Comparative Assessment of the Range of Spectral Regulation of Reactivity Margin for Fuel Burnup in Pressurized Water Reactors Using Zirconium Displacers for Uranium and Thorium Fuel Cycles

The excess reactivity in WWER-type pressurized water reactors is compensated using strong neutron absorbers. This leads to useless neutron consumption and reduce the breeding ratio and fuel burnup. In this work, one of the methods of spectral regulation of reactivity margin for fuel burnup was considered, namely, variation of the water-to-fuel ratio by inserting hollow cylindrical zirconium rods between fuel elements in the fuel assembly. Calculations were performed for a thorium–uranium-233 fuel. The range of change in the water-to-fuel ratio was estimated as a function of the diameter of inserted hollow zirconium rods. A comparison was made with the results of similar calculations for a uranium fuel at equal (3.7%) weight concentrations of fissile isotopes. The concentrations of the raw and fissile isotopes in the fuel in both fuel cycles were studied. In the Th–U-233 fuel cycle, with decreasing water-to-fuel ratio, the coefficient of accumulation of fissile isotopes can reach 0.75. The changes in the concentrations of the fission products in both fuel cycles were compared. The fuel and moderator temperature coefficients of reactivity and the control rod worth were estimated at all the considered water-to-fuel ratios. The safety parameters in the Th–U-233 fuel cycle were better than those in the UO 2 fuel cycle. It was shown that, at equal weight concentrations of the fissile isotope in the fuel, the insertion of hollow zirconium rods in the UO 2 fuel cycle changes the reactivity over a wider range than in the Th–U-233 fuel cycle.