Corrosion Testing of Stainless Steel-Zirconium: Metal Waste Forms
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ABSTRACT Stainless steel-zirconium (SS-Zr) alloys have been developed as waste forms for the disposal of metallic waste generated during the electrometallurgical treatment of spent nuclear fuel. The waste forms incorporate irradiated cladding hulls, components of the alloy fuel, noble metal fission products, and actinide elements. The baseline waste form is a stainless steel- 15 wt% zirconium (SS-I 5Zr) alloy. This article presents microstructures and some of the corrosion studies being conducted on the waste form alloys. Electrochemical corrosion, immersion corrosion, and vapor hydration tests have been performed on various alloy compositions to evaluate corrosion behavior and resistance to selective leaching of simulated fission products. The SS-Zr waste forms immobilize and retain fission products very effectively and show potential for acceptance as high-level nuclear waste forms.
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INTRODUCTION
Stainless steel-zirconium alloys have been developed for the disposition of metallic waste, generated during the electrometallurgical treatment of spent nuclear fuel, from the Experimental Breeder Reactor-II (EBR-II) located at the Argonne National Laboratory site in Idaho (ANL-W) [ 1, 2]. In the electrometallurgical process, chopped driver or blanket fuel segments are placed into the anode baskets of an electrorefiner. When a potential is applied, uranium, active fission products, and transuranic elements dissolve at the anode into the molten salt electrolyte, while uranium is deposited onto a steel cathode [3]. The irradiated fuel cladding, assembly hardware, zirconium from the alloy fuel, noble metal' fission products (NMFP) (e.g., Tc, Rh, Ru, Pd, and Nb), and remnant actinides left behind in the anodic dissolution baskets are melted together to make a metal waste form (MWF) intended for disposal in a geologic repository. The baseline waste form for EBR-II spent fuels is the stainless steel-15 wt% zirconium (SS-15Zr) alloy [1, 2]. However, the zirconium content of MWF alloys may vary from 5 to 20 wt% Zr depending on the composition of the starting fuel. The noble metal content of the waste forms depends on the burnup of the treated fuel; the actinide content depends on the efficiency of the electrorefining process. The waste forms may contain up to 4 wt% NMFP and up to 10 wt% actinides (mainly uranium). This article briefly describes the microstructures and some of the corrosion studies being conducted on representative, but nonradioactive, MWF alloys. Electrochemical corrosion measurements have been conducted at various solution pH values to obtain relative values of corrosion rate for the various MWF alloy compositions. Immersion tests in deionized water and in simulated J- 13 groundwater (representative of the proposed Yucca Mountain geologic repository in Nevada) have been performed to evaluate the release of fission product elements from the alloys. Tests in a saturated steam environment have been conducted to determine the nature and thickness of corrosion layers that form on the metallic alloys. Corrosion
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