This paper deals with the development of an approximation method to evaluate the isotopic composition of burnt VVER fuel rods (with U-Gd) by engineering codes (TVS-M, PERMAK-A) and taking into account fuel rods depletion conditions. Burn-up conditions for fuel rods differ from each other during operation due to the presence of water cavities, absorber and other within FA. Therefore, the isotopes evolution during burn-up in different fuel rods goes on in different manner even with identical values of fuel burn-up (MW · day/kg U). The isotopic content evolution can be described in detail by spectral codes. However, a full-size core calculation takes much running time. The approximation method, which takes into account fuel rods spectral differences during operation, was considered. The method is based on spectral functionality (a non asymptotic factor) usage, which allows to take into account the spectral conditions and history of exposure.