Potential of the VVER reactor spectral regulation with regard for fuel burn-up
Neutron spectrum control in a nuclear reactor makes it possible to monitor excessive reactivity through the absorption of excess neutrons in the absorber or by changing the uranium-water ratio. The paper deals with investigating the potential of the VVER reactor neutron spectrum control with the use of displacers. Zirconium rods of different diameters (Zr rods) were used as the displacers. The Zr rods were positioned between fuel rods. Introducing the Zr rods between the fuel rods and increasing their diameter leads to a decrease in the amount of water inside of the reactor and to a reduction in the multiplication factor. Excessive reactivity is made up for in current VVER reactors through the dissolution of boric acid in water. The results of comparing the potential efficiency of displacer utilization against the use of boric regulation are presented. The burn-up calculations have shown that adding Zr rods to the VVER-1000 geometry reduces the U-235 burning rate and enhances the Pu-239 production. The concentration of plutonium isotopes increases with the Zr rod diameter increase. Important safety parameters involved in the change of the moderator-fuel ratio are density and Doppler reactivity coefficients. These coefficients were therefore calculated with the obtained results demonstrating the potential safety of using displacers in VVER reactors instead of boric regulation. This has proved that the use of Zr rods as the neutron spectrum regulator makes it possible to maintain the neutron economy and to monitor excessive reactivity inside of VVERs.
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