In this study application of modified CANDLE burn-up scheme for medium-sized (800-1500 MWt) long-life Pb-Bi cooled fast reactors with natural uranium as fuel cycle input has been performed. The reactor cores are subdivided into several parts with equal volume in the axial directions. The natural uranium is initially put in region 1, after one cycle of 10 years of burn-up it is shifted to region 2, and 10 years after that it is shifted to region 3. This concept is applied to all regions, i.e. shifted the core of I-th region into I+1 region after the end of 10 years burn-up cycle. The first region 1 is filled by fresh natural uranium fuel. The core radial size is optimised to give maximum power density around 400 W/cc.