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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 of stainless steel cladding hulls that contain undissolved metal fission products such as Tc, Ru, Rh, Pd, and Ag; a small amount of undissolved actinides (U, Np, Pu) also remains with the hulls. These wastes will be immobilized in a waste form whose baseline composition is stainless steel alloyed with 15 wt% Zr (SS-15Zr). Scanning electron microscope (SEM) observations of simulated metal waste forms (SS-15Zr with added actinides) show eutectic intergrowths of iron solid-solution (''steel'') and Fe-Zr-Cr-Ni (''intermetallic'') materials. The actinide elements are almost entirely in the intermetallic materials, where they occur in concentrations as high as 20 at%. Neutron- and electron-diffraction studies of the simulated waste forms show materials with structures similar to those of Fe{sub 2}Zr and Fe{sub 23}Zr{sub 6}. New TEM observations of simulated waste form samples with compositions SS-15Zr-2Np, SS-15Zr-5U, SS-15Zr-11U-0.6Ru-0.3Tc-0.1Pd, and SS-15Zr-10Pu suggest that the major U- and Pu-bearing phase has a structure similar to that of the C15 (cubic, MgCu{sub 2}-type) polymorph of Fe{sub 2}Zr. Materials with this structure exhibit significant variability in chemical compositions and actinide concentrations up to 20 at% (normalized so that atomic fractions of Cr, Ni, Fe, and Zr add up to 1). A U-bearing material similar to the C36 (dihexagonal, MgNi{sub 2}-type) polymorph of Fe{sub 2}Zr was also observed. Chemical variability in materials with the C36 Fe{sub 2}Zr structure is smaller than in those with the C15 Fe{sub 2}Zr structure, and U concentrations are less than 5 at%. Uranium concentrations up to 5 at.% were observed in materials with the Fe{sub 23}Zr{sub 6} (cubic, Mn{sub 23}Th{sub 6}-type) structure. Microstructures similar to those produced during experimental deformation of Fe-10 at% Zr alloys were observed in intermetallic materials in all of the simulated waste form samples. Stacking faults and associated dislocations are common in samples with U, but rarely observed in those with Np and Pu, while twins occur in all samples. Previously reported differences in dissolution behavior between samples with different actinides may be related to increased defect-assisted dissolution in samples with U

Characterization of host phases for actinides in simulated metallic waste forms by transmission electron microscopy.