[en] The strap-type mixing grid design was substantiated that possesses the required efficiency and is intended for the VVER-1000 TVSA. The TVSA-ALFAs with mixing grids are in operation at Kalinin NPP Unit 1 since 2008. The TVSA-T design utilizes the combined grids (spacer grid + mixing grid). Currently, cores in Temelin NPP Unit 1 and 2 are in operation fully loaded with the TVSA-Ts. The TVSA design is characterized by high performance in terms of the operational reliability, burnup, fuel life and makes it possible to ensure, with the use of mixing grids (TVSA-12), high technical-economic characteristics for operating and future VVERs. The maximum burnup of 63 MW·d/kgU was achieved for the TVSA; 72 MW·d/kgU, for the fuel rod. The TVSA-12 design utilizes a highly efficient mixing grid that ensures an increase in the coolant mixing coefficient by 5 times and an increase in the critical heat fluxes by 30%. The TVSA-12 design with 12 spacer grids having greater height enhances thermohydraulic characteristics and thermomechanical properties of cassettes, which is favorable for the application of a fuel rod with the thin cladding 0.57 mm thick. The development of the TVSA and TVSA-12 aims at the increase in the uranium content and introduction of the 5 1 efficient fuel cycle of ∼ 320 effective days. With the fuel pellet φ=7.8/0 mm introduced, the baseline TVSA design surpasses the known counterparts in terms of the Uranium content