The transition to a closed fuel cycle after several years of operation of the BN-800 with oxide uranium fuel in an open fuel cycle is examined. It is shown that there is an advantage to using new fuel assemblies with 91 fuel elements with diameter 8.6 mm in a regime with four refuelings. On the basis of new fuel assemblies with mixed uranium-plutonium oxide fuel, transitional recycling to a closed fuel cycle without separating uranium and plutonium and without external plutonium makeup is examined. It is confirmed that a negative sodium void effect of reactivity is achieved with admissible values of the linear power density of a fuel element. It is shown that a regime with four refuelings can be obtained by adding uranium with enrichment no higher than 15% to replace the poison which is removed.Efficient utilization of fast reactors requires a closed fuel cycle, whose creation is a long and expensive process. Important aspects of this cycle are self-fueling, cost minimization, competitiveness, and nonproliferation of fissile materials. The adoption of a closed fuel cycle will make it possible to expand substantially the resource base for nuclear power by converting 238 U into the fissile isotopes 239 Pu and 241 Pu isotopes (thorium in 233 U).A commercial trial of a closed fuel cycle is to be based on the BN-800 reactor after it is started up. In principle, there are two possible approaches to solving this problem:• loading mixed uranium-plutonium oxide fuel (UO 2 -PuO 2 ) during BN-800 startup;• loading uranium oxide fuel during BN-800 startup (similarly to BN-600), making the transition to a closed fuel cycle after enough plutonium generated in the reactor has been accumulated and used to fabricate fuel. The optimal variant must be determined by technical-economic indicators and based on a comparative analysis taking account of the actual readiness of the production components of the closed fuel cycle.The present article examines the adoption of a closed fuel cycle after startup and several years of operation of BN-800 on uranium-enriched oxide fuel in an open fuel cycle in a regime with four refuelings (transitional stage to a closed fuel cycle). The main characteristics of the open fuel cycle of BN-800 with oxide uranium fuel as well as the accumulation of fissile plutonium isotopes and the possibility of achieving criticality with fuel recycling in a closed fuel cycle without sep-

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The results of neutron-physical investigations of a fast reactor using high-density fuel (UC) at the initial stage of the transition to a closed fuel cycle are presented. Validation is given for the possibility of making the transition to a closed fuel cycle with self-supply of fissile nuclei starting with the first recycle.The high density of the fuel used in fast reactors makes it possible to improve the neutron-physical characteristics and to use the fuel more efficiently.In some western countries, India, China, and South Korea intensive research is being conducted to develop fast reactors using different kinds of fuel. Experiments performed on the Phoenix reactor (France) studying nitride and carbide fuels showed the advantage of the latter type. Investigations of carbide fuel in a closed fuel cycle are continuing as part of the Generation IV international program as well as in a program with France, USA, and Japan collaborating to build FBR by 2020-2025 [2].India has made substantial advances in perfecting carbide fuel. Burnup 100-150 MW·days/kg has been achieved in the FBTR fast reactor with uranium-plutonium carbide fuel, irradiated fuel with burnup 25, 50, and 100 MW·days/kg has been reprocessed using a novel technology based on the Purex process [3]. Further improvements will require using metallic (alloyed) fuel in a closed fuel cycle: PFBR-500 (startup by 2020, India) [4] and KALIMER-600 (South Korea) [5].Different promising fuel compositions including carbide fuel (UC, PuC, UPuC) have been investigated in our country. A series of experiments with carbide fuel has been performed at the Research Institute for Nuclear Reactors and burnup 10.4% h.a. has been achieved [6]. Reassuring results have also been obtained for metallic alloyed uranium-plutonium-zirconium fuel, and a technology for fabricating fuel elements with such fuel has been perfected [7]. A federal targeted program has the BN-800 reactor starting up in 2012. The implementation of a closed fuel cycle based on this reactor is still not completely clear. Consequently, in order to transition to a closed fuel cycle taking account of the operation of this reactor in an open cycle it is helpful to compare the neutron-physical characteristics of the core with several types of fuel in order to determine its dimensions and the optimal parameters of the fuel assemblies.The present article together with [8] presents a conceptual investigation of this question for oxide and carbide fuels. In [8], optimization of fuel-assembly composition and core characteristics was performed for oxide duel and BN-800 oper-

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