Abstract: The paper investigates various computational modelling issues associated with VVER-440 fuel depletion, relevant to burnup credit. The well-known SCALE system and the TRITON sequence are used for the calculations. The effects of variations in depletion parameters and used calculation methods on the isotopic vectors are investigated. The burnup behaviour of Gadolinium, a burnable poison in nuclear fuel, is quite important in actual core analysis, but its behaviour is somewhat complicated, requiring special treatment in numerical modelling and calculations. Therefore, a special part of the paper is devoted to the treatment of Gadolinium-bearing fuels. Moreover, some discussions on power normalization, which plays an inevitable role in burnup calculations, are also included. The analyses in the paper include determination and ranking of the most important actinides and fission products and while emphasis is put on the fuel temperature distribution and its influence on the final isotopic vector of depleted fuel. To assess the acquired modelling experience used to predict the VVER-440 spent fuel nuclide composition, the measured compositions of Novovoronezh NPP irradiated fuel assembly are compared to data calculated by TRITON sequence. The samples of fuel assembly with 3.6 wt. % U-235 enrichment underwent 4-cycle campaign of totally 1109 days in the core and cooling period of 1-13 years. Calculated concentrations are compared to measured values burdened with their experimental uncertainties for totally 47 nuclides. The calculated results show overall a good agreement for all nuclides, differences from measured are pointed out and discussed in the paper.
Keywords: burnup, VVER 440, modelling, SCALE
