Nuclear energy systems are necessary to assure sufficient energy resources without harming the environment. Fast reactor (FR) systems are especially important taking into account the limited uranium resources and the nuclear sustainability. As the FR system is still under development, FR deployment start-time and rate are unclear. On the other hand, it is desirable to reduce light water reactor (LWR) spent fuel due to the difficulties of storage and disposal (retrievable) site determination. Reprocessing is one of the effective methods to reduce LWR spent fuel but the recovery and long-term storage of plutonium, even with uranium, is undesirable for the aspect of proliferation resistance. The authors propose the new system named Flexible Fuel Cycle Initiative (FFCI), which recovers only uranium (∼90%) from LWR spent fuel and stores the residual material (∼5% U, ∼1% Pu, ∼4% other nuclides) for the future FR deployment. Residual material named recycle material (RM) is suitable for FR fresh fuel preparation due to its high Pu concentration and similar Pu/U ratio to FR core fuel, and for proliferation resistance due to its high concentrations of fission products (FP) and minor actinides (MA). The volume of RM is about 1/10 of that of LWR spent fuel. However RM needs sufficient heat removal, radiation shielding and criticality safety. After the FR development is finished and several years before the commercial FR deployment start-time, Pu and U will be recovered from the RM that might be stored liquid or solid state. Many well known methods can be applied for U recovery such as solvent extraction, crystallization, precipitation, electro refining, and fluoride volatilization. As recovered U has slightly higher U-235 concentration than natural U, its re-enrichment and recycling in LWRs seems to be effective for ultimate utilization of nuclear resources. In this case fluoride volatility U recovery method is most preferable because the product is UF6 that is the supply material for enrichment. Quantitative evaluations have been carried out for several fuel cycle systems including FFCI with parameters such as spent fuel amounts, facility capacity and Pu balance, which revealed the feasibility and flexibility of FFCI for LWR spent fuel reduction, high facility capacity factors and sufficient (no excess) Pu supply to FR.

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