Comparative Study of Spectral Regulation Range of Excess Reactivity Control in Pressurized Water Reactors Using Zirconium Displacers for Uranium and Thorium Fuel Cycles

The compensation for the excess reactivity in the pressurized water reactors WWER is realized with high neutron absorber materials. The traditional excess reactivity regulation methods lead to unfeasible neutron utilization and reduce the breeding coefficient and fuel burnup. In the current work, the change of moderator-to-fuel ratio is investigated as one of the spectral regulation methods for excess reactivity control and its effect on the fuel burnup. Cylindrical Zirconium rods (Zr rods) are used to fulfill the moderator-to-fuel ratio change. The Zr rods are placed between fuel rods in WWER-1000 fuel assembly. The current work calculations are performed for the thorium fuel cycle (Th-U233). The change of the Zr rods diameter leads to the variation in moderator-to-fuel ratio. A comparison between the Zr rods as a reactivity regulator in WWER-1000 fuel assembly for both fuel cycles UO₂ and Th-U233. The concentration of the fertile and fissile fuel components for both fuel cycles has been analyzed. The fissile isotopes accumulation coefficient can reach 0.75 with the decrease of the moderator-to-fuel ratio in the Th-U233 fuel cycle. The primary safety parameters such as the Control rods worth, Doppler Effect reactivity coefficient, and Moderator Temperature reactivity Coefficient have been studied at different moderator-to-fuel ratio values. The safety parameters in the Th-U233 fuel cycle have higher values more than the UO₂ fuel cycle with the insertion of Zr rods. From the comparison between the Zr rods effect in both fuel cycles, it is clearly shown that Zr rods in the UO₂ fuel cycle have a more influential role in regulating the WWER-1000 core reactivity compared with its effect in the Th-U233 fuel cycle.

water displacers, reactivity coefficients, WWER-1000, thorium fuel cycle, excess reactivity, spectral reactivity regulation

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