2017 | AFC ACCOMPLISHMENTS 166 Figure 1. Temperature dependence in formation of M23C6 carbides in the N-doped HT9 steel. The 12Cr-1MoVW (HT9) steels with tempered-martensitic structures have been used as core structure materials for the sodium-cooled fast reactors as those exhibit high resistance to irradiation damage and good compatibility with liquid sodium coolant. Although the core components of typical fast reactors were designed for use in the 350–550°C temperature range, ongoing development efforts for higher performance reactors require significantly improved capabilities of core materials under high dose (i.e., high burnup) and high temperature conditions. In particular, therefore, the core materials for future fast reactors have to demonstrate excellent creep and swelling resistance in high temperature (> 500°C) and long term irradiation as well as to retain good mechanical performance to avoid embrittlement induced by low temperature (< ~430°C) neutron irradiation. In this research, new processing routes and nitrogen doping are applied to improve the high temperature performance of HT9 steels. Mechanical Properties of Original and N-doped HT9 Steels after Modified Thermomechanical Processing Principal Investigator:T.S. Byun Collaborators: J.P. Choi, K. Mattlin/ PNNL, S.A. Maloy/LANL