Dissolution Testing of a Metallic Waste Form in Chloride Brine

Abstract

Argonne National Laboratory has developed an electrometallurgical process for conditioning spent sodium-bonded metallic reactor fuel from the Experimental Breeder Reactor II (EBR-II). One waste stream from this process consists of a metal waste form (MWF) whose baseline composition is stainless steel alloyed with 15 wt% Zr (SS-15Zr) and whose microstructure is a eutectic intergrowth of iron solid solutions and Fe-Zr-Cr-Ni intermetallics. This paper reports scanning electron microscope (SEM) observations of corrosion products formed during static immersion tests in which coupons of surrogate MWF containing 10 wt% U (SS-15Zr-10U) were immersed in solutions with nominal pH values of 3 and 4 and 1000 ppm added chloride for 70 days at 50 °C. Although the majority of the surface areas of the coupons appear unchanged, linear areas with localized corrosion products apparently consisting of porous materials overlying corrosion-product-filled channels formed on both coupons, cross-cutting phase boundaries in the original eutectic microstructures. Many of the linear areas intersected the sample edge at notches present before the tests or followed linear flaws visible in pre-test images. Compositions of corrosion products differed significantly from the bulk composition, and the maximum observed concentration of U in corrosion products (~25 at%) slightly exceeded the highest reported values in actinide-bearing phases in uncorroded surrogate MWF samples with comparable concentrations of U (~17-19 at%). INTRODUCTIONArgonne National Laboratory has developed an electrometallurgical process for conditioning spent sodium-bonded metallic reactor fuel from the Experimental Breeder Reactor II (EBR-II) prior to disposal at the planned federal high-level radioactive waste repository to be constructed at Yucca Mountain, Nevada. In this process, chopped spent fuel is electrolytically partially dissolved in molten LiCl-KCl eutectic salt. Undissolved material (primarily fuel cladding hulls but also including zirconium, metallic fission products, and a low concentration of actinide elements) will be immobilized in a metal waste form (MWF) whose baseline composition is stainless steel alloyed with 15 wt% Zr (SS-15Zr). Scanning electron microscope (SEM) observations of metallic waste forms show eutectic intergrowths consisting of ~50 volume% actinide-free iron solid solutions (commonly referred to as "steels") and ~50 volume% of actinide-bearing Fe-Zr-Cr-Ni intermetallic phases (e.g. References [1] and [2]). Mo is present in low concentrations in both steels and intermetallics [3].An extensive series of static leach tests was conducted by immersing surrogate MWF ingots with 10 wt% U (SS-15Zr-10U) for 70 days at temperatures of 50, 70, and 90 °C in solutions with pH values from 3 to 12 and 0 to 10,000 ppm added Cl. The test results support the use of the HLW glass degradation model to represent MWF degradation for purposes of performance assessment for the proposed repository [4]. Studies of apparently uncorroded surfaces of surrogate...