Segregation of rare earth elements in the melting process for injection casting of U–Zr metallic fuels

We investigated the segregation behavior of rare earth elements (RE = La, Ce, Pr, and Nd) in the injection casting process of U–Zr metallic fuel using the characterization methods such as density measurement, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and inductively coupled plasma−atomic emission spectroscopy. Injection casting of the metallic fuels with nominal compositions of U–10Zr−xRE (in wt%, x = 5, 7, and 10) resulted in the fuel slugs exhibiting similar compositions regardless of the RE content charged in the melting crucible. The microstructure observations revealed that RE-segregated layer occupied the upper region of the melt-residue in case of injection casting of U–10Zr−7RE and U–10Zr−10RE while the melt-residue of U–10Zr−5RE was relatively homogenous in terms of RE distribution. The segregation was caused by the immiscibility of RE in U–Zr alloys. When the RE content was higher than 5 wt%, the thickness of the segregation layer increased with the amount of the charged RE. RE inclusions migrated toward the upper region of the molten alloy owing to the different densities and specific gravities as compared to that of the U–Zr alloy, which led to the non-uniformity in the composition of the melt-residue after injection casting of U–10Zr−7RE and U–10Zr−10RE. The migration behavior of RE during melting was suggested to be induced by the fractional crystallization owing to the immiscibility and the difference in solidification temperature.