Current HT-9 creep and deformation models in the BISON framework consistently predict unacceptable discrepancies between simulated and experimentally observed HT-9 cladding behavior due to a lack of experimental data at prototypical reactor conditions. Critical factors that must be considered for more accurate cladding prediction include fuel-cladding chemical interaction (FCCI), precipitate evolution, intermetallic phase formation, and radiation damage. To obtain mechanical property information more relevant to simulations used to predict in-pile fuel-cladding behavior, this work consists of micro-tensile tests on the HT-9 cladding of a U-10Zr fuel irradiated to a burnup of 8.1 at. % with a peak inner cladding temperature of 612°C in the Fast Flux Test Facility (FFTF) in addition to a sample of fresh, unirradiated HT-9 at both room-temperature and high-temperature (600°C) followed by advanced transmission electron microscopy characterization of deformed specimens. This experiment will help define prototypical mechanical properties of HT-9 cladding, clarify temperature and radiation effects on mechanical properties, and highlight structure-property relationships in the bulk and FCCI-affected regions of HT-9. Ultimately, the development of more robust fuel performance models used for qualifying HT-9 clad U-Zr-based fuels will benefit from this work.

The development of a correlative relationship between microstructure and mechanical behavior in irradiated HT-9 cladding has been rarely achieved. The technical objective of this RTE is to define mechanical properties of HT-9 cladding at prototypical reactor conditions using in-situ high temperature micro-tensile tests. Considering the scale of these tests, regions with small, heterogeneously mixed microstructure can be characterized by their mechanical behavior. Building off previous currently ongoing efforts of small-scale mechanical testing in HT-9 clad U-Zr-based metallic fuels, stronger correlations between individual phases, precipitates, radiation-induced defects, and observed mechanical properties can be developed. The results of this work will greatly benefit fuel performance modeling efforts such as those being developed within INL’s BISON framework by providing relevant experimental data for models to be refined towards accurately predicting metallic fuel-cladding behavior under extreme conditions. Ultimately, this study contributes towards the Advanced Fuels Campaign (AFC) program mission and advanced nuclear power industry efforts (i.e. TerraPower’s Natrium project) -all of which are primarily funded by DOE-NE.