[en] Numerical solution scheme and a program for calculating the fuel burnup fraction distribution in a three-dimensional polycell of a heterogeneous reactor are presented. The program is intended for the calculation of burnup fraction in the reactor polycell operating in the regime of continuous fueling as well as for modelling the fuel burnup process in the case of reactor fueling, performed at large time intervals. A spatial distribution is described with three-dimensional equations of the heterogeneous theory. Calculational results for fuel burnup fraction distributions in the three-dimensional polycell of a heavy water cooled reactor operating in the regime of continuous fueling are given as an example