Karen Wright

This work utilizes electron microscopy-based techniques to examine the radial behavior of solid fission products in plutonium (Pu) bearing mixed oxide (MOX) fuel irradiated to a burnup of 13.7% fissions per initial metal atom (FIMA). Metallic precipitates primarily consist of five fission products: ruthenium (Ru), rhodium (Rh), technetium (Tc), molybdenum (Mo), and palladium (Pd). The five metal precipitates (FMPs) examined in this work have low concentrations of Pd and Mo, with no major compositional differences along the fuel radius. A secondary Pd–Te metallic phase forms in cooler regions of the pellet, likely due to the diffusion of gaseous species away from the central void. X-ray chemical maps indicate that the Pd–Te phase can nucleate on the surface of FMPs before precipitating into separate particles. These particles were also found to alloy with iron (Fe) near the surface of the fuel pellet due to interdiffusion with the stainless-steel cladding. The insoluble perovskite oxide phase was found to form near the central void and at intermediate radial positions, but not at the fuel edge. These findings suggest that solid fission product phases form at varying counts and compositions along the fuel pellet radius, and thus should be considered when describing the thermal behavior of the fuel.

This work utilizes electron microscopy-based techniques to examine the radial behavior of solid fission products in plutonium (Pu) bearing mixed oxide (MOX) fuel irradiated to a burnup of 13.7% fissions per initial metal atom (FIMA). Metallic precipitates primarily consist of five fission products: ruthenium (Ru), rhodium (Rh), technetium (Tc), molybdenum (Mo), and palladium (Pd). The five metal precipitates (FMPs) examined in this work have low concentrations of Pd and Mo, with no major compositional differences along the fuel radius. A secondary Pd–Te metallic phase forms in cooler regions of the pellet, likely due to the diffusion of gaseous species away from the central void. X-ray chemical maps indicate that the Pd–Te phase can nucleate on the surface of FMPs before precipitating into separate particles. These particles were also found to alloy with iron (Fe) near the surface of the fuel pellet due to interdiffusion with the stainless-steel cladding. The insoluble perovskite oxide phase was found to form near the central void and at intermediate radial positions, but not at the fuel edge. These findings suggest that solid fission product phases form at varying counts and compositions along the fuel pellet radius, and thus should be considered when describing the thermal behavior of the fuel.

Electron microprobe examinations were performed to characterize the chemical features of a full cross section of irradiated nuclear fuel from the FUTURIX-FTA experiment. This experiment investigated the nuclear fuel performance of a candidate fuel alloy intended for the transmutation of long-lived minor actinides in a fast neutron spectrum. The irradiated fuel, designated FUTURIX-FTA DOE1, was composed of 34.1U-28.3Pu-3.8Am-2.1Np-31.7Zr (where the preceding numbers indicate concentrations in weight %). The fuel was irradiated in the Phénix sodium fast reactor in France to a measured burnup of 9.5% fissions per initial heavy metal atom (FIMA), and experienced a peak cladding temperature of 550 °C. Microprobe analysis showed elemental redistribution of Zr and U where Zr has increased in concentration in the fuel center from an initially fabricated content of 31.7 wt % to 41.5 wt%, and U decreased from 34.1 wt% to 24.8 wt%. From the center of the fuel extending out radially approximately 1 mm, the fuel represented dominantly a single phase. Beyond this region to the fuel periphery, the fuel separated into two major phases, descibed by their composition as a (U, Np, Pu) Zr2-like phase and a high uranium content-low zirconium content phase. From the outer radius of the fuel extending approximately 1.7 mm radially into the fuel, americium, lanthanide elements, and actinide elements precipitated in a phase whose chemical analysis resembles Nd7(Pd, Rh)3. In addition, americium occurred as a dissolved species in the major fuel phases. Sm and Am penetrated up to 15 μm into the cladding along presumed grain boundaries, while major cladding elements Fe, Ni, and Cr penetrated at least 30 μm into the fuel. No phase formation between cladding elements and fuel elements was observed as the result of cladding element diffusion into the fuel.