Neutronic analysis of VVER-1000 MOX fuel assembly with burnable absorber Gadolinia and Erbia

Highlights: • Erbia-Gadolinia can be used as an alternative burnable absorber in the VVER-1000 MOX assembly. • IBA rods with 3.6% enriched UO₂, 0.9% Gd₂O₃ and 5.0% Er₂O₃ shows better neutronic performance. • Erbia-Gadolinia IBA rods lower the excess reactivity and power peaking factor (PPF). • The LLFPs and MAs are slightly reduced. This paper represents the neutronic characteristics and feasibility of using Gadolinia and Erbia in integral burnable absorber (IBA) rods for the proposed MOX fuel assembly of VVER-1000 reactor by Nuclear Energy Agency (NEA). For comparison the 4.0% Gd₂O₃ IBA is considered as a reference. The assembly was burned to 40 GWd/t, and by neutronic calculation, the optimum percentage of Gd₂O₃ and Er₂O₃ was determined using SRAC-2006 code and JEFF-3.1 data library. The burnup dependent reactivity of a MOX assembly containing 12 UO₂-Gd₂O₃-Er₂O₃ (U-Gd-Er) IBA rods with 0.9% Gd₂O_3, and 5.0% Er₂O₃ was found almost identical to the reference assembly with 4.0% Gd₂O₃ in IBA rods. However, the assembly with U-Gd-Er IBA rods has a flatter pin-wise power distribution and lower power peaking factor (PPF). Long-lived fission products, poisons, and minor actinides are also investigated for choosing Gd₂O₃ and Er₂O₃ as IBA over only Gd₂O₃ for the VVER-1000 MOX fuel assembly.