The present work investigates the neutronic characteristics of VVER reactor assembly with various burnable poisons. The MCNPX code is used to design VVER-1000 LEU Assembly model. At first gadolinium oxide (Gd2O3) as integral burnable absorber is used, the results are compared with the results of the reference benchmark. Different fuels (uranium oxide, thorium oxide and thorium plutonium oxide) and five various burnable poisons are used in the study. Gadolinium Oxide (Gd2O3) and Erbium Oxide (Er2O3) are used together in the same assembly as IBAs that are mixed homogenously with 30 uranium fuel rods and Dysprosium Titanite (Dy2O3–TiO2) is coated the fuel rod with 0.001 cm thickness. Also, Iridium Oxide (IrO2) used in guide tube as BPR and finally Minor Actinides are used as both IBAs or BPRs. A number of parameters are determined at BOL such as thermal neutron flux, axial power distribution and pin power distribution. Also, four physical parameters are studied, Keff, βeff, MTC and FTC. The burnup calculation is performed for VVER-1200. The comparison of the multiplication factor variation with depletion (for 730 days) illustrated that Iridium Oxide in Th–F assembly has a faster rate of depletion than other Burnable absorbers. Using Iridium Oxide shortens the core life time due to the transformation process of Ir-191 to Ir-192. Moreover, the concentration of actinides for the different assemblies at EOC were discussed. In addition, the effect of using BPs in different fuel assemblies on the concentration of 135Xe and 149Sm production are also performed.