The status and prospects for increasing the fuel utilization efficiency of VVÉR-1000 reactors are reviewed. It is shown that the main trends in the development of water moderated and cooled reactors are reflected in an improved design with a four-year fuel run, different variants of which are now being implemented in nuclear power plants operating in Russia, Ukraine, and Bulgaria: weakly neutron-absorbing materials are used in the fuel assemblies, part of the excess reactivity of the core is compensated by an absorber (gadolinium) which is integrated with the fuel, the fuel load is designed with a reduced radial neutron leakage, and the number of refuelings is increased. Promising directions for improving fuel utilization are noted: increasing the energy content of the fuel load, operating the reactor with reduced values of the parameters at the end of a run after the reactivity excess has been exhausted on burnup, and reusing (recycling) the uranium and plutonium contained in the spent fuel.The nuclear fuel utilization regime of a reactor has changed greatly over the last 25 years from the moment the first VVÉR-1000 reactor was put into operation. The initial fuel-utilization design was intended for fuel assemblies operating for two reactor runs, each run lasting for about 300 eff. days with average makeup-fuel enrichment about ~3.3 mass% 235 U. The structural members of the fuel assemblies -the spacing lattices and guide channels, intended for moving the absorbing members of the mechanical control and safety system -were manufactured using corrosion-resistance steel, and the fuel was moved into the core primarily according to the periphery-center scheme (Fig. 1a). An absorber (boron in boric acid) dissolved in the coolant compensated the excess reactivity, so that safe conditions for starting up the reactor and increasing power (with respect to the sign of the temperature coefficient of reactivity) were achieved only because several groups of control-andsafety rods were lowered into the core. The fuel utilization efficiency was low according to current standards -the average burnup of the off-loaded fuel in a stationary refueling regime did not exceed 30 MW·days/kg and the specific consumption of natural uranium with 0.3% 235 U contained in the wastes was ~270 g/(MW·day). Until recently, it is these values that western specialists preferred to use as a way to characterize the fuel-utilization of VVÉR reactors.A three-year fuel run was adopted in the late 1980s in almost all power-generating units with VVÉR-1000 reactors. According to the design, the average and maximum admissible fuel burnup of the fuel assemblies were 41 and 49 MW·days/kg, respectively. A change in the fuel-pellet geometry made it possible to increase the admissible burnup-increasing the diameter of the central opening in a fuel pellet from 1.4 to 2.4 mm enlarged the gas cavity and created the conditions needed to decrease the gas pressure beneath the cladding and lower the temperature at the center of a fuel pellet. Since the mass of the fuel...

Problems related to use of Erbium as burnable poison for VVER are discussed. Comparison is made between neutronics characteristics of Uranium-Gadolinium and Uranium-Erbium fuel cycles. The study shows that use of Erbium as burnable poison allows decreasing the peaking factor in the core. Meanwhile residual Erbium at the end of the fuel cycle makes it necessary to increase fuel enrichment. There is made the conclusion of prospects of using Erbium as burnable poison for VVER.

The purpose of the work is to calculate VVER-1200 neutron-physical characteristics that are important for safety using MCU Monte Carlo code for an independent verification of the accuracy of design calculations of the first fuel cycle. The calculations were carried out before the startup of the first VVER-1200 power unit. A full-scale computer model of VVER-1200 was developed using the design documentation of the fuel assemblies and the reactor facility. Special attention was given to the accurate specification of the geometry and material description of the core and its immediate environment. The full-scale VVER-1200 model allows performing Monte Carlo calculation of some safety-relevant characteristics for which it is impossible or extremely difficult to carry out full-scale reactor experiments. In total, more than 110 different states were calculated; five states were calculated taking feedbacks into account. For all calculations, the neutron-physical characteristics obtained by means of the MCU code were used to verify the BIPR-7A and the PERMAK-A codes.

Two types of calculations were made to compare BIPR-7A and MCU results for 3D full-scale models. First EPS (emergency protection system) efficiency and in-core power distributions were analyzed for an equilibrium fuel load of VVER-1000 assuming its operation within an 18-month cycle. Computations were performed without feedbacks and with fuel burnup distributed over the core. After 3D infinite lattices of full-scale VVER-1000 fuel assemblies (FA's) with uranium fuel 4.4 % enrichment and uranium-erbium fuel 4.4 % enrichment and Er2O3 1 % wt were considered. Computations were performed with feedbacks and fuel burnup at the constant power level. For different time moments effective multiplication factor and power distribution were obtained. EPS efficiency and reactivity effects at chosen time moments were analyzed.

JSC TVEL has carried out a technical and economic study with the involvement of the National Research Centre “Kurchatov Institute” in the use of nuclear fuel enriched above the current limit of 5 wt% for VVER-1000/1200. The article presents neutronic characteristics of developed 18- and 24-month fuel cycles based on fuel enriched above 5 wt% and assessment of nuclear safety for fabrication and handling with high enriched fuel.