The results of a verification of the BONUS method as regards predictions of the time dependence of the mass and activity of fission products produced in thermal reactors are presented. Different standard regimes of fuel irradiation in VVER-1000 are examined, and the results calculated using the autonomous version of the BONUS code and as a module integrated into SOCRAT code are compared with the results obtained using other codes, including precision program complexes. On the whole, the calculation shows good agreement between BONUS and alternative codes; the standard deviation variance is 6-13.5% for fission product mass and activity, which is comparable to the discrepancies between different variants of the alternative calculations themselves.It is obvious that the moduli of a well-balanced integrated code that describes the complex physicochemical processes occurring in a reactor facility in standard and emergency situations must conform to an overall targeted accuracy. Specifically, considering the simplification of the models used to describe fission-product transport in a reactor facility as well as the uncertainty of the parameters and input data, high-accuracy cumbersome computational methods can hardly be regarded as the optimal choice for describing effects associated with radioactive intertransmuting actinides and fission products. In this connection, the BONUS method was developed at the Institute of Problems in the Safe Development of Nuclear Energy (IBRAE) to perform simplified calculations of the time dependence of the concentration, activity, and energy release of fission products. The method is implemented as a separate program module to be included in computer codes, such as the MFPR code [1,2], that simulate the behavior of fuel and the yield of fission products in different operating regimes or in integrated codes, such as SOCRAT [3], that simulate processes occurring inside and outside vessels during anticipated and unanticipated accidents.The BONUS method describes in the one-group approximation the change of the concentration of actinides under neutron irradiation as well as the concentration of fission products taking account of their production as a result of fission, γ and β decay, and radiative capture of slow neutrons. Of course, the evolution of the radionuclide composition of the fuel after the reactor is stopped is also simulated. Actually, the BONUS module incorporated in an external code is used to calculate the production of actinides and fission products in an individual spatial cell. It is supposed that the time dependence of the specific energy release in a given cell or the thermal-neutron flux density is given in at the irradiation stage. The par-
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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