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Multiphase Alloy Nuclear Waste Forms Developed for Pyrochemical U-10Mo Scrap Recovery Waste Streams

The US High Performance Research Reactor (USHPRR) Conversion program is developing an Al-clad Zr-bonded U-10Mo nuclear fuel foil to enable the conversion of research reactors to high-assay low-enriched uranium (HALEU) alloy fuel. Fabrication scrap from the fuel manufacturing will be electrorefined to recover the HALEU, which will generate waste streams consisting of Zr, Mo, and residual U retained in the stainless steel anode basket used in the electrorefiner. Four alloys were formulated with different relative amounts of Zr, Mo, and 316L-SS to represent the expected range of waste compositions. Laboratory-scale ingots were made and metallurgically characterized to assess differences in the microstructures and distributions of the surrogate waste metals. Different amounts of the same dominant phases with similar compositions were formed in most materials: a γ-austenite based matrix, two ZrFe2 intermetallic phases that could host residual uranium, an FeCrMo sigma (σ) intermetallic phase, a chi (χ) phase, and a secondary austenite (γ2) phase. Electrochemical tests were conducted to compare the corrosion behaviors in acidic and alkaline brine solutions for a range of simulated environmental redox conditions. Corrosion of the Mo-rich γ and γ2 solid solutions and the σ and χ intermetallic phases occurred. The multiphase alloy waste form formulated with the lowest molybdenum and highest zirconium contents showed the highest corrosion resistance and has sufficient amounts of ZrFe2 to immobilize trace amounts of residual uranium in the waste stream. The alloy formulation strategy and testing approach will be presented with key results.
 Work conducted at Argonne National Laboratory is operated for the U.S. Department of Energy, Office of Science under contact DE-AC02-06CH11357.