Host phases for actinides in simulated metallic waste forms

Argonne National Laboratory has developed an electrometallurgical process for conditioning spent sodium-bonded metallic reactor fuel prior to disposal. A waste stream from this process consists primarily of stainless steel cladding hulls containing undissolved metal fission products and a low concentration of actinide elements. This waste will be immobilized in a metallic waste form whose baseline composition is stainless steel alloyed with 15 wt% Zr (SS–15Zr). This paper presents transmission electron microscope, energy-dispersive X-ray spectroscopy, and electron diffraction observations of SS–15Zr alloys containing 2–11 wt% U, Np, or Pu. The major U- and Pu-bearing materials are Cr–Fe–Ni–Zr intermetallics with structures similar to that of the C15 polymorph of Fe2Zr, significant variability in chemical compositions, and 0–20 at.% actinides. A U-bearing material similar to the C36 polymorph of Fe2Zr had more restricted chemical variability and 0–5 at.% U. Uranium concentrations between 0 and 5 at.% were observed in materials with the Fe23Zr6 structure.