Comparison of various spectral control schemes using displacers

In this paper, one of the possible approaches to compensate for excessive reactivity at the beginning of the campaign is considered – spectral regulation. Its implementation is based on the use of displacement rods that change the water-fuel ratio during the fuel campaign. To assess the effect on the neutron-physical characteristics that arise in the assembly when displacers are introduced into its design, various models are proposed that differ in the placement of rods in the fuel assembly. The comparison takes place with the original version of the fuel assembly, which has a similar geometry to the VVER-1200 fuel assembly. Based on a single scheme for regulating the water-fuel ratio, the contribution to the extension of the fuel campaign for models with displacers is estimated. The comparison is carried out with the initial fuel assemblies, where the fuel campaign is 49 MW * day/kg for model 1 and 78 MW * day/ kg for model 2. The results showed the fundamental possibility of extending the fuel campaign due to the phased extraction of displacer rods, which tighten the spectrum. In a model with displacers in the intertwined space, the campaign extension was 8 MW*day/kg, which is equivalent to 16%; in the model where the displacers are placed in the guide channels, the extension was 6 MW* day/kg, which is equivalent to 5%. Complete replacement of boric acid in the versions under consideration turned out to be impossible, while the required concentration of H3BO3 in the coolant to compensate for excessive reactivity at the beginning of the campaign increased due to a decrease in boron efficiency with a more rigid neutron spectrum in the models. Keywords: spectral regulation, fuel assemblies, boric acid, fuel campaign, light-water reactor.