Separation of metallic residues from the dissolution of a high-burnup BWR fuel using nitrogen trifluoride

• The insoluble residue from dissolved used nuclear fuel is a high surface area material (1–5 nm size distribution).
• The residue contains an alloy: Mo, Tc, Ru, Rh, Pd, and minor elements Zr, Te, Se, Ag, I, having undetermined speciation.
• Fluorination separates the alloy into volatile fractions: (1) MoF6, TcF6, (2) RuF5, and solid components Pd and RhF3.
• Unusually high exothermicity was recorded for release of the gaseous fractions and is correlated to high surface area.
• Examination of the post fluorination residue supports the presence of an intermediate Ru–Rh–Pd alloy.

Nitrogen trifluoride (NF3) was used to fluorinate the metallic residue from the dissolution of a high burnup, boiling water reactor fuel (∼70 MWd/kgU). The washed residue included the noble-metal phase (containing ruthenium, rhodium, palladium, technetium, and molybdenum), smaller amounts of zirconium, selenium, tellurium, and silver, along with trace quantities of plutonium, uranium, cesium, cobalt, europium, and americium, likely as their oxides. Exposing the noble metal phase to 10% NF3 in argon, between 400 and 550 °C, removed molybdenum and technetium near 400 °C as their volatile fluorides, and ruthenium near 500 °C as its volatile fluoride. The events were thermally and temporally distinct and the conditions specified provide a recipe to separate these transition metals from each other and from the nonvolatile residue. Depletion of the volatile fluorides resulted in substantial exothermicity. Thermal excursion behavior was recorded with the thermal gravimetric instrument operated in a non-adiabatic, isothermal mode; conditions that typically minimize heat release. Physical characterization of the noble-metal phase and its thermal behavior are consistent with high kinetic velocity reactions encouraged by the nanoparticulate phase or perhaps catalytic influences of the mixed platinum metals with nearly pure phase structure. Post-fluorination, only two products were present in the residual nonvolatile fraction. These were identified as a nano-crystalline, metallic palladium cubic phase and a hexagonal rhodium trifluoride (RhF3) phase. The two phases were distinct as the sub-μm crystallites of metallic palladium were in contrast to the RhF3 phase, which grew from the parent, nano-crystalline noble-metal phase during fluorination, to acicular crystals exceeding 20-μm in length.

Selective separation of volatiles: Mo, Tc, and Ru, from nanoparticulate noble metal (hcp) phase using NF3 and the non-volatile residue; acicular, micron crystallites of RhF3 and nanoparticulate, fcc Pd metal.Figure optionsDownload as PowerPoint slide