Passivation Layer Stability of a Metallic Alloy Waste Form

Abstract

SUMMARYAlloy waste form development under the Waste Forms Campaign of the DOE-NE Fuel Cycle Research & Development program includes the process development and characterization of an alloy system to incorporate metal species from the waste streams generated during nuclear fuel recycling. This report describes the tests and results from the FY10 activities to further investigate an Fe-based waste form that uses 300-series stainless steel as the base alloy in an induction furnace melt process to incorporate the waste species from a closed nuclear fuel recycle separations scheme. a This report is focused on the initial activities to investigate the formation of oxyhydroxide layer(s) that would be expected to develop on the Fe-based waste form as it corrodes under aqueous repository conditions. Corrosion tests were used to evaluate the stability of the layer(s) that can act as a passivation layer against further corrosion and would affect waste form durability in a disposal environment. Alloy Waste Form with 300-Series Type Stainless Steel -Processing and StructureInduction furnace melts (~ 50 gram per batch) were made under vacuum at a process temperature of 1600°C for compositions with non-radioactive surrogates of Zr, Mo, Re (a surrogate for Tc-99), Ru, Rh, Pd, and Sn in the relative ratios expected from separations processing of a reference fuel b and with a 47 wt% loading of 304L stainless steel or 316L stainless steel that yields an effective 53% waste-specie loading. All surrogate species dissolved into the melt at the process condition and were incorporated into intermetallic or solid solution phases as the melt cooled.A total of four primary phases was common to the melts with 304L stainless steel and 316L stainless steel as identified by EDS analysis. The phases were characterized as Fe-Mo, Zr-Mo, Fe-rich, and Pd-rich phases that are most likely the Laves phase Fe 2 Mo; C36 and C15 polytypes of Fe 2 Zr; J-Fe solid solution phase, and a Pd-rich phase (perhaps Pd 2 Zr) based on similar compositions with the phases identified in the previous melts made with pure Fe. All four phases from the pure Fe melts were present in the melts made with the stainless steels. The preliminary results suggest however that the Fe phase is J-Fe (fcc) due to its high nickel content > 10 at% that remains with this phase in the assemblage and that would stabilize austenite. The D-Fe (bcc) phase is the Fe solid solution phase that formed in the Fe-based waste form material made with pure Fe in FY09. Similar to the alloy melts made with pure Fe, each of the surrogate waste metal species from the stainless steel melts were incorporated into one or more of these phases in concentrations nearly identical to those in materials made with pure Fe.A stabilization treatment was added to the processing conditions for the Fe-based alloy. A 1-hour hold at 1000°C was added during the melt cool-down. This resulted in a coarse microstructure (larger grains) compared to the microstructure of phases that formed during the continuous cool-down ramp ...

Molten salt reactors and electrochemical reprocessing: synthesis and chemical durability of potential waste forms for metal and salt waste streams

“…Williamson and co-workers also demonstrated the formation of a passive corrosion film on a SS316based metal waste form with Zr, Mo, Ru, Rh, Pd, and Re (as a Tc surrogate) [82]. Samples were fabricated in a vacuum induction furnace at 1600°C and exposed to steam for 7, 14, and 28 days at 200°C.…”