Diverse approaches to manage early excess reactivity of VVER 1200 reactor

The objective of this study is to manage the initial excess reactivity of the VVER-1200 reactor through different approaches, including the use of burnable absorbers and control rods in fuel assemblies with varying enrichments (from 4.95 wt.% to 2.0 wt.%) by using the Monte Carlo code Serpent. It is found that 30 integrated burnable absorbers (IBAs) rods are required to reduce the infinite multiplication factor (k inf ) from 1.32408 to 1.06983 in assemblies containing 4.95 wt.% enriched fuel. On the other hand, assemblies with enrichment below 2.5 wt.% do not require any IBA rods. Control rods (CRs) can also effectively reduce excess reactivity at the beginning of life (BOL), provided they are inserted more than 80% of the assembly’s height. When fully inserted, the CRs provide a reactivity worth of 85.5 pcm/cm in 4.95 wt.% enriched assemblies at BOL. Fuel enrichment has a significant impact on delayed neutron fraction as well as the FTC, and MTC. While the effect of IBA rods on these parameters is found to be insignificant; however they do harden the neutron spectrum at the BOL due to the thermal neutron absorption by isotopes 155 Gd and 157 Gd.