Microstructural Evolution and Dislocation Density Analysis of HT9 Steel Irradiated in the FFTF

HT9 steel is a ferritic/martensitic alloy being considered for use in fuel cladding in fast reactors in order to achieve high fuel burnup. The microstructural evolution, texture, and dislocation density of an HT9 duct were examined following the six-year irradiation of a fuel assembly in the Fast Flux Test Reactor Facility (FFTF). Material obtained from several positions within the assembly produced samples with a wide range of irradiation dose and irradiation temperature history. Synchrotron x-ray diffraction measurements were performed at beamline 1-ID at the Advanced Photon Source. Using the Convolutional Multiple Whole Profile (CMWP) fitting technique of evaluating dislocation contribution to line broadening, we show that irradiation temperature, rather than dose, most significantly controls dislocation density, with higher temperatures correlating to lower dislocation densities. Further, neutron diffraction texture measurements show larger texture evolution at higher doses. The results of synchrotron XRD deformation experiments and annealing experiments will also be presented.