Research of the influence of axial profiling of fuel assemblies (on the example of the Z49A2 model) on the effects of reactivity

The paper presents the results of studying the effect of axial profiling of the burnup absorber in the fuel rods of the fuel assemblies of the VVER-1200 reactor. Profiling was carried out by dividing the fuel rod into 5 equal zones in height, in which the shares of Gd were redistributed, based on data on the distribution of the neutron flux density along the height of the fuel rod of the original model. At the next stage, the concentrations of the remaining isotopes were recalculated so that the total number of nuclei of each isotope in the fuel element remained unchanged before and after the recalculation. Data were obtained on a decrease in the rate of reactivity loss, which allows us to put forward an assertion about the possibility of extending the fuel campaign. An assessment of the change in the conversion ratio was carried out, the influence of profiling on safety was investigated from the standpoint of reactivity effects, the proportion of delayed neutrons. The technology of fuel rod profiling did not have a significant effect on the CR and the proportion of delayed neutrons, the Doppler effect turned out to be almost identical, the density effect of reactivity at the beginning of the fuel campaign turned out to be more negative, at the end less. The accumulation of minor actinides (MA) is considered. The results showed that after profiling, the concentrations of MA increase. This aspect was justified by the calculation of the TVEG cell, which showed that after profiling, the neutron spectrum is warmed by 50%, as a result of which the rates of MA accumulation reactions increase. The data taken in parallel on the energy release in neighboring tablets at the boundaries of the profiling zones showed a significant gradient of energy release closer to the periphery, about 15%. This result requires a safety justification from the position of thermomechanics.