supporting TRU-surrogate fuel development for fast-reactor fuel cycles.