2018 | AFC ACCOMPLISHMENTS 142 3.2 CLADDING DEVELOPMENT The sodium-cooled fast reactors have been designed for operation in a wide temperature range of 300–550°C and ongoing development efforts for a higher thermal efficiency require improved performance of core materials at higher temperatures.Thus far, the 12Cr-1MoVW (HT-9) steels with fully-tempered martensitic structures have been used as core structure materials for the fast reactors not only because they exhibit high resistance to irradiation-induced embrittlement, thermal and irradiation creep, and void swelling but also because they show little compatibility problem with liquid sodium coolant; however, the irradiation-induced embrittlement in low-temperature irradiation, void swelling at very high doses (> 150 dpa), and high-temperature strength still limit expansion of the capability of HT-9 steels to higher doses and temperatures required in advanced fast reactors. In this work package, therefore, significantly improved high- temperature mechanical performance with the HT-9 alloy has been pursued by developing new thermomechanical processing routes. Improvement of Mechanical Properties of HT-9 Steels by Modified Thermomechanical Treatment Principal Investigator:T.S. Byun Collaborators: J.P. Choi, T.G. Lach – PNNL, S.A. Maloy - LANL Project Description: This research aims to develop new processing routes for HT-9 steels that can deliver improved high- temperature mechanical properties required in advanced fast reactors. For the fiscal year, a series of new processing routes were designed and the samples after those processes were tested and compared in search of an optimized microstructure that can demonstrate both high strength and high fracture toughness in the reactor operation temperature range.Application of 16 different thermomechanical treatments (TMTs) to two HT-9 steels was performed considering the guidance from the comprehensive thermodynamics simulation performed earlier in the project. Finer microstructures usually demonstrate higher resistance to radiation damage as well as better mechanical performance.Among the numerous processing routes that have been explored in the steel development, only a limited number of processing methods, including the rapid cooling that can effectively refine the quenched lath structure, are feasible for processing