A semi-graphical model of nuclear fuel burn-up as a function of the core energy generation and operating conditions: АС curve – variation of the parent U-235 isotope mass; 0N curve – increment of the bred Pu-239 isotope mass; ΔGe – equilibrium mass of initial fuel and bred fuel; point 1 – in-core accumulation of fission fragments during reactor rated power operation and with the core’s normal thermophysical parameters; point 3 – accumulation of fission fragments during the reactor core operation at a power less than the rated power and with variation of the core’s thermophysical parameters – 0F curve