2017 | AFC ACCOMPLISHMENTS 167 Significant improvement of HT9 steel performance via an innovative thermomechanical treatment will help achieve much higher burnup in fast reactor fuels. Project Description: This work particularly aims to develop an improved thermomechanical processing (TMP) route for HT9 steels as they are currently the most probable candidates for the core materials of advanced fast reactors.These structural materials will have better resistance to radiation- induced damage if controllable microstructural features and their size scales can directly involve radiation damage processes such as formation of nanometer scale defect clusters, voids, and helium bubbles, creep deformation, elemental segregation, and phase changes. A principle used in this process improvement is the largely accepted idea that more defect recombination sites or higher defect sink density is required for higher radiation resistance.Therefore, refinement of lath structure and precipitates is the key method in the processing development of HT9 steels. In this fiscal year, new TMP routes were explored using two HT9 steels (one with and one without nitrogen addition) provided by the Los Alamos National Laboratory (LANL) team led by Stuart A. Maloy. The research activities specifically focused on testing a series of new processing routes to determine the microstructures of HT9 steels that lead to improved mechanical properties including high- temperature fracture toughness. First, a comprehensive thermodynamics simulation using Thermo-Calc Software was performed for the two HT9 steels to obtain key metallurgical information on equilibrium phase formation and elemental distributions within phases. Second, the first round of new heat-treatment routes, including both the typical normalization plus tempering and the normalization plus newly designed two-step tempering, have been applied to the two steels; the treated materials were then evaluated by tensile property testing over a wide temperature range of room temperature to 600°C.