Production, Characterization and Corrosion Analysis of Multiphase Alloy-Ceramic Composites

This study addressed the possible use of alloy/ceramic composite waste forms to immobilize metallic and oxide waste streams generated during the electrochemical reprocessing of spent reactor fuel using a single waste form. A representative composite material (AOC410) was made to evaluate the microstructure and corrosion behavior at alloy/ceramic interfaces by reacting 410 stainless steel with Zr, Mo, and a mixture of lanthanide oxides. Zr metal reacted with lanthanide oxides to generate lanthanide zirconates, which combined with the unreacted lanthanide oxides to form a porous ceramic network that filled with alloy to produce a composite puck. Alloy present in excess of the pore volume of the ceramic generated a metal bead on top of the puck. The alloys in the composite and forming the bead were both mixtures of martensite grains and ferrite grains bearing carbide precipitates; FeCrMo intermetallic phases also precipitated at ferrite grain boundaries within the composite puck. Micrometer-thick regions of ferrite surrounding the carbides were sensitized and corroded preferentially in electrochemical tests. The lanthanide oxides dissolved chemically, but the lanthanide zirconates did not dissolve and are suitable host phases. The presence of oxide phases did not affect corrosion of the neighboring alloy phases. The effects of added Ru and Pd on the microstructure and electrochemical behaviour of a composite material (AOC410N) made by melting those metals with AISI 410 stainless steel, Zr, Mo, and lanthanide oxides were assessed using electrochemical and microscopic methods. The noble metals alloyed with the steel to provide solid solution strengthening and inhibit carbide/nitride formation. A passive film formed during electrochemical tests in acidic NaCl solution, but became less effective as corrosion progressed and regions over the intermetallics eventually failed. A U-bearing composite (AOCU) was made for corrosion testing by reacting HT9 steel to represent fuel cladding, Zr and Mo to simulate metallic fuel waste, and a mixture of ZrO2, Nd2O3, and UO2 to represent oxide wastes. The Nd2O3 and some of the uranium reacted with Zr to form zirconates and the remaining uranium was reduced and incorporated in Fe-Zr-U intermetallics. Two Fe-Cr-Mo intermetallics also formed, which are expected to host Tc. The results of microstructure characterizations of the intermetallic and ceramic phases that were generated and tests conducted to evaluate their corrosion behaviors will be presented. Test results suggest composite waste forms will provide flexibility for immobilizing complex waste streams by accommodating both metallic and oxidized waste streams in durable host phases while lowering waste form production and disposal costs.