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Many technical problems must be solved in order to adopt advanced fuel cycles at nuclear power plants with VVÉR-1000 reactors. The TVS-2 structure was initially developed in the process of solving these problems; it gave high technical performance and cost-effectiveness and provided a base for subsequent upgrading. The TVS-2M structure with a larger fuel load was developed on the basis of tested and new technological solutions to maximize fuel utilization in VVÉR-1000 reactors.The industry program "Efficient use of fuel in nuclear power plants in the period 2002-2005 and in the future up to 2010" provides for the adoption at the Balakov nuclear power plant of fuel cycles which give a higher ICUF (Installed Capacity Utilization Factor). The first steps for reaching this goal were the development and adoption of fuel assemblies with a rigid framework -second-generation fuel assemblies, called TVS-2. It is designed for a four-year fuel cycle and 3 × 350 eff. days with burnup 55 MW·days/kg. The predicted buckling of the axis of a fuel assembly over the operating period does not exceed 5 mm.TVS-2 has the following basic structural solutions as compared with the currently used improved fuel assemblies: • a rigid welded framework (spacing lattices welded to guiding channels) to ensure resistance to bending and torsional loads was formed;• the alloy É-635 was used to fabricate the guiding channels and the central channel in order to increase the mechanical properties and the geometric stability during prolonged operation;• the bending stiffness of the spacing lattices was increased (the height of the cells was increased to 30 mm and the thickness of the tube walls of the cells was increased to 0.3 mm) to eliminate shape losses resulting from temperature and radiation induced elongation;• the cell geometry of the spacing lattices was optimized to ensure optimal forces for movement of the fuel elements in the cells; this decreased the thermomechanical loads on the elements of a fuel-element bundle while preserving their resistance to fretting-wear during operation;• the transverse stiffness of the head was increased (a guiding shell was introduced) to increase the stability and decrease the friction in its components;• the "wrench" size was increased to limit the maximum buckling in the core during operation; • the number and arrangement of spacing lattices along the height of a fuel-element bundle was optimized to decrease the metal contents, production labor, and provide the required hydraulic characteristics;• a bottom lattice with a centrosymmetric overflow opening was introduced to improve the flow hydrodynamics at the entrance into a fuel-element bundle; such a lattice is stronger without any decrease of the flow section for the coolant;• connecting pipes for temperature control were placed in the head to increase the representativeness of the regular temperature monitoring at the exit from a fuel assembly.

Hypothetical conditions with deep cooling of coolant are analyzed for serial Reactor Installation (RI) with WWER-1000. The beyond design basis accident (BDBA) scenarios are specially chosen, to reveal the effectiveness of internal properties of nuclear fuel and reactor emergency protection (EP) in conditions when criterial parameters are close to the acceptance criteria or violate them. Various phases of the accident were considered: ATWS (anticipated transients without scram), repeated criticality and ‘cold slug’. Plural failures of Control Rods of Control and Protection System (CRs CPS) were modeled at the operation of EP. The analysis was carried out by the coupled code KORSAR/GP [1–3]. At the first investigation stage the individual conservative variant was considered, from which the expediency of use of BEPU method is revealed. Then at the second investigation stage the analysis of the set of variants in BE approach with uncertainty analysis on the set of statistically varied parameters — neutron-physical, thermo-physical, thermo-hydraulic and hydrodynamic was carried out with PANDA+KORSAR/GP [4] program complex. The advantages of Best Estimate Plus Uncertainty (BEPU) method for the safety margins revealing are demonstrated.