Study on neutronic behavior of VVER-1000 fuel assembly with duplex fuel rod design

Various studies suggested that duplex fuel rods have better neutronic characteristics than ordinary single-oxide fuel pellets. A duplex fuel rod consists of an inner uranium dioxide (UO2) layer and an outer thorium dioxide (ThO2) layer. This paper aims to study the neutronic behavior of the VVER-1000 fuel assembly with duplex fuel rod design. The duplex fuel rods were loaded into the fuel assembly in three configurations: outermost region, mid-region, and central region of the assembly. Three compositions with volume fractions of 75% UO2-25% ThO2, 70% UO2-30% ThO2, and 65% UO2-35% ThO2 were considered. A series of numerical calculations were conducted using the MCNP6 code and ENDF/B-VII.1 library. In general, there is an improvement in DTC and MTC value for fuel assembly using Duplex fuel rods. However, βeff and ℓ values tend to decrease while the value of Λ tends to increase which may affect fuel assembly response during transient scenarios. The results show that Configuration I can maintain its criticality for up to 15 MWd/kg for all fractions of UO2-ThO2 being used in duplex fuel, while Configuration II and Configuration III only reach below 15 MWd/kg when using duplex fuel rods. The Doppler temperature coefficient (DTC) and the moderator temperature coefficient (MTC) become stronger with the increasing ThO2 volume fraction within the VVER fuel assembly. Compared with non-duplex fuel, the amount of 239Pu was lower when using duplex fuel rods, but minor actinides was slightly higher at 40 MWd/kg. It can be concluded that the VVER-1000 with a duplex fuel rod could improve its safety characteristics besides that increasing fissile material content might be crucial to optimize VVER-1000 fuel management.