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The sensitivity study of the RBMK-1500 spent nuclear fuel (SNF) nuclide composition modelling was performed by investigating the essential physical reactor performance parameters (influence of coolant density, fuel and graphite temperatures, the fuel irradiation history, axial fuel assembly power profile) and specific model parameters depending on the computational code (model geometry description, neutron flux convergence criteria, evaluated nuclear data libraries and energy intervals used, resonance self-shielding parameters, etc.). SNF nuclide composition modelling was performed by using T-DEPL sequence from the SCALE 5 code package. The study showed that some of analysed parameters can influence the calculated SNF nuclide composition significantly, especially for minor actinides and fission products with high neutron capture cross-sections. It has been found that the coolant density and axial power profile have the largest influence on irradiated fuel inventory and in order to obtain the more precise RBMK-1500 SNF nuclide composition these parameters should be modelled as close to real conditions as possible. The lattice cell pitch used for resonance self-shielding can have a significant effect on calculated actinide activities. The correct parameter could be obtained from the experimental data that are presently unavailable.

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Benchmark calculations of RBMK reactor spent nuclear fuel isotopic composition were performed using the MCNP and ORIGEN codes system. The RBMK assembly is explicitly modelled in order to investigate the differences of spent nuclear fuel burn-up due to different coolant density and the fuel rod position (inner or outer ring in the assembly). The modelling results are compared with experimentally measured data of RBMK-1000 fuel isotopic composition with 2% enrichment 235 U fuel at different fuel burn-up. The comparison of the experimentally measured and calculated isotopic composition for the main actinides and fission products allowed validation of MCNP and ORIGEN codes for RBMK spent nuclear fuel calculations. Variations of RBMK spent nuclear fuel burn-up and plutonium isotopic ratios depending on fuel location in the assembly were determined.

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