Actinide distribution in a stainless steel–15 wt% zirconium high-level nuclear waste form

Abstract

Actinide-bearing waste forms are being produced from metallic remnants resulting from the electrometallurgical extraction of uranium from EBR-II spent fuel. The baseline metal waste form (MWF) is a stainless steel–15 wt% zirconium (SS–15Zr) alloy that may contain up to 10 wt% actinides, mostly in the form of uranium. This article presents the results of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and neutron diffraction on SS–15Zr alloys containing uranium, plutonium, and neptunium. Neutron diffraction results showed that the addition of uranium to SS–15Zr does not result in the formation of discrete uranium-rich phases. The lattice parameters of the ZrFe 2-type intermetallics are larger in uranium-containing SS–15Zr alloys and are consistent with the substitution of uranium at zirconium sites of the ZrFe 2 lattice. SEM studies showed that actinides are present only in the ZrFe 2-type intermetallics; moreover, both actinide-rich and actinide-deficient areas are found within the Laves compound. TEM showed that the simultaneous presence of multiple Laves polytypes, each with a different preference for the uranium atom, results in the uranium concentration gradients observed within the Laves intermetallics.