The transition from light water reactors (LWR) to fast breeder reactors (FBR) is indispensable for sustainable nuclear electricity supply but has various Unpredictable factors such as introduction speed and time of FBR. In order to manage these unpredictable factors. the authors propose advanced fuel cycle system which consists of LWR spent fuel reprocessing (uranium removal) and FBR spent fuel reprocessing and fabrication. LWR fuel reprocessing only carries out about 90% uranium removal from LWR spent fuel. then the composition of remaining spent fuel (recycle material) is about 50% uranium, 15% plutonium and 35% fission products + minor actinides. Recycle material is transferred to FBR spent fuel reprocessing to recover plutonium and uranium followed by mixed Oxide (MOX) fuel fabrication for FBR. Depending on the introduction time of FBR. recycle material (about 1/10 volume of original LWR spent fuel) may be stored until the construction of FBR. The proposed fuel cycle system called "flexible fuel cycle initiative (FFC1)'' has some characteristics compared with ordinary (reference) system which consists of full reprocessing facilities of LWR and FBR spent fuels. that is smaller LWR reprocessing facility, higher plutonium content in recycle material for FBR. compact storage volume Of recycle material with high proliferation resistance in case of FBR introduction delay. By the evaluation of several transition scenarios from LWR to FBR. the FFC1 system was clarified to supply enough and no surplus plutonium for FBR. (c) 2006 Elsevier Ltd. All rights reserved.