This paper presents the results of the influence of reactivity on fuel temperature distribution for the neutron-physical characteristics of WWER 1000-1200 cores, which in turn affects the parameters of Xenon oscillation. The improved method for calculating Resonance Absorption in a 238 UO 2 (Doppler reactivity effect) with a Nonuniform Temperature Profile are developed. The parameters calculation results for Xenon oscillations have been calculated using complex program PROSTOR. The validation of the improved method was examined by comparing the calculated xenon parameters to measurement results obtained during commissioning tests of WWER-1000 reactor power units. Results showed a more accurate calculation for the Resonance Absorption in a 238 UO2, they significantly reduce the stabilization of Doppler Effect broadening, hence these facts, are vital importance in the case of load following mode accompanied by Xenon processes in the core. The improved method is implemented into the computer-software of the Full-Scale Simulator of the 3 rd Power Unit of the Rostov NPP and the Full-Scale Simulator of the 4 th Power Unit of the Kalinin NPP. The method was tested during acceptance tests of these Full-Scale Simulators.