Analysis and Optimization of Intrafuel Assembly Nonuniform Power Density and Burnup of WWER-1000 Fuel

This paper considers and systematizes fuel assembly variants for the WWER-1000 reactor. A fuel assembly of infinite height with six U49G6-type U-Gd fuel rods used in large-capacity WWER-1000 fuel loads is simulated in the Serpent neutronic code. As a result of the calculations, U-Gd fuel rod arrangement with less uneven power density along the fuel assembly is selected, and the effect of the number of U-Gd fuel rods and their arrangement on reactivity is assessed. It is shown that the arrangement of U-Gd fuel rods in the fourth ring is optimal in terms of uneven power density per fuel rod and burnup. The effect of fuel assembly arrangement on the fuel burnup for individual groups of U-Gd fuel rods is considered. An option is proposed for reducing the cost of computing resources by identifying the groups of fuel rods with the greatest differences in power density and burnup. The studied assemblies are compared with the existing U49G6 fuel assembly. A method is proposed for estimating the underproduction of thermal energy in a fuel assembly due to the uneven burnup of fuel rods, while maintaining the maximum burnup for individual fuel rods. Based on the analysis of underproduction, an option is proposed for improving the fuel assembly layout with six U-Gd fuel rods to equalize the power density field and reduce unproductive fuel losses. For this purpose, enrichment in the first, second, and tenth fuel rod rings, counting from the central fuel assembly tube, can be reduced. As a result of such profiling, the burnup of the most burnt fuel rods in the fuel assembly can be reduced to 1.015 of the average burnup.