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

Actinide-bearing waste forms are being produced from metallic remnants resu lting from the electrometallurgical extraction of uranium from EBR-II spent fuel. The baseline metal waste form (MWF) is a stainless steel-15 wt% zirc onium (SS-15Zr) alloy that may contain up to 10 wt% actinides, mostly in th e form of uranium. This article presents the results of scanning electron m icroscopy (SEM), transmission electron microscopy (TEM), and neutron diffra ction on SS-15Zr alloys containing uranium, plutonium, and neptunium. Neutr on diffraction results showed that the addition of uranium to SS-15Zr does not result in the formation of discrete uranium-rich phases. The lattice pa rameters of the ZrFe2-type intermetallics are larger in uranium-containing SS-15Zr alloys and are consistent with the substitution of uranium at zirco nium sites of the ZrFe2 lattice. SEM studies showed that actinides are pres ent only in the ZrFe2-type intermetallics; moreover, both actinide-rich and actinide-deficient areas are found within the Laves compound. TEM showed t hat the simultaneous presence of multiple Laves polytypes, each with a diff erent preference for the uranium atom, results in the uranium concentration gradients observed within the Laves intermetallics. (C) 2000 Elsevier Scie nce B.V. All rights reserved.