b'Nuclear Science User Facilities Characterization of Neutron-Irradiated Zr-1Nb-OUsing Scanning Transmission Electron MicroscopyM Nedim CinbizIdaho National LaboratoryMahmut.cinbiz@inl.govevolved microstructure and localthat zirconium oxide encapsulated chemistry at the pellet/claddingfuel into small pockets, suggesting interface (PCI) for the developmentthe diffusion of zirconium was the of further experimental investigationsmain driving force. Mo-Tc-Ru-Rh-Pd and computational efforts. Thus, thisprecipitates were found not only in study investigated the microstructuralthe UO 2fuel region, but also in the characterization of high-burnup nuclearzirconium oxide region. The Mo-Tc-fuel at the PCI with the presence ofRu-Rh-Pd precipitates are known as HBS using analytical TEM. Several TEMfive metal precipitates in irradiated samples were prepared from a high- UO 2fuels [2]. Measurement of a large burnup nuclear fuel using focusedMo-Tc-Ru-Rh-Pd precipitate enabled ion beam milling. Phases present werethe determination of the composition, identified via electron-diffractionsuch as 37 at% Mo, 8.6 at% Tc, 29 patterns, and the local chemistry wasat% Ru, 1.4 at% Rh, and 14 at% Pd. M investigated by energy-dispersive x-rayIt should be noted that almost all the odern zirconium alloys show excellent water-sidespectroscopy (EDS). Findings willMo-Tc-Ru-Rh-Pd precipitates were corrosion resistance, lowbe informative for the developmentassociated with pores. hydrogen pickup, and exceptionalof materials computational models,Conclusionirradiation-resistance during light- especially for mesoscale approaches. This study characterized the PCI water reactor (LWR) operation asResults region of a high-burnup fuel from compared to Zircaloys [1]. ImprovedCharacterization of the PCI regionan LWR using analytical TEM. Results operational performance of theserevealed the presence of monoclinicrevealed that the PCI region is a alloys boosted activities to increaseand tetragonal zirconium oxide phasescomplex structure where zirconium current burnup limits for nuclear fuelformed at the fuel/cladding interface.oxide diffuses into oxide fuel. At rods from ~60 to ~70 GWd/t forMonoclinic phase is formed at thehigh burnup, the pellet/cladding LWRs. Burnup extension faces manyzirconium side of the PCI region, andgap is completely closed by forming design challenges from the licensingthe tetragonal phase was formed atan interlocked diffusion layer (see viewpoint, primarily related to high- the fuel side. This suggests monoclinicFigure 1). This implies that thermal burnup structure (HBS) formation asphase was formed prior to the forma- energy would be transported well as mitigation of pellet/claddingtion of the tetragonal phase. Elementalthroughout PCI layer, and the gap chemical and mechanical interactionmaps, as shown in Figure 1, showedthermal uncertainty would be null. during reactor transients. In orderthat no single-phase uranium-zirco- Modeling tools must incorporate to succeed, a detailed description ofnium-oxygen compound was detectedthis effect by including the thermal nuclear fuel is needed. This descriptionin the PCI region, but interlockedproperties of zirconium oxide phases. needs to be robust and accurate enoughseparate zirconium oxide and uraniumHowever, the thermal conductivity that computational models can beoxide phases were observed. Bothof zirconium oxide phases must be developed, modeling from nanoscalezirconium and uranium oxides exhib- incorporated to heat-transfer models. to engineering scale. State-of-the-artited nanograin structure. MonoclinicThis study offers a clear description characterization techniques, suchgrains were elongated while tetragonalof the PCI layer that can easily be as analytical transmission electrongrains were equiaxed. A notableincluded in mesoscale computation. microscopy (TEM), can elucidateobservation, shown in Figure 1, wasIn addition to that, the nanograined 56'