[en] An advanced mechanistic code SFPR for modeling of single fuel rod behavior under various regimes of LWR reactor operation (normal and off-normal, including severe accidents) is under development at IBRAE. The code is designed by coupling of two stand-alone mechanistic codes MFPR (for modeling of irradiated UO₂ fuel behavior and fission product release) and SVECHA/QUENCH, or S/Q (for modeling of Zr cladding thermo-mechanical and physico-chemical behavior). Both codes were initially designed for accident conditions (and for this reason, are rather mechanistic) and later extended to various normal operation conditions. The main physical models of the two codes were adapted and used in the Russian best-estimated integral code SOCRAT designed for mechanistic analysis of severe accidents, which was applied to the safety justification of NPP with the new generation of VVER type reactor such as Tyanvan NPP in China and Kudamkulam NPP in India. The code MFPR was developed for analysis of fission products release from irradiated UO₂ fuel by IBRAE in collaboration with IRSN (Cadarache, France). MFPR was thoroughly validated against extended matrix of out-of-pile analytical tests and, for this reason, includes detailed mechanistic models on fuel microstructure evolution (including, besides pores and fission gas bubbles, other point and extended defects, which become especially important for consideration of high-burnup fuel and transient conditions) and chemically active elements behavior. The mechanistic description of chemically active elements (fission-induced) is based on complex association of diffusion-vaporization mechanism involving multi-phase and multi-component thermo-chemical equilibrium at grain boundaries with accurate calculation of fuel oxidation. The latest version of the code was extended to full-scale systematic simulation of radioactive transmutations of fission products and their transport in the fuel. The S/Q was initially developed by IBRAE for the detailed modeling of re-flooding phenomena observed in the FZK (Germany) single-rod QUENCH rig tests, in close cooperation with FZK experimentalists, and also supported by various European organizations (IRSN, JRC/IE) within 4.-6. Framework Programs of EC. In the S/Q code, the main physical phenomena occurring during degradation of fuel rods are mechanistically considered: cladding oxidation, cladding mechanical deformation, hydrogen uptake and release by the cladding, heat conduction inside the fuel rod, heat and mass exchange with the surrounding two-phase media. Further development of the code in application to normal regimes of reactor operation was performed in collaboration with various Russian research and design organizations. The new code, SFPR, resulted from coupling of MFPR and S/Q allows new applications to reactor tests with instrumented fuel rods pre-irradiated in commercial reactors. On the base of thorough validation against various in-pile experiments (HALDEN, SILOE, MIR), development of an advanced fuel performance code for best estimate code calculations for both normal and off-normal LWR reactor operation regimes is foreseen. Extension of the code to description of fuel rods with MOX, TRISO, SFR fuel is important part of the development plan (e.g., started in collaboration with IRSN within the new International project SAMANTHA). As a result, it is anticipated to use the single-rod code SFPR as a constituent module of the Russian integral code SOCRAT in application to analysis and safety justification of the new-generation nuclear reactors. (authors)