Next-generation innovative reactors have a new value of their flexibility with variable renewable energy. A sodium-cooled fast reactor (SFR) can make flexibility by coupling a thermal energy storage (TES) system with molten salt. New challenges for the SFR coupled with TES are to develop a safety design approach and a heat exchanger (HX) between sodium and molten salt. On that account, since 2022, a three-year project has been performed to develop 1) a safety design approach and risk assessment methodology of the SFR with TES, 2) a performance evaluation technology of an HX between sodium and molten salt, and proposal of heat transfer improvement measures, and 3) an evaluation technology of chemical reaction characteristics between sodium and molten salt, and proposal of safety improvement measures. This paper describes the project overview and recent progress. For 1), this study investigated the record of operation and incidents or troubles of a TES system that uses molten salt as a heat storage medium for concentrating solar thermal power generation, which includes about 50 commercial facilities. These data could serve as a reliability database for risk assessment. For 2), this study selected a straight shell-and-tube type HX as a result of investigating several types of HXs. Using the intermediate HX of the experimental SFR in Japan as a reference, the heat transfer performance of the HX was evaluated by a simple method. This study developed the most promising HX type by improving the heat transfer performance with parameters of heat transfer tube diameter, heat transfer tube pitch ratio, heat transfer tube material change, primary side/secondary side replacement, and cross flow. For 3), this study preliminarily evaluated the chemical reaction characteristics using an equilibrium phase diagram with thermodynamic equilibrium calculation software. No database for the reaction of nitric acid molten salt and sodium was confirmed, so that a basic reaction experiment is necessary. This study prepared a glove box and reagents in 2022, and a thermal analysis-related equipment is also introduced in 2023.