Assessment of loading scheme and moderation impact on minor actinide transmutation in VVER-1000 fuel assembly

This study aims to investigate the impact of loading scheme and moderation on minor actinide (MA) transmutation in a thermal-spectrum VVER-1000 reactor. Six different assembly models, consisting of inner-coated, outer-coated and homogeneously mixed MA-containing fuel elements were simulated in OpenMC for a period of 600 EFPDs. Three of the models employed fuel elements cooled by light-water from the outside only, while the other three were dual-cooled. A uniform loading of minor actinides in each assembly was ensured. All models demonstrated successful transmutation of Np and Am isotopes, while the concentration of Cm increased with burnup. Model 5 with homogeneously mixed MAs exhibited the highest overall transmutation rate (TR) of approximately 23.139%/y, followed by Model 2 (21.405%/y) and Model 1 (21.202%/y) utilizing inner-coated MAs. The loading of MAs as outer coatings around fuel elements proved to be the least effective approach for transmutation. Additional moderation via dual cooling resulted in a slight decrease of TR, except for Model 2 with inner-coating of MAs directly in contact with the coolant flowing inside. MA transmutation was also accompanied by a penalty in fuel cycle performance, with Model 5 suffering a 27.85% reduction in discharge burnup and a 27.16% decrease in cycle length compared to the reference. However, both the fuel and moderator temperature coefficients of reactivity became more negative with minor actinide loading and increased moderation. The introduction of MAs also affected the power distribution in the assembly, with homogenous loadings raising the power peaking factor the most.