An advanced sodium-cooled fast reactor concept has been developed in Korea for transuranics (TRU) transmutation with an electricity output of 600 MW(electric) (called the KALIMER-600 TRU burner). The core design philosophy is primarily based on passive safety mechanisms to meet the Generation IV technology goals. Accordingly, metal fuel has been adopted to enhance its inherent passive safety characteristics. The charged fuel in a ternary metal alloy (U-TRU-Zr) consists of self-recycled TRU and TRU recovered from the spent nuclear fuels of current light water reactors through a pyro-metallurgical process, which is assumed to carry over 5% of the inventory of rare earth (RE) elements. It has been recognized that an additional amount of RE in the fuel would decrease the material attractiveness of the charged fuel with respect to proliferation resistance and physical protection (PR&PP). However, this may raise concerns because most of the reactor physics parameters will tend to negatively affect the passive safety features encoded in the original core concept. Thus, this study investigates the impact of the RE recovery fraction on the core physics performance and important safety parameters such as Doppler coefficients and sodium void reactivity. The results are expected to help provide guidance regarding the development of limiting conditions for RE contents to recycling technology flow sheet developers and ternary metal fuel developers, and to provide insight into optimizing the core passive safety characteristics under accident conditions should a significant amount of RE be needed to enhance PR&PP.