Stainless steel-zirconium (SS-Zr) alloys are being considered as waste forms for the disposal of metallic waste generated during the electrometallurgical treatment of spent nuclear fuel. The baseline waste form for spent fuels from the EBR-11 reactor is a stainless steel-15 wt. Yo zirconium (SS-15Zr) alloy. This article briefly reviews the microstructure of various SS-Zr waste form alloys and presents results of immersion corrosion and electrochemical corrosion tests performed on these alloys. The electrochemical tests show that the corrosion behavior of SS-Zr alloys is comparable to those of other alloys being considered for the Yucca Mountain geologic repository. The immersion tests demonstrate that the SS-Zr alloys are resistant to selective leaching of fission product elements and, hence, suitable as candidates for high-level nuclear waste forms.
Stainless steel-zirconium alloys are being evaluated for the disposal of metallic waste generated during the electrometallurgical treatment of spent nuclear fuel [1, 2]. In the electrometallurgical process, chopped driver or blanket fuel segments are placed into the anode baskets of an electro refiner. When a potential is applied, uranium, active fission products, and transuranic (TRU) elements dissolve at the anode into the molten salt electrolyte, while uranium 1is 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 actinides left behind in the anodic dissolution baskets are melted together to make a metal waste form (MWF).
The baseline waste form for spent fuels horn the Experimental Breeder Reactor-II (EBR- 11), located at the Argonne National Laboratory site in Idaho, is a stainless steel-15 wt% zirconium (SS-15Zr) alloy. 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 electro refining process. The waste forms may contain up to 4 WWO NMFP and up to 10 WtO/O actinides (mainly uranium).
To be acceptable in a geologic repository, waste forms must be chemically durable and be able to retain their radionuclide inventory [4]. The chemical durability of a waste form is dependent on various repository factors, which include solution composition, pH value, temperature, and radiolysis effects [5]. In situ studies are often conducted to identify phenomena associated with a specific repository environment. In addition, short-term laboratory experiments are used to identify corrosion mechanisms that could occur in a repository environment; this information may then be used to support material behavior models designed to evaluate long-term performance [4].
This article briefly describes the microstructure and some of the corrosion studies being conducted on representative, but non-radioactive, MWF alloys. Electrochemical corrosion measurements have been conducted at various pH values to obtain relative values of corrosion rate for various MWF compositions. Immersion tests in deionized water and in simulated J-13 groundwater2 (representative of the proposed Yucca Mountain geologic repository in Nevada) have been performed to evaluate the release of fission products from the alloys. Corrosion tests on actinide-containing materials and on actual radioactive waste forms arising from the treatment of EBR-11 fuels will be presented in future articles.
